- Email: [email protected]
An investigation of the dependence of atmospheric corrosion rate on temperature using printed-circuit iron cells
~ )
Corrosion Science, Vol. 36, No. 5, pp. 797-813, 1994
Pergamon
Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0010-938X/94 $6.(11)+ 11.00
0010-938X(93)E0007-E
AN INVESTIGATION OF THE DEPENDENCE OF ATMOSPHERIC CORROSION RATE ON TEMPERATURE USING PRINTED-CIRCUIT IRON CELLS A. M. G. PACHECOand M. G. S. FERREIRA Dept. Engenharia Quimica, Instituto Superior T6cnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal
Abstraet~Chloride-mediated atmospheric corrosion was simulated on multilaminar, printed-circuit cells, in order to ascertain the effect of temperature on iron corrosion rate (early stages). Tests were carried out at 25, 35 and 45°C, under salt contaminations equivalent to chloride deposition rates of 0.25, 1.00 and 4.00 mg dm 2 d a y - l ; relative humidity was kept at 85% throughout and surface wetness was allowed to result solely from moisture uptake. Electrochemical data generated by linear polarization and zero-resistance ammetry are discussed with reference to acceleration factors, energy barriers (Arrhenius model) and process control. Temperature has a direct, marked influence on the rate of corrosion and also seems to regulate the catalytic rate dependence on [C1 ] through Freundlich isotherms. The apparent activation energy goes up to 87.5 kJ mol 1, and gives almost perfect plots with [CI ] (semi-log) and electrode overpotential (linear). All features concerning energy barricrs suggest an overall corrosion process under mixed control, bearing a significant, if not predominant contribution from charge transfer. INTRODUCTION
THE IMPORTANCEof temperature as the major atmospheric variable has not yet found a sound counterpart in its evaluation as a risk factor for materials performing in the open. This may stem from mechanistic reasoning that, intrinsically, atmospheric corrosion ought to be a diffusion-controlled process, regardless of surfaces being hindered by films of any nature (electrolytes and/or solid products), thickness (monolayer to visual range), or deposition mode (adsorption, condensation, direct wetting); with this approach, only a minor influence upon the mass transfer rate of oxygen in solution can be anticipated, due to self-convection, viscosity, and Nernst layer effects. In addition, the lack of reliable, real-time monitoring of atmospheric corrosion phenomena has prevented, thus far, an accurate risk assessment for the variables per se, and a deeper understanding of their covariance. As pointed out by Haynie, 1 environmental factors are strongly covariant and subject to error: therefore, a strained use of multiple-regression analysis can be questionable as leading to regressors with poor predictive skill, and/or to a lack of significance for the regression coefficients within their correlation matrices. Despite some continuing expectations that temperature would appear as an important factor in atmospheric corrosion, e statistical inference has often denied or, at least, not confirmed such an assumption, not only because temperature and relative humidity are closely related to one another, but also due to the covariance of pollutant fluxes with temperature. 1'3-7 In these terms and in spite of some sound evidence presented Manuscript received 27 August 1993. 797
798
A.M.G. PACHECOand M.G.S. FERREIRA
FIG. 1.
Two-electrode, multilaminar cell for atmospheric corrosion experiments.
by Sereda 8 and Grossman,9 correlation studies based on outdoor data are likely to be misleading, either by masking the true significance of temperature on corrosion, or by making it include by proxy the effects of other variables. This might have prevented temperature being given an explicit and/or independent weight in the formulation of some current corrosion models, to,11 This paper addresses the two items (process control and real-time monitoring) as part of a research program designed to investigate the early stages of iron corroding in chloride-containing atmospheres, whose gravimetric and structural aspects will be dealt with in forthcoming papers. Temperature-humidity effects have been examined by Preston12 and Sereda.8 After all, the comparatively little attention paid to the influence of temperature on atmospheric corrosion is by no means an exlusive of this topic: one should bear in mind that such an influence on galvanic corrosion was suggested by Doyle and Godard t3 only very late in the 60s. The situation for marine environments has been steadily reversed in a more or less explicit manner, t4-22 with temperature now being considered a primary factor of corrosivity, along with humidity and salt, in such atmospheres. 23 However a sizeable body of direct evidence has been seldom supported by statistical inference based on environmental data, although field observations go back even prior to Sereda, 8 for example Dychko and Dychko: 24 this fact still casts some doubt on temperature effects. This paper presents a laboratory simulation of atmospheric corrosion phenomena, studied by means of electrochemical techniques, in order to prevent cross-effects between process variables and to obtain a fast response from changes in temperature alone. EXPERIMENTAL METHOD Electrochemical testing (polarization resistance measurements and zero-resistance ammetry) was performed upon multilaminar, printed-circuit cells, whose layout is depicted in Fig. 1. Cells were engraved from pinhole-free, 50/~m thick iron foils, the chemical composition (major impurities, wt%) of which was as follows: C < 0.08, Mn < 0.03, P < 0.04, S < 0.05, Si < 0.10. After tight bonding to an epoxy laminate board 1.60 mm thick, the two-electrode configuration was obtained by processing a photoresist
A n investigation of atmospheric corrosion rate
799
(dry film) in aqueous m e d i u m , and by removing the exposed, unwanted parts with 5.0 M HC1 + 0.3 M FcCI 3. As shown in Fig. 1, each sensor consists of 40 metal lines, 30.00 _+ 0.10 m m × 0.50 + 0.05 m m , every other one externally shorted by two lateral tracks 5 m m wide. The whole assembly is 40 m m wide, with a line spacing of 200 ~tm. After etching, and prior to work, as-printed devices were stored in desiccators containing silica gel. Immediately before use, they were pickled for about 5 min in Clarke's solution, thoroughly rinsed under running distilled water, degreased with ethanol, and allowed to dry in a stream of cold air: then, all metallic parts but the sensor lines (i.e. external tracks, lead contacts) were insulated, thus leaving an exposed surface area of 3 cm 2 for corrosion testing. The composition of Clarke's solution is as follows: (a) hydrochloric acid (HCI, sp. gr. 1.19), 1 I; (b) antimony trioxide (Sb203) , 20 g; and (c) stannous chloride (Sn CI2), 50 g. Stock solutions of salt were prepared from reagent grade NaCI and de-ionized water (16-18 M~O.cm), in order to simulate chloride deposition rates of 0.25, 1.00, and 4.00 mg dm 2 day ~: these figures stand for all the categories of airborne salinity but So (background concentration), according to 1SO,2~ and their whole range roughly coincides with the reference one by Mattsson. 26 The initial pH value was approx. 6. I, a slightly acidic value similar to those often found for actual atmospheric films in clean marine environments or, at most, under low levels of SOx .27 The exposed sensors' parts were contaminated only once with equal volumes (168 /xl) of the corresponding salt solutions to avoid disturbing the temperature--humidity conditions and disrupting the m e a s u r e m e n t s during each test. Aliquots were poured on those parts, spread evenly all over them with the aid of a glass rod to a nominal thickness of 200/~m, and fully evaporated in a drying oven at 80°C for approx. 3 min. As soon as room temperature was recovered, the all-solid devices were transferred into small, tight-closed containers with a free volume < 10 dm 3, where they were held at constant temperaturc and relative humidity throughout the experiments; run onset was always referred to 15 rain of elapsed time from the closure. Relative humidity was created/maintained by the equilibrium vapour pressure of glycerol aqueous solutions placed in the bottom of the containers and it was regularly checked with a digital hygrometer with a capacitive probe. T e m p e r a t u r e control was achieved by m e a n s of a water bath: both the water and the enclosed air were continuously monitored by remote thermography using mercury probes. Throughout the whole operation, the m a x i m u m observed fluctuations in the former parameters lay within _+0.3% R H and +0.5°C for all the work atmospheres. The present cells are sclf-moistening devices, whose performance relics entirely on their hygroscopic capability toward a given environment, i.e. on moisture uptake and aqueous films developed thereupon, and not on any kind of m a n - m a d e wetting prior to testing. Moreover, it should be emphasized that the thickness of 200/~m stood merely for a reasonable compromise between easy spreading and quick drying: it bears no relation whichsoever to the extent of water adsorption on to technical metals, an issue first introduced by T o m a s h o v , 2s then semi-quantified by Barton et al. 29 and most discussed by Mikhailovskii and co-workers. 3° 32 The major experimental equipment included precision potcntiostats, a digital (16 bit) ramp gcnerator, high-impedance ( > 1012 o h m ) voltmeters, null-load a m m e t e r s - - h o m e - m a d e from low-noise (0.01 pA/X/Hzz), chopper-stabilized operational amplifiers - - and an electronic thermostat (+0.02°C). Linear polarization tests were carried out at 25, 35 and 45°C. Each factorial run of 16 cyclic scans lasted for 96 h at a constant humidity of 85% R H , and a salinity up to 4.00 mg dm -2 day i Ct . Raw data acquisition over a + 15 m V range around the open-circuit potential, soft data analysis within 5 mV shifts, a conversion factor (B) of 20 mV and fresh (clean and dry) cells were applied throughout. Short-circuit currents were m e a s u r e d in a pre-corroded cell, operated in both galvanic and electrolytic (AVcxt -- 30, 60, 120 mV) modes: dynamical temperature range was 25-55°C, with steps of ca I°C every 4 min and the relative humidity being kept to 85%, as before. For comparison purposes, analysis of cell output was focused on to the 25-45°C interval; it should be emphasized that the actual upper limit for trials of 55°C is not unrealistic: even for moderate climates, the temperature of rusted steel surfaces may reach 60-65°C, ~3 and figurcs far beyond these have also been quotcd. 34
EXPERIMENTAL Linear
polarization
daily basis thenceforth. squares
procedure:
scans were
RESULTS made
Experimental
AND
DISCUSSION
hourly for the first twelve hours
data were linearly fitted by means
l i n e s o f b e s t fit a v o i d t h e u n c e r t a i n t y
and on a of a least-
arisen from manipulating
800
A.M.G. PACHECOand M.G.S. FERREIRA
7
I
I
I
I
I
I
I
I
I
6 5
2
__
--
_
,.
m
1 0
0
2'0
4'0
6'0 t
8'0
1 O0
(h)
Fl~. 2. Cumulative charge passed in printed-circuit cells kept under simulated atmospheric conditions, as determined from linear polarization scans; [CI-] = 0.25 mg dm -2 day-~; R H = 85%; T = 25°C ( A ) , 35°C ( I ) , 45°C (O).
graphical supports, and the propagation of errors through algebraic and/or numerical algorithms, without misrepresenting the formal concept of polarization resistance (Rp). Furthermore, there is no need for tailoring any error function because not only the intrinsic reliability and the predictive skill of regression lines, but also the standard errors in their parameters can be easily derived from correlation analysis. Current-potential values were found to be linearly correlated at the 0.1 percent level (ct) for nearly all the 144 polarization data subsets: as a matter of fact, only two of these point to a lower level of significance (a = 0.01), namely the second- and eighth-hour scans of the 25°C/1.00 mg dm -2 day -1 C1- run. Still, the corresponding regression lines account for not less than 95% of the total variance to be explained, which means that, as a whole, results are highly significant, inasmuch as the remaining coefficients of determination lie very close to 100%. The standard errors in estimating the regression coefficients range from 0.4 to 13.6%, with an average of 2.9% ; however, should one trim this distribution's positive tail by simply disregarding the two aforementioned poorer fits, its skewness would be drastically reduced, with the maximum error of the slope variates then being 7.5% (trimmed average: 2.7%). In these terms and given such indices, there is no statistically significant difference between the regression line grade and the tangent inclination at the polarization plot origin for each scan. As the differences between corresponding slopes of fitted and drawn lines are not significant from zero at the uppermost probability level (a = 0.001), the intrinsic (statistical) reliability of Re -l values obtained from regressing current on potential seems hardly questionable; extrinsic (comparative) reliability will be discussed later. Reciprocals of polarization resistance were converted into corrosion currents through the Stern-Geary relationship (B = 20 mV); after numerical integration, the coulombic charge passed in each factorial experiment is shown against time elapsed (cumulative mode) in Figs 2-4. The relative acceleration induced by temperature may be better appreciated in terms of the iron average corrosion rate (Iacr) over the entire period of an experiment (96 h); such rates were obtained from ultimate
An investigation of atmospheric corrosion rate
801
12 10 8
4
2 0 0
20
40
60
80
100
t (h) Fro. 3.
S a m e a s F i g . 2, but[Cl ] = 1 . 0 0 m g d m 2day-a.
cumulative charges (Qt) using Faraday's law for iron, and their values are displayed on Table 1. The comparison between electrochemical estimates and gravimetric determinations (weight-loss measurements from as-exposed plates) yielded an efficiency, the so-called cell factor, of about 9%: this value was found to hold rather still throughout the experiments, and is consistent with the findings of Mansfield et al. 35 when evaluating the reproducibility of such measurements. The non-erratic performance of single-metal, printed-circuit iron cells can hardly be emphasized. Laboratory and outdoor studies have shown that electrochemical data underestimate true corrosion rates: 36'37 therefore, constant efficiency seems much more important an asset than any utopian search for an asymptotic behaviour toward unity. On the other hand, that constancy may be taken as an a posteriori validation of the option for an a p r i o r i B value and the conversion of Rp- 1thereby. Apart from the
40
I
I
I
I
I
I
I
I
I
35 30 25
15 10 5 0
20
40
60 t
Fro. 4.
80
100
(h)
Same as Fig. 2, but [CI-] = 4.00 mg dm-2 d a y - i.
802
A.M.G. PACHECOand M.G.S. FERREIRA TABLE 1. ELECTROCHEMICAL ESTIMATES OF IRON AVERAGE CORROSION RATE ( I a c r ) FROM TOTAL CHARGE ( Q t ) PASSED IN LINEAR POLARIZATION EXPERIMENTS
Factorial level (°C/mg dm -2 day 1CI-)
Qt (coul)
Iacr (rag dm 2 day-l)
25/0.25 25/1.00 25/4.00 35/0.25 35/1.00 35/4.00 45/0.25 45/1.00 45/4.00
0.706 1.905 9.811 1.981 6.283 19.993 6.506 11.042 37.333
1.70 4.60 23.66 4.78 15.15 48.22 15.69 26.63 90.04
figure itself, which appears to be good actually, 38 this procedure was almost turned into a standard by the few authors concerned with multilaminar cells as polarization resistance probes for atmospheric corrosion p h e n o m e n a . To our knowledge and since the early report by Kuznetsov et al. ,39 such data have been either converted this way, 4° or not converted at all, 41 but never by means of Tafel slopes: curiously enough, this possibility was disregarded even by researchers claiming access to the full extent of polarization curves, from which Tafel zones could be delimited. 42 Generally speaking, and in terms of Rp -1 conversion, our experience goes with Skinner's 43 r e c o m m e n d a t i o n for using judiciously chosen B values, not to mention the possibility of some apocryphal Tafel behaviour as observed by Fishman and Crowe, 44 and/or the scepticism expressed by M a n s f i e l d Y Particularly, and as far as the present study is concerned, neither absolute corrosion rates are at issue, nor is there any indication of mechanistic changes throughout the factorial design; also, both the intrinsic (statistical) and the extrinsic (comparative) reliability of linear polarization data were ascertained/discussed: therefore, and for the sake of simplicity, such electrochemical, apparent corrosion rates will be referred to as iron average corrosion rates (Iacr) , hereinafter, without further remarks on their true values. According to numbers in Table 1, raising the t e m p e r a t u r e from 25 to 45°C causes the rate of corrosion to increase by factors of about 9 (9.2), 6 (5.8), and 4 (3.8), under chloride contaminations of 0.25, 1.00, and 4.00 mg dm -2 day - l , respectively. That is to say, within a narrow interval of just 20°C, an increment of almost 10 times in atmospheric rate can be found for iron corroding in m o d e r a t e saline conditions, though the more severe these get, the less important that increment appears. Such results confirm the positive coefficient of acceleration in the early report by Sanyal and Bhadwar 46 and in the m o r e recent work by Shuvakhina et al. ,47 allowances being m a d e for test conditions that were somewhat different, the time scale especially. This last aspect may account for the steeper change in Iacr with respect to t e m p e r a t u r e observed here, as c o m p a r e d to similar ratios that can be inferred from both papers above: since one is dealing with corrosion rates averaged in different time intervals, initial fast kinetics smoothed down to different extents as well. On the other hand, the inverse synergy observed between the rate of chloride
An investigation of atmospheric corrosion rate
803
5
4
3
2 _= 1
0 0.0031
o.oo3
0.0033
o.oo34
T-I(K | ) FIG. 5. Arrhenius plots for iron average corrosion rate (lacr) from polarization resistance measurements; [CI-] = 0 . 2 5 m g d m - 2 d a y * ( A ) , 1 . 0 0 m g d m 2 day 1 ( m ) , 4 . 0 0 m g d m 2 day 1 ( 0 ) .
deposition and the magnitude of temperature-induced acceleration clearly substantiates some of this papers' opening remarks (cf. Introduction): namely, the covarlance of atmospheric variables and the need for a clearcut distinction of their intrinsic weight, in general terms, from their real impact in a particular situation. Otherwise, unimportant factors will keep on being misnomers for undetected effects: for instance, no appreciable influence of temperature on corrosion rate was found in the presence of chlorides by Mikhailovskii et al. 31 or by Hutchins and McKenzie,4~ probably because temperature variations were too small and/or their effects were overshadowed by others. The present study indicates that, depending on salinity, lacr may vary over a decade in response to an increment of just 20°C; the harsher the conditions get, the less the temperature coefficient shows up, though always remaining above those found for other technical metals (Cd, Cu, Zn): 47 therefore, temperature should be viewed as an important risk factor for iron in chloride-containing atmospheres. However, as pointed out by Brown and Masters, 49 depending on the specific characteristics of the environment and the metal in question, temperature changes have been reported to increase, decrease, or show no significant effect on atmospheric corrosion rates. This apparent ambiguity has led to a seeming assessment of unimportance, which is contradicted not only by the present results, but also by the knowledgeable assertion that, whenever the pollutant level is held constant for a given material, the important parameters remaining are relative humidity a n d temperature. 5° It is believed by us that there exists neither ambiguity nor contradiction, but rather a question of context probably ruled, in the final analysis, by corrosion layer features. On this subject, one should recall Vernon's 5t wise words some 40 years ago about atmospheric corrosion being pre-eminently the resultant of film formation and film breakdown, or simply pay attention to a recent study by Askey et al. 52 on iron and zinc in HCl-containing atmospheres. Other than its sign and magnitude, thermal acceleration may be evaluated in terms of its model. Figure 5 shows Iacr to comply with an Arrhenius-type behaviour; however simple, such a model still represents the best description of the change in rate of a process with respect to temperature, provided that both the frequency (pre-
804
A . M . G . PACHECO a n d M . G . S . FERREIRA TABLE 2. ACTIVATION ENERGIES (Ea) , PEARSON COEFFICIENTS (r), AND VARIANCE REMOVALS ( l ~ r ) , WITH REGARD TO THE ARRHENIUS LINES IN FIG. 5 CI 0.25 1.00 4.00
( m g d m -2 day - l )
E a (kJ tool -1)
r
var ( % )
87.5 69.5 52.7
-0.998 -0.983 - 1.000
99.7 96.6 100.0
exponential) factor and the activation energy (Ea) are constant within a given temperature range. Accordingly, an Arrhenius dependence of corrosion rate on temperature implies a thermally activated process, and so the corresponding test interval may be thought as encompassing the same basic mechanism, i.e. in practical terms, the same type of process control. Table 2 lists the relevant information concerning the Arrhenius plots in Fig. 5. From Fig. 5 and from the corresponding data in Table 2, it can be ascertained that the quality of fit is not alike: 3.4% of the total variation in the results obtained under 1.00 mg d m - : day -1 CI- remain to be explained, whereas the high-chloride regression is significant at any level by the criteria adhered to in this work, i.e., all coefficients of determination and correlation being rounded off to the nearest tenth and thousandth, respectively. Nevertheless, the analysis of residuals does not indicate lack-of-fit, and their pooled variance is about 1.2% ; on the other hand, an inadequacy of the model just for the mid-range contamination seems rather unlikely to occur in view of its statistical performance on either side. Therefore, in addition to sign and magnitude features, the acceleration caused by temperature is found to follow an Arrhenius pattern, reliable enough to yield unique, meaningful values of the activation energy for the initial stages of iron corroding under various salt burdens. This uniqueness clearly indicates temperature as the sole acceleration factor, otherwise some kind of deviation from linear Arrhenius behaviour should be expected; 53 also, it enables Ea values to be taken as representative for the temperature interval under consideration. Depending on salinity, the apparent activation energy (overall process) ranges from 52.7 to 87.5 kJ mol-l: despite the scarce data available from the literature, the former value in particular, may be seen to agree with the one that Manning et a1.,54 derived from operating mechanical probes under somewhat similar conditions. As those authors' comment on the reason for the paucity of Arrhenius data is to be endorsed, it seems worth putting some emphasis on such an agreement, since the corresponding acquisition modes are completely different; besides, the present results are consistent with a few others that have been reported for processes concerning chlorides. 55-57 Up to 87.5 kJ mol-1, apparent activation energy places itself within the range of charge-transfer control, albeit that its figures suggest a gradual transition toward values currently associated with mass-transfer control, as expected from the growth of chloride-enhanced, thick yet highly irregular rust phases on the iron surface. However, even with heavily corroded, rather hindered surfaces, energy barriers cannot be unambiguously assigned to transport phenomena: on the contrary, they seem too high not to include a significant if not predominant contribution from
An investigationof atmospheric corrosion rate
805
charge-transfer requirements. Although the data are not extensive, and E a values are not to be taken literally, 58 such a predominance should not be ruled out, notably for the lower chloride levels: diffusion barriers may reach 50-60 kJ mol-1, 54 but, in most cases, figures far behind these have been reported, 57'59~1 and, at least in one case, a thermal activation energy of 40 kJ mo1-1 was found to be consistent with rate determination by an interfacial reaction. 6~ In these terms and irrespective of the exact mechanism involved, Ea values point to a mixed control of the corrosion process; as chloride increases, they seem to approach those pertaining to single-ion transport through solid films, 55'56 whereas they become wider than those derived from other film failure models based on chloride-enhanced incorporation and/or chloride-induced mechanical disruption. 63'6a A rise in the degree of surface contamination pushes Ea into a range of values roughly between bulk diffusion in a liquid medium and individual progression through a solid phase: on phenomenological grounds, this may be seen to comply with the variable extent of diffusional limitations existing for an electrolyte soaked into the countless flaws (pores, cracks and various imperfections) of highly irregular and flaky, multiphase rust systems like the present one. However noticeable it may appear, the energy drop does not seem quite enough to fully discard interface control, even when the chloride deposition rate is at its maximum (4.00 mg dm 2 d a y - l ) . It should be noted that the top contamination in this study is not an upper boundary value for wet-candle salinity: non-episodic figures of 12.5 through c a 16.0 mg dm 2 day-1 CI can be found in the literature. 6s'66 Still, it represents an impressive salt burden, likely to bind all but a few of the airborne chloride deposition events; on the other hand, a value of 0.25 mg dm -2 d a y - l C1 is by no means a background rate. There are, of course, cases of extreme chloride pollution such as salt mists and direct wetting by marine spray and splash, even though, strictly speaking, the latter cannot be referred to as atmospheric phenomena; also, the importance of some abnormal salt episodes in the triggering and/or enhancing of corrosion processes further inland should not be overlooked. 67 Whatever happens, airborne salinity is strongly dependent on the geophysical factors influencing the dynamics of tropospheric marine aerosols, which leads to a very high variability inherent in the sea-salt profiles inland. Even taking all these aspects into account, the point is that an average flux of 4.00 mg dm -2 day -1 CI still ranks above most data from both wet and dry collection procedures, and that only at such a rate, Ea values become quasi-typical of diffusional limitations: in these terms, it seems most unlikely that chloride-induced atmospheric corrosion of iron should invariably be diffusion-controlled, at least in its early stages. Another interesting feature of the present data is illustrated by the steady decrease of the apparent activation energy with the increase in salt contamination, leading to an almost perfect negative correlation between the former variates and the logarithm of [Cl-], as shown in Fig. 6 (the three decimal Pearson coefficient of - 1.000, is significant at all levels). To our knowledge, such a relationship has never been reported to exist, probably because traditional methods of determining atmospheric corrosion rates may not be sensitive enough to enable such relationships to be searched for; yet, the present one would hardly result from chance alone, since when E~ is regressed to [Ci-] in half-logarithmic mode, the corresponding line leaves practically no residual variance to be accounted for. The highly significant correlation found between an electrodic quantity (E~), so to speak, directly derived from
806
A.M.G. PACHECOand M.G.S. FERREIRA
100.0
-"~,~.80.060.0
40.0 -2.00
-1100
0.00
1.00
2.00
ln(lCl- l/todd) FIG. 6.
Apparent activation energy over the range 25-45°C as a function of chloride
deposition rate. cell output and the simulated rate of chloride deposition that is ultimately a bulk concentration, suggests a catalytic process following a quantitative contact adsorption of the halide ions reaching the bare metal surface. On the other hand, the kinetic order of the corrosion process with respect to chloride ion (0 In Rp-1/c3 In [C1-])x,, where x i stands for any other process variable, may be stated in terms of the average observed rate (d In Iacr/d In [Cl-]) and seems to approach unity as t e m p e r a t u r e descends through 25°C. In fact, from Fig. 7, a linear bilogarithmic relationship between corrosion rate and salinity can be seen to exist for each test temperature: this means that the rate is proportional to the nth power of the chloride ion concentration, where n reads 0.63, 0.83 and 0.95, for 45, 35 and 25°C, respectively. These exponents correspond to the most probable values of regression coefficients in the logarithmic space of Fig. 7. As an average, the fitted lines fail to explain about 2.3% of the total variance in the results: although an examination of their residuals indicates no significant lack-of-fit, the corresponding slopes are likely to be no m o r e accurate than _ 0.03.
5.00 4.00 3.00 2.00 1.00 0.00 -2.00
-I ioo
o.'0o
t .oo
2.oo
ln([Cl- ]/mdd) FIG. 7.
Effect of chloride contamination on iron average corrosion rate (Iacr) from polarization resistance measurements; T = 25°C ( , ) , 35°C (I), 45°C (O).
An investigation of atmospheric corrosion rate
807
Such partial orders point to a strong rate dependence on [C1-], which seems to develop into first-order kinetics with respect to this ion as t e m p e r a t u r e decreases. The progressive deviation from unity observed for higher thermal steps (particularly for 45°C) suggests that the tendency for contact adsorption onto the metal surface is being opposed by an enhancement of random motions in the outer electrolyte layers. Moreover, the functional dependencies depicted in Fig. 7 can be viewed as a set of Freundlich isotherms. Many examples of bulk solute adsorption have been found to follow Freundlich-type equations 6s and the very fact that such a model can be seen to underlie the present data suggests, once more, that transport features are to be associated with diffusion processes in a liquid phase, rather than in the solid state. Otherwise, should young rusts act as an effective barrier to the migration of chloride ions inwards, neither a unit power dependence of corrosion rate on [CI ] would have been approached, nor a Freundlich-like behaviour could have been observed. Lastly, the catalytic role tentatively entertained for CI- seems to be the only one that conforms to both the aforesaid sharp dependence and the analysis of corrosion products yet performed: indeed, no chloride-containing rust phases were identified by X-ray diffraction and vibrational (infrared and R a m a n ) spectroscopies throughout this study, i.e., up to a deposition rate of 4.00 mg dm -2 day -1 CI-. Some evidence o f f l - F e O O H (a rust analogue of akagandite) emerging in early experimental rusts was given elsewhere 69 but its formation required much harsher conditions, namely [CI ] -> 8.00 mg dm 2 day I, and was never observed below this level; also, no other form of structural chloride could be identified. To this point, a thermal acceleration of the overall process (initial stages) of iron corroding in chloride-mediated conditions was clearly evidenced, and a catalytic action of the halide could be envisaged on an ab initio basis of rate dependence without rust prevalence. However, the kind of control acting on that process still does not appear to lie beyond reasonable doubt, that is whether charge-transfer features or abnormally severe transport limitations are showing through E~, values, An attempt was made to gain deeper insight into the last issue, A printed-circuit cell that had been tested at 45°C under 4.00 mg dm -2 day 1 C I - , was held at 25°C for eight days with no further contamination; for this period of time, the electrodes remained externally shorted by a 100 k ~ resistor, and corrosion was allowed to proceed in order to create a suitable device for direct current measurements. By driving such a load, the cell is polarized to an extent that will enable a net current, as small as it may appear, to be acquired later on without applying an external tension. This is an advisable procedure to maintain the signal of current during the rest period, which would otherwise tend to fluctuate randomly through zero. On the other hand, a well-rusted surface leads to an enhanced moisture uptake: with such a well-wetted "sponge", not only the cell signal could be ameliorated without losing its significance, TM but also any misevaluation (overestimation) of thermal acceleration factors as a result of drying-out effects v1'72 would be prevented. After the rest period, the electrodes were put through a zero-resistance a m m e t e r ( n a n o a m m e t e r ) and the cell was repeatedly cycled between 25 and 55°C, always under 85% RH. Following each t e m p e r a t u r e ramp at a sweep rate of ca 0.25°C m i n - i the cell was allowed to recover back to 25°C and this t e m p e r a t u r e was kept for about 2 h prior to the next sweep. The cell was first operated in a galvanic mode (AV = 0), and then in an electrolytic one (AV = 30, 60, 120 mV), AV being the external e m f that was successively applied between the two electrodes. Short-circuit currents
808
A . M . G . PACHECOand M. G.S. FERREIRA TABLE 3. REGRESSION (bl), CORRELATION (r), AND DETERMINATION (var) COEFFICIENTS, ABSOLUTE (abs) AND RELATIVE (%) STANDARD ERRORS (S.E.) OF SLOPE ESTIMATES, AND ACTIVATIONENERGIES (Ea) , WITH REGARDTO THE ARRHENIUS LINES IN FIG. 9 AV (mV) 0 30 60 120
b1
S.E. (abs)
S.E. (%)
r
var (%)
Ea(kJ mol - l )
-4737.3 -2634.0 -2463.2 -2149.0
148.7 128.9 146.7 36.8
3.1 4.9 6.0 1.7
-0.996 -0.989 -0.984 -0.999
99.1 97.9 96.9 99.7
39.4 21.9 20.5 17.9
in the operative range (25-55°C) are shown in Fig. 8 and Arrhenius plots for such currents (25--45°C) are depicted in Fig. 9; each point in the former figure stands for an average of triplicate runs. Table 3 lists the corresponding Arrhenius slopes (bl), and the absolute (abs) or relative (%) magnitude of their standard errors (S.E.); all the other symbols are trivial. There is an obvious temperature effect on the currents in Fig. 8, which act as proxy for iron corrosion rates through the thermal range. Such an acceleration results from both galvanic and forced cell output in a similar manner, though a current gap may be seen to exist between each type of data: this feature is even more noticeable when currents are plotted in Arrhenius mode (Fig. 9), and/or when it is translated into energy barriers (Table 3). Before going any further on this subject, it should be noted that short-circuit currents are estimates of instantaneous corrosion rates at best and thus they must not be directly matched to former average corrosion rates, insofar as these are integral values; also, in principle, they are likely to convey more bias than the reciprocals of polarization resistance. In these terms, thermal acceleration is consistently evidenced from both approaches, but it would be unwise to carry out any direct numerical comparison between them. On the other hand, current data fit in an
700
600 500
400 300
200 100 0 20
30
40
5O
60
T (C) FIG. 8. Short-circuit current output (25-55°C) of a pre-corroded cell from zero-resistance a m m e t r y m e a s u r e m e n t s ; AVext = 0 m V (D), 30 m V ( & ) , 60 m V ( I ) , 120 m V (O).
An investigation of atmospheric corrosion rate
809
6.5
6.0 5.5
5.0 4.5
4.0 0.0031
0.0032
0.0033
0.0034
T I(K 1) FIG. 9.
Arrhenius plots (25--45°C) for the short-circuit currents in Fig. 8 (same symbology).
Arrhenius model as before: accounting for nine degrees of freedom, all the correlation coefficients are significant far beyond the 0.001 level, and the standard errors of the regression coefficients (slope estimates) are expected not to exceed 6% at most, which enables activation energy to be determined with fair precision. As a whole, Ea values stand below those from linear polarization experiments; yet, they go up to 39.4 kJ mo1-1, a value that seems to be consistent with the former one by the same cell (52.7 kJ mol-1), allowing for the time elapsed, the surface conditions and the dissimilar parental measurements. As mentioned above, apparent activation energy shows some discontinuous behaviour with regard to the applied emf: there is a cut down to almost a half (21.9 kJ mo1-1) for the lower external input and a comparatively weaker response to higher voltage steps. All this happens as if a substantial amount of energy requirements could be withdrawn from the process by an external field, leaving behind a component much more insensitive to the same field. This suggests a dual source for those requirements, that is, a mixed control as hypothesized earlier: in these terms, the overall corrosion process would be under both anodie control by an interracial reaction and cathodic control by oxygen diffusion, with the former component getting more relevant for shorter times and/or lower salinity. There are several reasons in favour of this model. First, rate processes are likely to be field-assisted to a much greater extent than transport ones. Second, cathodic control by charge-transfer is very unlikely to occur in a near-neutral aqueous medium. Thirdly, it does not seem too hard to accept charge-transfer to become relatively less important as corrosion proceeds and/or ionic strength increases. Fourthly, the energy barriers remaining after what appears to be the removal of the charge-transfer share bear no further ambiguity, since their values are now most typical of aqueous diffusion. 73"74Finally and in connection with the last item, such field-assisted values would be expected to vary linearly with overpotentiai, 7s the slope being given by - 2 . 3 0 3 RT/b, where R is the gas constant, T is the absolute temperature, and b is the corresponding Tafel coefficient. The principle of an equal splitting of the external emf between electrodes was adhered hereto, as it has been done for small polarizations since Marsh 76 through this work. The quality of fit
810
A.M.G. PACHECOand M.G.S. FERREIRA 30.0
I
I
I
I
0
20.0
10.0
0
1 '5
3'0
4'5
6'0
75
Iql (mV)
Fro. 10. Apparent activation energy over the range 25-45°C as a function of electrode overpotential.
shown in Fig. 10 suggests that such an assumption remains valid as far as it goes, that is for twin electrodes polarized at a small-to-medium extent (Ir/I -< 60 mV), with the (conventional) working one being displaced in the negative direction, i.e. toward more negative potentials. The standard error in estimating the slope of the line in Fig. 10 is about 2.1%: this is quite an uncommon deviation from a regression which only minimal degrees of freedom are allowed for. Using the mid-point of the Arrhenius fitting range (35°C) as the reference temperature in the slope expression, a cathodic Tafel coefficient in excess of 66 V per decade is obtained, thus, in practical terms, an infinity. This result is consistent with the assignment of field-assisted energy barriers to aqueous diffusion, and it confirms an overall corrosion process under partial cathodic control; however, the relative magnitude of Ea values found through this work indicates that in self-driven atmospheric corrosion, the process is also partially, if not mainly controlled by charge-transfer. These findings refer to the initial stages of chlorideinduced atmospheric corrosion of iron, but, to some extent, they contradict some knowledge in the field (see, for instance, Rozenfeld 77 and references therein) that almost irrespective of both the substrate and the environment, atmospheric corrosion of metals would be essentially, if not exclusively controlled by the cathodic process of oxygen diffusing under short supply. CONCLUSIONS 1. As judged from simulated laboratory experiments, temperature may be considered an important variable affecting the corrosion of iron in chloridecontaining atmospheres. Factorial design and electrochemical monitoring are likely to prevent the covariance of environmental factors and to enable the process to be followed on a real-time basis: these are precisely the major drawbacks of traditional (outdoor) exposure tests. 2. Depending on salinity, Iacr may vary over a decade as a response to an increment of just 20°C in a common interval (25-45°C): the harsher the conditions get, the less the temperature coefficient shows up, though always remaining above those that have been found for other technical metals (Cd, Cu, Zn).
An investigation of atmospheric corrosion rate
811
3. A p p a r e n t activation energies go up to 87.5 kJ mol 1, thus placing themselves within the range of charge-transfer control; even with heavily corroded, rather hindered surfaces, energy barriers cannot be assigned to transport processes unambiguously. On the contrary, E~ values point to some mixed control, as they seem to include an ever significant contribution not derived from mass-transfer features: this assumption is based on the sharp energy drop due to any first input of external tension, and on the continuous, linear behaviour observed for higher voltage steps. Such a kind of disruption may be thought as corresponding to the removal of the charge-transfer quota, inasmuch as further evolution is almost steepless. 4. Field-assisted E a values strongly suggest an overall process under partial cathodic control, probably by the rate of oxygen diffusion in a stagnant, near-neutral electrolyte: there is a highly significant regression of activation energy to overpotential, and a cathodic Tafel slope of several tens of volt per decade (i.e. actually infinite) results thereupon. 5. Given the last two items and taking into account the relative magnitude of the energy barriers before (40-90 kJ mo1-1, roughly) and after (around 20 kJ mol - I ) applying an external emf, it seems most unlikely that self-driven, chloride-induced atmospheric corrosion of iron can be solely diffusion controlled, at least in its early stages. 6. T e m p e r a t u r e regulates the quantitative adsorption of CI- seemingly through Freundlich isotherms. Qualitatively, this pattern is consistent with the catalytic role envisaged for chloride ions, as suggested by a strong rate dependence with no structural rust appearance; also, quantitatively, it is consistent with a kinetic order (partial order) of the corrosion process with respect to [CI-] that was found to lie between 0.6 and 1.0 and to approach unity as t e m p e r a t u r e decreases. Besides, Freundlich behaviour agrees with the trivial morphology of chloride-induced rust layers that is likely to allow an almost unimpeded access inwards; otherwise, a model for solute adsorption from bulk solutions onto surfaces such as Freundlich's would hardly apply. 7. In general terms, the performance of printed-circuit cells as d.c. electrochemical probes appears as reliable, unbiased, and particularly suited for real-time monitoring, leading to results that are not only self-consistent, but also compare well to the scarce data available in the literature.
l. 2. 3. 4. 5. 6.
REFERENCES F. H. HAYNIE,Atmospheric Corrosion of Metals A S T M STP 767,286 (1982). P.J. SEREDA,Corrosion in Natural Environments A S T M STP 558, 7 (1974). K. OMA,T. SU~ANOand Y. HIRAI,Corros. Eng. (Boshoku Gi]utsu) 14, 16 (1965). F. H. HAYNIEand J. B. UPHAM,Corrosion in Natural Environments A S T M STP 558, 33 (1974). P. ZANNETTI,P. MELL1and E. RUNCA,Atmos. Envir. ll, 605 (1977). F. H. HAYNIE,Durability of Building Materials and Components ASTM STP 691,157 (1980).
7. P. BOURBON, M. GIROUX, J. ALARY, M. JULLIAT, M. C. GENSAC, M. F. BERGEAL, C. BALSA, M. CARPENTIERand J. M. FA~E, Tribune du CEBEDEA U 33(434), 3 (1980). ~. P.J. SEREDA, 1rid. Engng Chem. 52,157 (1961/). 9. P. R. GRoSSMAN, Atmospheric Factors Affecting the Corrosion of Engineering Metals A S T M STP 646,
5 (1978). 10. T. F. OTERO,A. PORROand A. S. ELOLA,Corrosion 48, 785 (1992). 11. J. W. SPENCE,F. H. HAYNIE,F. W. LIPFERT,S. D. CRAMERand L. G. McDONALD,Corrosion 48, 1009 (1992). 12. R. ST. J. PRESTOY,J. Iron Steel Inst. 160, 286 (1948).
812 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56.
A.M.G. PACHECOand M.G.S. FERREIRA D. P. DOYLE and H. P. GODARD, Mater. Prot. 8(8), 40 (1969). C. P. LARRABEE, Trans. electrochem. Soc. 87,161 (1945). J. C. HUDSON, Werkst. Korros. 15, 363 (1964). L. CERVENY,Tribune du CEBEDEA U 23(304), 114 (1969). J. F. STANNERS,Br. Corros. J. 5, 117 (1970). J. F. STANNERS,Tribune du CEBEDEAU24(324), 512 (1970). R. K. TRIPATHI, U. S. AGNIHOTRIand J. N. NANDA, Br. Corros. J. 7, 212 (1972). K. K. CHIN, S. D. RAMASWAMYand S. K. Roy, Proc. Conf. Tall Buildings, Kuala Lumpur, p. 6 (1974). J. F. STANNERS,Corrosion in Natural Environments A S T M STP 558, 23 (1974). S. K. RoY, Corrosion 39, 291 (1983). E. A. BAKER, Degradation o f Metals in the Atmosphere A S T M STP 965, 125 (1988). A. A. DYCHKO and K. A. DYCHKO, J. appl. Chem. USSR 30,261 (1957). ISO International Standard 9223, Corrosion o f Metals and Alloys--Classification o f Corrosivity o f Atmospheres. E. MATTSSON,Mater. Performance 21(7), 9 (1982). D. KNOTKOVA,-CERMAKOV~and J. VLICKOVA,,Werkst. Korros. 21, 16 (1970). N. D. TOMASHOV,Corrosion 20, 7t (1964). K. BARTON,S. BARTONOV.~and E. BER~NEK, Werkst. Korros. 25,659 (1974). Yu. N. MIKHAILOVSKn,V. V. AGAFONOVand V. A. SAIqKO, Protection o f Metals (Zashch. Met.) 13, 429 (1977). Yu. N. MIKHAILOVSKII,P. V. STREKALOVand V. V. AGAFONOV,Protection o f Metals (Zashch. Met.) 16,308 (1980). P. V. STREKALOV,YU. N. MIKHAILOVSKII,G. KH. DZHINCHARADZE, D. KNOTKOV~and J. VLCKOV~, Protection o f Metals (Zashch. Met.) 22,397 (1986). H. GUTrMAN, Metal Corrosion in the Atmosphere A S T M STP 435,223, (1968). Yu. M. PANCHENKO,P. V. STREKALOVand Yu. N. MIKHAILOVSKn,ProtectionofMetals(Zashch. Met.) 25,438 (1989). F. MANSFELD,S. TSA1, S. JEANJAQUET,E. MEYER, K. FERTIGand C. OGDEN, Atmospheric Corrosion of Metals A S T M STP 767,309 (1982). V. KUCERAand M. COLLIN,Proc. EUROCOR, '77, London, p. 189 (1977). F. MANSFELD and S. TSAI, Corros. Sci. 20,853 (1980). E. HEITZ and W. SCHWENK, Br. Corros. J. l l , 74 (1976). V. A. KUZNETSOV, S. G. POLYAKOV,YU. S. GERASIMENKOand Yu. G. KOTLOV, Protection o f Metals (Zashch. Met.) 12, 578 (1976). J. A. GONZ~,LEZ, Werkst. Korros. 29,456 (1978). F. MANSFELD, S. L. JEANJAQUET,M. W. KENDIG and D. K. ROE, Atmos. Envir. 20, 1179 (1986). J. A. GONZALEZ, E. OTERO, C. CABAIqASand J. M. BASTIDAS,Br. Corros. J. 19, 89 (1984). W. SKINNER, Br. Corros. J. 22, 172 (1987). S. G. FISHMANand C. R. CROWE, Corros. Sci. 17, 27 (1977). F. MANSFELD, Atmospheric Corrosion (ed. W. H. AILOR), p. 139. John Wiley and Sons Inc., New York (1982). B. SANYALand D. B. BHADWAR,J. Sci. Ind. Res. (India) 21A, 243 (1962). L. A. SHUVAKHINA,Yu. N. MIKHAILOVSKII,N. F. SHARONOVAand V. S. SEDOVA,Protection of Metals (Zashch. Met.) 13, 130 (1977). J. S. HUTCHINSand M. MCKENZlE, Br. Corros. J. 9, 158 (1974). P. W. BROWN and L. W. MASTERS, in Atmospheric Corrosion (ed. W. H. AILOR), p. 31. John Wiley and Sons Inc., New York (1982). S. P. SHARMA,J. H. THOMASIII and F. E. BADER, J. electrochem. Soc. 125, 2002 (1978). W. H. J. VERNON, Research 5, 54 (1952). A. ASKEY, S. B. LYON, G. E. THOMPSON,J. B. JOHNSON, G. C. WooD, M. COOKEand P. SAGE,Corros. Sci. 34,233 (1993). A. MILCH and P. TASAICO,J. eleetrochem. Soc. 127, 884 (1980). M. I. MANNING, A. G. CROUCHand B. LLOYD, Mater. Performance 27(9), 74 (1988). R. FOSTERand T. P. HOAR, Localized Corrosion (eds, C. L. MCBEE and J. KRUGER), p. 698. NACE, Houston (1972). M. S. DE SA,, C. M. RANGELand C. A. C. SEQUEIRA,Br. Corros. J. 23, 186 (1988).
An investigation of atmospheric corrosion rate 57. 58. 59. 60. 6l. 62. 63. 64. 65. 66. 67. 68. 69.
70. 71. 72. 73. 74. 75. 76. 77.
813
J.-R. PARKand D. D. MACDONALD,Corrosion 45,563 (1989). N. SCHWARTZand D. D. BACON,J. electrochem. Soc. 125, 1487 (1978). N. SATOand M. COUEN,J. electrochem. Soc. 111,512 (1964). J. W. SPENCE,F. H. HAYNIE, D. C. STILESand E. O. EDNEY, Degradation of Metals in the Atmosphere A S T M STP 965,339 (1988). A. A. OLOWE,B. PAURONand J. M. R. G~NIN, Corros. Sci. 32,985 (1991). J. ROBERTSONand J. E. FORREST,Corros. Sei. 32,521 (1991). T. P. HOARand W. R. JACOB,Nature 216, 1299 (1967). J. L. DAWSONand M. G. S. FERREIRA,Corros. Sci. 26, 1009 (1986). A. K. SINGH, T. ERICSSON,L. HAGGSTROMand J. GULLMAN,Corros. Sci. 25,931 (1985). A . A . BRAGARDand H. E. BONNARENS,Atmospheric Corrosion of Metals A S T M STP 767,339 (1982). B. LLOYDand M. I. MANNING, Corros. Sci. 30, 77 (1990). A. K. GAt,WEY, Chemistry o f Solids, Science Paperbacks, Chapman and Hall Ltd, London (1967). A. M. G. PACttECO, Initial Stages of the Chloride-induced Atmospheric Corrosion of Iron: Gravimetric, Electrochemical, and Structural Aspects (Doctoral Thesis). Technical University of Lisbon, Lisbon (1991). F. MANSFELDand J. V. KENKEL,Corrosion 33, 13 (1977). M. STRATMANN,K. BOHNENKAMPand H.-J. ENGELL, Werkst. Korros. 34,604 (1983). M. J. Jusxo and M. G. S. FERREIRA,Corros. Sci. 29, 1353 (1989). K. J. LAIDLER, Chemical Kinetics. McGraw-Hill Book Co., New York (1965). J. M. WEST, Basic Corrosion and Oxidation. Ellis Horwood, London (1980). E. GILEAD1, E. KIROWA-EISNERand J. PENC1NER, Interracial Electrochemistry: An Experimental Approach. Addison-Wesley Publishing Co. Inc., Reading, MA (1975). G. A. MARSH, Proc. 2nd Int. Cong. Metallic Corrosion, New York, p. 936 (1963). I. L. ROZENFELD,Atmospheric Corrosion o f Metals, NACE, Houston (1972).
Corrosion Science, Vol. 36, No. 5, pp. 797-813, 1994
Pergamon
Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0010-938X/94 $6.(11)+ 11.00
0010-938X(93)E0007-E
AN INVESTIGATION OF THE DEPENDENCE OF ATMOSPHERIC CORROSION RATE ON TEMPERATURE USING PRINTED-CIRCUIT IRON CELLS A. M. G. PACHECOand M. G. S. FERREIRA Dept. Engenharia Quimica, Instituto Superior T6cnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal
Abstraet~Chloride-mediated atmospheric corrosion was simulated on multilaminar, printed-circuit cells, in order to ascertain the effect of temperature on iron corrosion rate (early stages). Tests were carried out at 25, 35 and 45°C, under salt contaminations equivalent to chloride deposition rates of 0.25, 1.00 and 4.00 mg dm 2 d a y - l ; relative humidity was kept at 85% throughout and surface wetness was allowed to result solely from moisture uptake. Electrochemical data generated by linear polarization and zero-resistance ammetry are discussed with reference to acceleration factors, energy barriers (Arrhenius model) and process control. Temperature has a direct, marked influence on the rate of corrosion and also seems to regulate the catalytic rate dependence on [C1 ] through Freundlich isotherms. The apparent activation energy goes up to 87.5 kJ mol 1, and gives almost perfect plots with [CI ] (semi-log) and electrode overpotential (linear). All features concerning energy barricrs suggest an overall corrosion process under mixed control, bearing a significant, if not predominant contribution from charge transfer. INTRODUCTION
THE IMPORTANCEof temperature as the major atmospheric variable has not yet found a sound counterpart in its evaluation as a risk factor for materials performing in the open. This may stem from mechanistic reasoning that, intrinsically, atmospheric corrosion ought to be a diffusion-controlled process, regardless of surfaces being hindered by films of any nature (electrolytes and/or solid products), thickness (monolayer to visual range), or deposition mode (adsorption, condensation, direct wetting); with this approach, only a minor influence upon the mass transfer rate of oxygen in solution can be anticipated, due to self-convection, viscosity, and Nernst layer effects. In addition, the lack of reliable, real-time monitoring of atmospheric corrosion phenomena has prevented, thus far, an accurate risk assessment for the variables per se, and a deeper understanding of their covariance. As pointed out by Haynie, 1 environmental factors are strongly covariant and subject to error: therefore, a strained use of multiple-regression analysis can be questionable as leading to regressors with poor predictive skill, and/or to a lack of significance for the regression coefficients within their correlation matrices. Despite some continuing expectations that temperature would appear as an important factor in atmospheric corrosion, e statistical inference has often denied or, at least, not confirmed such an assumption, not only because temperature and relative humidity are closely related to one another, but also due to the covariance of pollutant fluxes with temperature. 1'3-7 In these terms and in spite of some sound evidence presented Manuscript received 27 August 1993. 797
798
A.M.G. PACHECOand M.G.S. FERREIRA
FIG. 1.
Two-electrode, multilaminar cell for atmospheric corrosion experiments.
by Sereda 8 and Grossman,9 correlation studies based on outdoor data are likely to be misleading, either by masking the true significance of temperature on corrosion, or by making it include by proxy the effects of other variables. This might have prevented temperature being given an explicit and/or independent weight in the formulation of some current corrosion models, to,11 This paper addresses the two items (process control and real-time monitoring) as part of a research program designed to investigate the early stages of iron corroding in chloride-containing atmospheres, whose gravimetric and structural aspects will be dealt with in forthcoming papers. Temperature-humidity effects have been examined by Preston12 and Sereda.8 After all, the comparatively little attention paid to the influence of temperature on atmospheric corrosion is by no means an exlusive of this topic: one should bear in mind that such an influence on galvanic corrosion was suggested by Doyle and Godard t3 only very late in the 60s. The situation for marine environments has been steadily reversed in a more or less explicit manner, t4-22 with temperature now being considered a primary factor of corrosivity, along with humidity and salt, in such atmospheres. 23 However a sizeable body of direct evidence has been seldom supported by statistical inference based on environmental data, although field observations go back even prior to Sereda, 8 for example Dychko and Dychko: 24 this fact still casts some doubt on temperature effects. This paper presents a laboratory simulation of atmospheric corrosion phenomena, studied by means of electrochemical techniques, in order to prevent cross-effects between process variables and to obtain a fast response from changes in temperature alone. EXPERIMENTAL METHOD Electrochemical testing (polarization resistance measurements and zero-resistance ammetry) was performed upon multilaminar, printed-circuit cells, whose layout is depicted in Fig. 1. Cells were engraved from pinhole-free, 50/~m thick iron foils, the chemical composition (major impurities, wt%) of which was as follows: C < 0.08, Mn < 0.03, P < 0.04, S < 0.05, Si < 0.10. After tight bonding to an epoxy laminate board 1.60 mm thick, the two-electrode configuration was obtained by processing a photoresist
A n investigation of atmospheric corrosion rate
799
(dry film) in aqueous m e d i u m , and by removing the exposed, unwanted parts with 5.0 M HC1 + 0.3 M FcCI 3. As shown in Fig. 1, each sensor consists of 40 metal lines, 30.00 _+ 0.10 m m × 0.50 + 0.05 m m , every other one externally shorted by two lateral tracks 5 m m wide. The whole assembly is 40 m m wide, with a line spacing of 200 ~tm. After etching, and prior to work, as-printed devices were stored in desiccators containing silica gel. Immediately before use, they were pickled for about 5 min in Clarke's solution, thoroughly rinsed under running distilled water, degreased with ethanol, and allowed to dry in a stream of cold air: then, all metallic parts but the sensor lines (i.e. external tracks, lead contacts) were insulated, thus leaving an exposed surface area of 3 cm 2 for corrosion testing. The composition of Clarke's solution is as follows: (a) hydrochloric acid (HCI, sp. gr. 1.19), 1 I; (b) antimony trioxide (Sb203) , 20 g; and (c) stannous chloride (Sn CI2), 50 g. Stock solutions of salt were prepared from reagent grade NaCI and de-ionized water (16-18 M~O.cm), in order to simulate chloride deposition rates of 0.25, 1.00, and 4.00 mg dm 2 day ~: these figures stand for all the categories of airborne salinity but So (background concentration), according to 1SO,2~ and their whole range roughly coincides with the reference one by Mattsson. 26 The initial pH value was approx. 6. I, a slightly acidic value similar to those often found for actual atmospheric films in clean marine environments or, at most, under low levels of SOx .27 The exposed sensors' parts were contaminated only once with equal volumes (168 /xl) of the corresponding salt solutions to avoid disturbing the temperature--humidity conditions and disrupting the m e a s u r e m e n t s during each test. Aliquots were poured on those parts, spread evenly all over them with the aid of a glass rod to a nominal thickness of 200/~m, and fully evaporated in a drying oven at 80°C for approx. 3 min. As soon as room temperature was recovered, the all-solid devices were transferred into small, tight-closed containers with a free volume < 10 dm 3, where they were held at constant temperaturc and relative humidity throughout the experiments; run onset was always referred to 15 rain of elapsed time from the closure. Relative humidity was created/maintained by the equilibrium vapour pressure of glycerol aqueous solutions placed in the bottom of the containers and it was regularly checked with a digital hygrometer with a capacitive probe. T e m p e r a t u r e control was achieved by m e a n s of a water bath: both the water and the enclosed air were continuously monitored by remote thermography using mercury probes. Throughout the whole operation, the m a x i m u m observed fluctuations in the former parameters lay within _+0.3% R H and +0.5°C for all the work atmospheres. The present cells are sclf-moistening devices, whose performance relics entirely on their hygroscopic capability toward a given environment, i.e. on moisture uptake and aqueous films developed thereupon, and not on any kind of m a n - m a d e wetting prior to testing. Moreover, it should be emphasized that the thickness of 200/~m stood merely for a reasonable compromise between easy spreading and quick drying: it bears no relation whichsoever to the extent of water adsorption on to technical metals, an issue first introduced by T o m a s h o v , 2s then semi-quantified by Barton et al. 29 and most discussed by Mikhailovskii and co-workers. 3° 32 The major experimental equipment included precision potcntiostats, a digital (16 bit) ramp gcnerator, high-impedance ( > 1012 o h m ) voltmeters, null-load a m m e t e r s - - h o m e - m a d e from low-noise (0.01 pA/X/Hzz), chopper-stabilized operational amplifiers - - and an electronic thermostat (+0.02°C). Linear polarization tests were carried out at 25, 35 and 45°C. Each factorial run of 16 cyclic scans lasted for 96 h at a constant humidity of 85% R H , and a salinity up to 4.00 mg dm -2 day i Ct . Raw data acquisition over a + 15 m V range around the open-circuit potential, soft data analysis within 5 mV shifts, a conversion factor (B) of 20 mV and fresh (clean and dry) cells were applied throughout. Short-circuit currents were m e a s u r e d in a pre-corroded cell, operated in both galvanic and electrolytic (AVcxt -- 30, 60, 120 mV) modes: dynamical temperature range was 25-55°C, with steps of ca I°C every 4 min and the relative humidity being kept to 85%, as before. For comparison purposes, analysis of cell output was focused on to the 25-45°C interval; it should be emphasized that the actual upper limit for trials of 55°C is not unrealistic: even for moderate climates, the temperature of rusted steel surfaces may reach 60-65°C, ~3 and figurcs far beyond these have also been quotcd. 34
EXPERIMENTAL Linear
polarization
daily basis thenceforth. squares
procedure:
scans were
RESULTS made
Experimental
AND
DISCUSSION
hourly for the first twelve hours
data were linearly fitted by means
l i n e s o f b e s t fit a v o i d t h e u n c e r t a i n t y
and on a of a least-
arisen from manipulating
800
A.M.G. PACHECOand M.G.S. FERREIRA
7
I
I
I
I
I
I
I
I
I
6 5
2
__
--
_
,.
m
1 0
0
2'0
4'0
6'0 t
8'0
1 O0
(h)
Fl~. 2. Cumulative charge passed in printed-circuit cells kept under simulated atmospheric conditions, as determined from linear polarization scans; [CI-] = 0.25 mg dm -2 day-~; R H = 85%; T = 25°C ( A ) , 35°C ( I ) , 45°C (O).
graphical supports, and the propagation of errors through algebraic and/or numerical algorithms, without misrepresenting the formal concept of polarization resistance (Rp). Furthermore, there is no need for tailoring any error function because not only the intrinsic reliability and the predictive skill of regression lines, but also the standard errors in their parameters can be easily derived from correlation analysis. Current-potential values were found to be linearly correlated at the 0.1 percent level (ct) for nearly all the 144 polarization data subsets: as a matter of fact, only two of these point to a lower level of significance (a = 0.01), namely the second- and eighth-hour scans of the 25°C/1.00 mg dm -2 day -1 C1- run. Still, the corresponding regression lines account for not less than 95% of the total variance to be explained, which means that, as a whole, results are highly significant, inasmuch as the remaining coefficients of determination lie very close to 100%. The standard errors in estimating the regression coefficients range from 0.4 to 13.6%, with an average of 2.9% ; however, should one trim this distribution's positive tail by simply disregarding the two aforementioned poorer fits, its skewness would be drastically reduced, with the maximum error of the slope variates then being 7.5% (trimmed average: 2.7%). In these terms and given such indices, there is no statistically significant difference between the regression line grade and the tangent inclination at the polarization plot origin for each scan. As the differences between corresponding slopes of fitted and drawn lines are not significant from zero at the uppermost probability level (a = 0.001), the intrinsic (statistical) reliability of Re -l values obtained from regressing current on potential seems hardly questionable; extrinsic (comparative) reliability will be discussed later. Reciprocals of polarization resistance were converted into corrosion currents through the Stern-Geary relationship (B = 20 mV); after numerical integration, the coulombic charge passed in each factorial experiment is shown against time elapsed (cumulative mode) in Figs 2-4. The relative acceleration induced by temperature may be better appreciated in terms of the iron average corrosion rate (Iacr) over the entire period of an experiment (96 h); such rates were obtained from ultimate
An investigation of atmospheric corrosion rate
801
12 10 8
4
2 0 0
20
40
60
80
100
t (h) Fro. 3.
S a m e a s F i g . 2, but[Cl ] = 1 . 0 0 m g d m 2day-a.
cumulative charges (Qt) using Faraday's law for iron, and their values are displayed on Table 1. The comparison between electrochemical estimates and gravimetric determinations (weight-loss measurements from as-exposed plates) yielded an efficiency, the so-called cell factor, of about 9%: this value was found to hold rather still throughout the experiments, and is consistent with the findings of Mansfield et al. 35 when evaluating the reproducibility of such measurements. The non-erratic performance of single-metal, printed-circuit iron cells can hardly be emphasized. Laboratory and outdoor studies have shown that electrochemical data underestimate true corrosion rates: 36'37 therefore, constant efficiency seems much more important an asset than any utopian search for an asymptotic behaviour toward unity. On the other hand, that constancy may be taken as an a posteriori validation of the option for an a p r i o r i B value and the conversion of Rp- 1thereby. Apart from the
40
I
I
I
I
I
I
I
I
I
35 30 25
15 10 5 0
20
40
60 t
Fro. 4.
80
100
(h)
Same as Fig. 2, but [CI-] = 4.00 mg dm-2 d a y - i.
802
A.M.G. PACHECOand M.G.S. FERREIRA TABLE 1. ELECTROCHEMICAL ESTIMATES OF IRON AVERAGE CORROSION RATE ( I a c r ) FROM TOTAL CHARGE ( Q t ) PASSED IN LINEAR POLARIZATION EXPERIMENTS
Factorial level (°C/mg dm -2 day 1CI-)
Qt (coul)
Iacr (rag dm 2 day-l)
25/0.25 25/1.00 25/4.00 35/0.25 35/1.00 35/4.00 45/0.25 45/1.00 45/4.00
0.706 1.905 9.811 1.981 6.283 19.993 6.506 11.042 37.333
1.70 4.60 23.66 4.78 15.15 48.22 15.69 26.63 90.04
figure itself, which appears to be good actually, 38 this procedure was almost turned into a standard by the few authors concerned with multilaminar cells as polarization resistance probes for atmospheric corrosion p h e n o m e n a . To our knowledge and since the early report by Kuznetsov et al. ,39 such data have been either converted this way, 4° or not converted at all, 41 but never by means of Tafel slopes: curiously enough, this possibility was disregarded even by researchers claiming access to the full extent of polarization curves, from which Tafel zones could be delimited. 42 Generally speaking, and in terms of Rp -1 conversion, our experience goes with Skinner's 43 r e c o m m e n d a t i o n for using judiciously chosen B values, not to mention the possibility of some apocryphal Tafel behaviour as observed by Fishman and Crowe, 44 and/or the scepticism expressed by M a n s f i e l d Y Particularly, and as far as the present study is concerned, neither absolute corrosion rates are at issue, nor is there any indication of mechanistic changes throughout the factorial design; also, both the intrinsic (statistical) and the extrinsic (comparative) reliability of linear polarization data were ascertained/discussed: therefore, and for the sake of simplicity, such electrochemical, apparent corrosion rates will be referred to as iron average corrosion rates (Iacr) , hereinafter, without further remarks on their true values. According to numbers in Table 1, raising the t e m p e r a t u r e from 25 to 45°C causes the rate of corrosion to increase by factors of about 9 (9.2), 6 (5.8), and 4 (3.8), under chloride contaminations of 0.25, 1.00, and 4.00 mg dm -2 day - l , respectively. That is to say, within a narrow interval of just 20°C, an increment of almost 10 times in atmospheric rate can be found for iron corroding in m o d e r a t e saline conditions, though the more severe these get, the less important that increment appears. Such results confirm the positive coefficient of acceleration in the early report by Sanyal and Bhadwar 46 and in the m o r e recent work by Shuvakhina et al. ,47 allowances being m a d e for test conditions that were somewhat different, the time scale especially. This last aspect may account for the steeper change in Iacr with respect to t e m p e r a t u r e observed here, as c o m p a r e d to similar ratios that can be inferred from both papers above: since one is dealing with corrosion rates averaged in different time intervals, initial fast kinetics smoothed down to different extents as well. On the other hand, the inverse synergy observed between the rate of chloride
An investigation of atmospheric corrosion rate
803
5
4
3
2 _= 1
0 0.0031
o.oo3
0.0033
o.oo34
T-I(K | ) FIG. 5. Arrhenius plots for iron average corrosion rate (lacr) from polarization resistance measurements; [CI-] = 0 . 2 5 m g d m - 2 d a y * ( A ) , 1 . 0 0 m g d m 2 day 1 ( m ) , 4 . 0 0 m g d m 2 day 1 ( 0 ) .
deposition and the magnitude of temperature-induced acceleration clearly substantiates some of this papers' opening remarks (cf. Introduction): namely, the covarlance of atmospheric variables and the need for a clearcut distinction of their intrinsic weight, in general terms, from their real impact in a particular situation. Otherwise, unimportant factors will keep on being misnomers for undetected effects: for instance, no appreciable influence of temperature on corrosion rate was found in the presence of chlorides by Mikhailovskii et al. 31 or by Hutchins and McKenzie,4~ probably because temperature variations were too small and/or their effects were overshadowed by others. The present study indicates that, depending on salinity, lacr may vary over a decade in response to an increment of just 20°C; the harsher the conditions get, the less the temperature coefficient shows up, though always remaining above those found for other technical metals (Cd, Cu, Zn): 47 therefore, temperature should be viewed as an important risk factor for iron in chloride-containing atmospheres. However, as pointed out by Brown and Masters, 49 depending on the specific characteristics of the environment and the metal in question, temperature changes have been reported to increase, decrease, or show no significant effect on atmospheric corrosion rates. This apparent ambiguity has led to a seeming assessment of unimportance, which is contradicted not only by the present results, but also by the knowledgeable assertion that, whenever the pollutant level is held constant for a given material, the important parameters remaining are relative humidity a n d temperature. 5° It is believed by us that there exists neither ambiguity nor contradiction, but rather a question of context probably ruled, in the final analysis, by corrosion layer features. On this subject, one should recall Vernon's 5t wise words some 40 years ago about atmospheric corrosion being pre-eminently the resultant of film formation and film breakdown, or simply pay attention to a recent study by Askey et al. 52 on iron and zinc in HCl-containing atmospheres. Other than its sign and magnitude, thermal acceleration may be evaluated in terms of its model. Figure 5 shows Iacr to comply with an Arrhenius-type behaviour; however simple, such a model still represents the best description of the change in rate of a process with respect to temperature, provided that both the frequency (pre-
804
A . M . G . PACHECO a n d M . G . S . FERREIRA TABLE 2. ACTIVATION ENERGIES (Ea) , PEARSON COEFFICIENTS (r), AND VARIANCE REMOVALS ( l ~ r ) , WITH REGARD TO THE ARRHENIUS LINES IN FIG. 5 CI 0.25 1.00 4.00
( m g d m -2 day - l )
E a (kJ tool -1)
r
var ( % )
87.5 69.5 52.7
-0.998 -0.983 - 1.000
99.7 96.6 100.0
exponential) factor and the activation energy (Ea) are constant within a given temperature range. Accordingly, an Arrhenius dependence of corrosion rate on temperature implies a thermally activated process, and so the corresponding test interval may be thought as encompassing the same basic mechanism, i.e. in practical terms, the same type of process control. Table 2 lists the relevant information concerning the Arrhenius plots in Fig. 5. From Fig. 5 and from the corresponding data in Table 2, it can be ascertained that the quality of fit is not alike: 3.4% of the total variation in the results obtained under 1.00 mg d m - : day -1 CI- remain to be explained, whereas the high-chloride regression is significant at any level by the criteria adhered to in this work, i.e., all coefficients of determination and correlation being rounded off to the nearest tenth and thousandth, respectively. Nevertheless, the analysis of residuals does not indicate lack-of-fit, and their pooled variance is about 1.2% ; on the other hand, an inadequacy of the model just for the mid-range contamination seems rather unlikely to occur in view of its statistical performance on either side. Therefore, in addition to sign and magnitude features, the acceleration caused by temperature is found to follow an Arrhenius pattern, reliable enough to yield unique, meaningful values of the activation energy for the initial stages of iron corroding under various salt burdens. This uniqueness clearly indicates temperature as the sole acceleration factor, otherwise some kind of deviation from linear Arrhenius behaviour should be expected; 53 also, it enables Ea values to be taken as representative for the temperature interval under consideration. Depending on salinity, the apparent activation energy (overall process) ranges from 52.7 to 87.5 kJ mol-l: despite the scarce data available from the literature, the former value in particular, may be seen to agree with the one that Manning et a1.,54 derived from operating mechanical probes under somewhat similar conditions. As those authors' comment on the reason for the paucity of Arrhenius data is to be endorsed, it seems worth putting some emphasis on such an agreement, since the corresponding acquisition modes are completely different; besides, the present results are consistent with a few others that have been reported for processes concerning chlorides. 55-57 Up to 87.5 kJ mol-1, apparent activation energy places itself within the range of charge-transfer control, albeit that its figures suggest a gradual transition toward values currently associated with mass-transfer control, as expected from the growth of chloride-enhanced, thick yet highly irregular rust phases on the iron surface. However, even with heavily corroded, rather hindered surfaces, energy barriers cannot be unambiguously assigned to transport phenomena: on the contrary, they seem too high not to include a significant if not predominant contribution from
An investigationof atmospheric corrosion rate
805
charge-transfer requirements. Although the data are not extensive, and E a values are not to be taken literally, 58 such a predominance should not be ruled out, notably for the lower chloride levels: diffusion barriers may reach 50-60 kJ mol-1, 54 but, in most cases, figures far behind these have been reported, 57'59~1 and, at least in one case, a thermal activation energy of 40 kJ mo1-1 was found to be consistent with rate determination by an interfacial reaction. 6~ In these terms and irrespective of the exact mechanism involved, Ea values point to a mixed control of the corrosion process; as chloride increases, they seem to approach those pertaining to single-ion transport through solid films, 55'56 whereas they become wider than those derived from other film failure models based on chloride-enhanced incorporation and/or chloride-induced mechanical disruption. 63'6a A rise in the degree of surface contamination pushes Ea into a range of values roughly between bulk diffusion in a liquid medium and individual progression through a solid phase: on phenomenological grounds, this may be seen to comply with the variable extent of diffusional limitations existing for an electrolyte soaked into the countless flaws (pores, cracks and various imperfections) of highly irregular and flaky, multiphase rust systems like the present one. However noticeable it may appear, the energy drop does not seem quite enough to fully discard interface control, even when the chloride deposition rate is at its maximum (4.00 mg dm 2 d a y - l ) . It should be noted that the top contamination in this study is not an upper boundary value for wet-candle salinity: non-episodic figures of 12.5 through c a 16.0 mg dm 2 day-1 CI can be found in the literature. 6s'66 Still, it represents an impressive salt burden, likely to bind all but a few of the airborne chloride deposition events; on the other hand, a value of 0.25 mg dm -2 d a y - l C1 is by no means a background rate. There are, of course, cases of extreme chloride pollution such as salt mists and direct wetting by marine spray and splash, even though, strictly speaking, the latter cannot be referred to as atmospheric phenomena; also, the importance of some abnormal salt episodes in the triggering and/or enhancing of corrosion processes further inland should not be overlooked. 67 Whatever happens, airborne salinity is strongly dependent on the geophysical factors influencing the dynamics of tropospheric marine aerosols, which leads to a very high variability inherent in the sea-salt profiles inland. Even taking all these aspects into account, the point is that an average flux of 4.00 mg dm -2 day -1 CI still ranks above most data from both wet and dry collection procedures, and that only at such a rate, Ea values become quasi-typical of diffusional limitations: in these terms, it seems most unlikely that chloride-induced atmospheric corrosion of iron should invariably be diffusion-controlled, at least in its early stages. Another interesting feature of the present data is illustrated by the steady decrease of the apparent activation energy with the increase in salt contamination, leading to an almost perfect negative correlation between the former variates and the logarithm of [Cl-], as shown in Fig. 6 (the three decimal Pearson coefficient of - 1.000, is significant at all levels). To our knowledge, such a relationship has never been reported to exist, probably because traditional methods of determining atmospheric corrosion rates may not be sensitive enough to enable such relationships to be searched for; yet, the present one would hardly result from chance alone, since when E~ is regressed to [Ci-] in half-logarithmic mode, the corresponding line leaves practically no residual variance to be accounted for. The highly significant correlation found between an electrodic quantity (E~), so to speak, directly derived from
806
A.M.G. PACHECOand M.G.S. FERREIRA
100.0
-"~,~.80.060.0
40.0 -2.00
-1100
0.00
1.00
2.00
ln(lCl- l/todd) FIG. 6.
Apparent activation energy over the range 25-45°C as a function of chloride
deposition rate. cell output and the simulated rate of chloride deposition that is ultimately a bulk concentration, suggests a catalytic process following a quantitative contact adsorption of the halide ions reaching the bare metal surface. On the other hand, the kinetic order of the corrosion process with respect to chloride ion (0 In Rp-1/c3 In [C1-])x,, where x i stands for any other process variable, may be stated in terms of the average observed rate (d In Iacr/d In [Cl-]) and seems to approach unity as t e m p e r a t u r e descends through 25°C. In fact, from Fig. 7, a linear bilogarithmic relationship between corrosion rate and salinity can be seen to exist for each test temperature: this means that the rate is proportional to the nth power of the chloride ion concentration, where n reads 0.63, 0.83 and 0.95, for 45, 35 and 25°C, respectively. These exponents correspond to the most probable values of regression coefficients in the logarithmic space of Fig. 7. As an average, the fitted lines fail to explain about 2.3% of the total variance in the results: although an examination of their residuals indicates no significant lack-of-fit, the corresponding slopes are likely to be no m o r e accurate than _ 0.03.
5.00 4.00 3.00 2.00 1.00 0.00 -2.00
-I ioo
o.'0o
t .oo
2.oo
ln([Cl- ]/mdd) FIG. 7.
Effect of chloride contamination on iron average corrosion rate (Iacr) from polarization resistance measurements; T = 25°C ( , ) , 35°C (I), 45°C (O).
An investigation of atmospheric corrosion rate
807
Such partial orders point to a strong rate dependence on [C1-], which seems to develop into first-order kinetics with respect to this ion as t e m p e r a t u r e decreases. The progressive deviation from unity observed for higher thermal steps (particularly for 45°C) suggests that the tendency for contact adsorption onto the metal surface is being opposed by an enhancement of random motions in the outer electrolyte layers. Moreover, the functional dependencies depicted in Fig. 7 can be viewed as a set of Freundlich isotherms. Many examples of bulk solute adsorption have been found to follow Freundlich-type equations 6s and the very fact that such a model can be seen to underlie the present data suggests, once more, that transport features are to be associated with diffusion processes in a liquid phase, rather than in the solid state. Otherwise, should young rusts act as an effective barrier to the migration of chloride ions inwards, neither a unit power dependence of corrosion rate on [CI ] would have been approached, nor a Freundlich-like behaviour could have been observed. Lastly, the catalytic role tentatively entertained for CI- seems to be the only one that conforms to both the aforesaid sharp dependence and the analysis of corrosion products yet performed: indeed, no chloride-containing rust phases were identified by X-ray diffraction and vibrational (infrared and R a m a n ) spectroscopies throughout this study, i.e., up to a deposition rate of 4.00 mg dm -2 day -1 CI-. Some evidence o f f l - F e O O H (a rust analogue of akagandite) emerging in early experimental rusts was given elsewhere 69 but its formation required much harsher conditions, namely [CI ] -> 8.00 mg dm 2 day I, and was never observed below this level; also, no other form of structural chloride could be identified. To this point, a thermal acceleration of the overall process (initial stages) of iron corroding in chloride-mediated conditions was clearly evidenced, and a catalytic action of the halide could be envisaged on an ab initio basis of rate dependence without rust prevalence. However, the kind of control acting on that process still does not appear to lie beyond reasonable doubt, that is whether charge-transfer features or abnormally severe transport limitations are showing through E~, values, An attempt was made to gain deeper insight into the last issue, A printed-circuit cell that had been tested at 45°C under 4.00 mg dm -2 day 1 C I - , was held at 25°C for eight days with no further contamination; for this period of time, the electrodes remained externally shorted by a 100 k ~ resistor, and corrosion was allowed to proceed in order to create a suitable device for direct current measurements. By driving such a load, the cell is polarized to an extent that will enable a net current, as small as it may appear, to be acquired later on without applying an external tension. This is an advisable procedure to maintain the signal of current during the rest period, which would otherwise tend to fluctuate randomly through zero. On the other hand, a well-rusted surface leads to an enhanced moisture uptake: with such a well-wetted "sponge", not only the cell signal could be ameliorated without losing its significance, TM but also any misevaluation (overestimation) of thermal acceleration factors as a result of drying-out effects v1'72 would be prevented. After the rest period, the electrodes were put through a zero-resistance a m m e t e r ( n a n o a m m e t e r ) and the cell was repeatedly cycled between 25 and 55°C, always under 85% RH. Following each t e m p e r a t u r e ramp at a sweep rate of ca 0.25°C m i n - i the cell was allowed to recover back to 25°C and this t e m p e r a t u r e was kept for about 2 h prior to the next sweep. The cell was first operated in a galvanic mode (AV = 0), and then in an electrolytic one (AV = 30, 60, 120 mV), AV being the external e m f that was successively applied between the two electrodes. Short-circuit currents
808
A . M . G . PACHECOand M. G.S. FERREIRA TABLE 3. REGRESSION (bl), CORRELATION (r), AND DETERMINATION (var) COEFFICIENTS, ABSOLUTE (abs) AND RELATIVE (%) STANDARD ERRORS (S.E.) OF SLOPE ESTIMATES, AND ACTIVATIONENERGIES (Ea) , WITH REGARDTO THE ARRHENIUS LINES IN FIG. 9 AV (mV) 0 30 60 120
b1
S.E. (abs)
S.E. (%)
r
var (%)
Ea(kJ mol - l )
-4737.3 -2634.0 -2463.2 -2149.0
148.7 128.9 146.7 36.8
3.1 4.9 6.0 1.7
-0.996 -0.989 -0.984 -0.999
99.1 97.9 96.9 99.7
39.4 21.9 20.5 17.9
in the operative range (25-55°C) are shown in Fig. 8 and Arrhenius plots for such currents (25--45°C) are depicted in Fig. 9; each point in the former figure stands for an average of triplicate runs. Table 3 lists the corresponding Arrhenius slopes (bl), and the absolute (abs) or relative (%) magnitude of their standard errors (S.E.); all the other symbols are trivial. There is an obvious temperature effect on the currents in Fig. 8, which act as proxy for iron corrosion rates through the thermal range. Such an acceleration results from both galvanic and forced cell output in a similar manner, though a current gap may be seen to exist between each type of data: this feature is even more noticeable when currents are plotted in Arrhenius mode (Fig. 9), and/or when it is translated into energy barriers (Table 3). Before going any further on this subject, it should be noted that short-circuit currents are estimates of instantaneous corrosion rates at best and thus they must not be directly matched to former average corrosion rates, insofar as these are integral values; also, in principle, they are likely to convey more bias than the reciprocals of polarization resistance. In these terms, thermal acceleration is consistently evidenced from both approaches, but it would be unwise to carry out any direct numerical comparison between them. On the other hand, current data fit in an
700
600 500
400 300
200 100 0 20
30
40
5O
60
T (C) FIG. 8. Short-circuit current output (25-55°C) of a pre-corroded cell from zero-resistance a m m e t r y m e a s u r e m e n t s ; AVext = 0 m V (D), 30 m V ( & ) , 60 m V ( I ) , 120 m V (O).
An investigation of atmospheric corrosion rate
809
6.5
6.0 5.5
5.0 4.5
4.0 0.0031
0.0032
0.0033
0.0034
T I(K 1) FIG. 9.
Arrhenius plots (25--45°C) for the short-circuit currents in Fig. 8 (same symbology).
Arrhenius model as before: accounting for nine degrees of freedom, all the correlation coefficients are significant far beyond the 0.001 level, and the standard errors of the regression coefficients (slope estimates) are expected not to exceed 6% at most, which enables activation energy to be determined with fair precision. As a whole, Ea values stand below those from linear polarization experiments; yet, they go up to 39.4 kJ mo1-1, a value that seems to be consistent with the former one by the same cell (52.7 kJ mol-1), allowing for the time elapsed, the surface conditions and the dissimilar parental measurements. As mentioned above, apparent activation energy shows some discontinuous behaviour with regard to the applied emf: there is a cut down to almost a half (21.9 kJ mo1-1) for the lower external input and a comparatively weaker response to higher voltage steps. All this happens as if a substantial amount of energy requirements could be withdrawn from the process by an external field, leaving behind a component much more insensitive to the same field. This suggests a dual source for those requirements, that is, a mixed control as hypothesized earlier: in these terms, the overall corrosion process would be under both anodie control by an interracial reaction and cathodic control by oxygen diffusion, with the former component getting more relevant for shorter times and/or lower salinity. There are several reasons in favour of this model. First, rate processes are likely to be field-assisted to a much greater extent than transport ones. Second, cathodic control by charge-transfer is very unlikely to occur in a near-neutral aqueous medium. Thirdly, it does not seem too hard to accept charge-transfer to become relatively less important as corrosion proceeds and/or ionic strength increases. Fourthly, the energy barriers remaining after what appears to be the removal of the charge-transfer share bear no further ambiguity, since their values are now most typical of aqueous diffusion. 73"74Finally and in connection with the last item, such field-assisted values would be expected to vary linearly with overpotentiai, 7s the slope being given by - 2 . 3 0 3 RT/b, where R is the gas constant, T is the absolute temperature, and b is the corresponding Tafel coefficient. The principle of an equal splitting of the external emf between electrodes was adhered hereto, as it has been done for small polarizations since Marsh 76 through this work. The quality of fit
810
A.M.G. PACHECOand M.G.S. FERREIRA 30.0
I
I
I
I
0
20.0
10.0
0
1 '5
3'0
4'5
6'0
75
Iql (mV)
Fro. 10. Apparent activation energy over the range 25-45°C as a function of electrode overpotential.
shown in Fig. 10 suggests that such an assumption remains valid as far as it goes, that is for twin electrodes polarized at a small-to-medium extent (Ir/I -< 60 mV), with the (conventional) working one being displaced in the negative direction, i.e. toward more negative potentials. The standard error in estimating the slope of the line in Fig. 10 is about 2.1%: this is quite an uncommon deviation from a regression which only minimal degrees of freedom are allowed for. Using the mid-point of the Arrhenius fitting range (35°C) as the reference temperature in the slope expression, a cathodic Tafel coefficient in excess of 66 V per decade is obtained, thus, in practical terms, an infinity. This result is consistent with the assignment of field-assisted energy barriers to aqueous diffusion, and it confirms an overall corrosion process under partial cathodic control; however, the relative magnitude of Ea values found through this work indicates that in self-driven atmospheric corrosion, the process is also partially, if not mainly controlled by charge-transfer. These findings refer to the initial stages of chlorideinduced atmospheric corrosion of iron, but, to some extent, they contradict some knowledge in the field (see, for instance, Rozenfeld 77 and references therein) that almost irrespective of both the substrate and the environment, atmospheric corrosion of metals would be essentially, if not exclusively controlled by the cathodic process of oxygen diffusing under short supply. CONCLUSIONS 1. As judged from simulated laboratory experiments, temperature may be considered an important variable affecting the corrosion of iron in chloridecontaining atmospheres. Factorial design and electrochemical monitoring are likely to prevent the covariance of environmental factors and to enable the process to be followed on a real-time basis: these are precisely the major drawbacks of traditional (outdoor) exposure tests. 2. Depending on salinity, Iacr may vary over a decade as a response to an increment of just 20°C in a common interval (25-45°C): the harsher the conditions get, the less the temperature coefficient shows up, though always remaining above those that have been found for other technical metals (Cd, Cu, Zn).
An investigation of atmospheric corrosion rate
811
3. A p p a r e n t activation energies go up to 87.5 kJ mol 1, thus placing themselves within the range of charge-transfer control; even with heavily corroded, rather hindered surfaces, energy barriers cannot be assigned to transport processes unambiguously. On the contrary, E~ values point to some mixed control, as they seem to include an ever significant contribution not derived from mass-transfer features: this assumption is based on the sharp energy drop due to any first input of external tension, and on the continuous, linear behaviour observed for higher voltage steps. Such a kind of disruption may be thought as corresponding to the removal of the charge-transfer quota, inasmuch as further evolution is almost steepless. 4. Field-assisted E a values strongly suggest an overall process under partial cathodic control, probably by the rate of oxygen diffusion in a stagnant, near-neutral electrolyte: there is a highly significant regression of activation energy to overpotential, and a cathodic Tafel slope of several tens of volt per decade (i.e. actually infinite) results thereupon. 5. Given the last two items and taking into account the relative magnitude of the energy barriers before (40-90 kJ mo1-1, roughly) and after (around 20 kJ mol - I ) applying an external emf, it seems most unlikely that self-driven, chloride-induced atmospheric corrosion of iron can be solely diffusion controlled, at least in its early stages. 6. T e m p e r a t u r e regulates the quantitative adsorption of CI- seemingly through Freundlich isotherms. Qualitatively, this pattern is consistent with the catalytic role envisaged for chloride ions, as suggested by a strong rate dependence with no structural rust appearance; also, quantitatively, it is consistent with a kinetic order (partial order) of the corrosion process with respect to [CI-] that was found to lie between 0.6 and 1.0 and to approach unity as t e m p e r a t u r e decreases. Besides, Freundlich behaviour agrees with the trivial morphology of chloride-induced rust layers that is likely to allow an almost unimpeded access inwards; otherwise, a model for solute adsorption from bulk solutions onto surfaces such as Freundlich's would hardly apply. 7. In general terms, the performance of printed-circuit cells as d.c. electrochemical probes appears as reliable, unbiased, and particularly suited for real-time monitoring, leading to results that are not only self-consistent, but also compare well to the scarce data available in the literature.
l. 2. 3. 4. 5. 6.
REFERENCES F. H. HAYNIE,Atmospheric Corrosion of Metals A S T M STP 767,286 (1982). P.J. SEREDA,Corrosion in Natural Environments A S T M STP 558, 7 (1974). K. OMA,T. SU~ANOand Y. HIRAI,Corros. Eng. (Boshoku Gi]utsu) 14, 16 (1965). F. H. HAYNIEand J. B. UPHAM,Corrosion in Natural Environments A S T M STP 558, 33 (1974). P. ZANNETTI,P. MELL1and E. RUNCA,Atmos. Envir. ll, 605 (1977). F. H. HAYNIE,Durability of Building Materials and Components ASTM STP 691,157 (1980).
7. P. BOURBON, M. GIROUX, J. ALARY, M. JULLIAT, M. C. GENSAC, M. F. BERGEAL, C. BALSA, M. CARPENTIERand J. M. FA~E, Tribune du CEBEDEA U 33(434), 3 (1980). ~. P.J. SEREDA, 1rid. Engng Chem. 52,157 (1961/). 9. P. R. GRoSSMAN, Atmospheric Factors Affecting the Corrosion of Engineering Metals A S T M STP 646,
5 (1978). 10. T. F. OTERO,A. PORROand A. S. ELOLA,Corrosion 48, 785 (1992). 11. J. W. SPENCE,F. H. HAYNIE,F. W. LIPFERT,S. D. CRAMERand L. G. McDONALD,Corrosion 48, 1009 (1992). 12. R. ST. J. PRESTOY,J. Iron Steel Inst. 160, 286 (1948).
812 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56.
A.M.G. PACHECOand M.G.S. FERREIRA D. P. DOYLE and H. P. GODARD, Mater. Prot. 8(8), 40 (1969). C. P. LARRABEE, Trans. electrochem. Soc. 87,161 (1945). J. C. HUDSON, Werkst. Korros. 15, 363 (1964). L. CERVENY,Tribune du CEBEDEA U 23(304), 114 (1969). J. F. STANNERS,Br. Corros. J. 5, 117 (1970). J. F. STANNERS,Tribune du CEBEDEAU24(324), 512 (1970). R. K. TRIPATHI, U. S. AGNIHOTRIand J. N. NANDA, Br. Corros. J. 7, 212 (1972). K. K. CHIN, S. D. RAMASWAMYand S. K. Roy, Proc. Conf. Tall Buildings, Kuala Lumpur, p. 6 (1974). J. F. STANNERS,Corrosion in Natural Environments A S T M STP 558, 23 (1974). S. K. RoY, Corrosion 39, 291 (1983). E. A. BAKER, Degradation o f Metals in the Atmosphere A S T M STP 965, 125 (1988). A. A. DYCHKO and K. A. DYCHKO, J. appl. Chem. USSR 30,261 (1957). ISO International Standard 9223, Corrosion o f Metals and Alloys--Classification o f Corrosivity o f Atmospheres. E. MATTSSON,Mater. Performance 21(7), 9 (1982). D. KNOTKOVA,-CERMAKOV~and J. VLICKOVA,,Werkst. Korros. 21, 16 (1970). N. D. TOMASHOV,Corrosion 20, 7t (1964). K. BARTON,S. BARTONOV.~and E. BER~NEK, Werkst. Korros. 25,659 (1974). Yu. N. MIKHAILOVSKn,V. V. AGAFONOVand V. A. SAIqKO, Protection o f Metals (Zashch. Met.) 13, 429 (1977). Yu. N. MIKHAILOVSKII,P. V. STREKALOVand V. V. AGAFONOV,Protection o f Metals (Zashch. Met.) 16,308 (1980). P. V. STREKALOV,YU. N. MIKHAILOVSKII,G. KH. DZHINCHARADZE, D. KNOTKOV~and J. VLCKOV~, Protection o f Metals (Zashch. Met.) 22,397 (1986). H. GUTrMAN, Metal Corrosion in the Atmosphere A S T M STP 435,223, (1968). Yu. M. PANCHENKO,P. V. STREKALOVand Yu. N. MIKHAILOVSKn,ProtectionofMetals(Zashch. Met.) 25,438 (1989). F. MANSFELD,S. TSA1, S. JEANJAQUET,E. MEYER, K. FERTIGand C. OGDEN, Atmospheric Corrosion of Metals A S T M STP 767,309 (1982). V. KUCERAand M. COLLIN,Proc. EUROCOR, '77, London, p. 189 (1977). F. MANSFELD and S. TSAI, Corros. Sci. 20,853 (1980). E. HEITZ and W. SCHWENK, Br. Corros. J. l l , 74 (1976). V. A. KUZNETSOV, S. G. POLYAKOV,YU. S. GERASIMENKOand Yu. G. KOTLOV, Protection o f Metals (Zashch. Met.) 12, 578 (1976). J. A. GONZ~,LEZ, Werkst. Korros. 29,456 (1978). F. MANSFELD, S. L. JEANJAQUET,M. W. KENDIG and D. K. ROE, Atmos. Envir. 20, 1179 (1986). J. A. GONZALEZ, E. OTERO, C. CABAIqASand J. M. BASTIDAS,Br. Corros. J. 19, 89 (1984). W. SKINNER, Br. Corros. J. 22, 172 (1987). S. G. FISHMANand C. R. CROWE, Corros. Sci. 17, 27 (1977). F. MANSFELD, Atmospheric Corrosion (ed. W. H. AILOR), p. 139. John Wiley and Sons Inc., New York (1982). B. SANYALand D. B. BHADWAR,J. Sci. Ind. Res. (India) 21A, 243 (1962). L. A. SHUVAKHINA,Yu. N. MIKHAILOVSKII,N. F. SHARONOVAand V. S. SEDOVA,Protection of Metals (Zashch. Met.) 13, 130 (1977). J. S. HUTCHINSand M. MCKENZlE, Br. Corros. J. 9, 158 (1974). P. W. BROWN and L. W. MASTERS, in Atmospheric Corrosion (ed. W. H. AILOR), p. 31. John Wiley and Sons Inc., New York (1982). S. P. SHARMA,J. H. THOMASIII and F. E. BADER, J. electrochem. Soc. 125, 2002 (1978). W. H. J. VERNON, Research 5, 54 (1952). A. ASKEY, S. B. LYON, G. E. THOMPSON,J. B. JOHNSON, G. C. WooD, M. COOKEand P. SAGE,Corros. Sci. 34,233 (1993). A. MILCH and P. TASAICO,J. eleetrochem. Soc. 127, 884 (1980). M. I. MANNING, A. G. CROUCHand B. LLOYD, Mater. Performance 27(9), 74 (1988). R. FOSTERand T. P. HOAR, Localized Corrosion (eds, C. L. MCBEE and J. KRUGER), p. 698. NACE, Houston (1972). M. S. DE SA,, C. M. RANGELand C. A. C. SEQUEIRA,Br. Corros. J. 23, 186 (1988).
An investigation of atmospheric corrosion rate 57. 58. 59. 60. 6l. 62. 63. 64. 65. 66. 67. 68. 69.
70. 71. 72. 73. 74. 75. 76. 77.
813
J.-R. PARKand D. D. MACDONALD,Corrosion 45,563 (1989). N. SCHWARTZand D. D. BACON,J. electrochem. Soc. 125, 1487 (1978). N. SATOand M. COUEN,J. electrochem. Soc. 111,512 (1964). J. W. SPENCE,F. H. HAYNIE, D. C. STILESand E. O. EDNEY, Degradation of Metals in the Atmosphere A S T M STP 965,339 (1988). A. A. OLOWE,B. PAURONand J. M. R. G~NIN, Corros. Sci. 32,985 (1991). J. ROBERTSONand J. E. FORREST,Corros. Sei. 32,521 (1991). T. P. HOARand W. R. JACOB,Nature 216, 1299 (1967). J. L. DAWSONand M. G. S. FERREIRA,Corros. Sci. 26, 1009 (1986). A. K. SINGH, T. ERICSSON,L. HAGGSTROMand J. GULLMAN,Corros. Sci. 25,931 (1985). A . A . BRAGARDand H. E. BONNARENS,Atmospheric Corrosion of Metals A S T M STP 767,339 (1982). B. LLOYDand M. I. MANNING, Corros. Sci. 30, 77 (1990). A. K. GAt,WEY, Chemistry o f Solids, Science Paperbacks, Chapman and Hall Ltd, London (1967). A. M. G. PACttECO, Initial Stages of the Chloride-induced Atmospheric Corrosion of Iron: Gravimetric, Electrochemical, and Structural Aspects (Doctoral Thesis). Technical University of Lisbon, Lisbon (1991). F. MANSFELDand J. V. KENKEL,Corrosion 33, 13 (1977). M. STRATMANN,K. BOHNENKAMPand H.-J. ENGELL, Werkst. Korros. 34,604 (1983). M. J. Jusxo and M. G. S. FERREIRA,Corros. Sci. 29, 1353 (1989). K. J. LAIDLER, Chemical Kinetics. McGraw-Hill Book Co., New York (1965). J. M. WEST, Basic Corrosion and Oxidation. Ellis Horwood, London (1980). E. GILEAD1, E. KIROWA-EISNERand J. PENC1NER, Interracial Electrochemistry: An Experimental Approach. Addison-Wesley Publishing Co. Inc., Reading, MA (1975). G. A. MARSH, Proc. 2nd Int. Cong. Metallic Corrosion, New York, p. 936 (1963). I. L. ROZENFELD,Atmospheric Corrosion o f Metals, NACE, Houston (1972).