A laboratory study of the effect of NO2 on the atmospheric corrosion of zinc

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Atmospheric Environment 41 (2007) 8681–8696 www.elsevier.com/locate/atmosenv

A laboratory study of the effect of NO2 on the atmospheric corrosion of zinc J.G. Castan˜oa, D. de la Fuenteb, M. Morcillob, a

Corrosion and Protection Group, Department of Materials Engineering, Universidad de Antioquia, P.O. Box 1226, Medellı´n, Colombia b National Centre for Metallurgical Research (CENIM/CSIC), Avda. Gregorio del Amo 8, 28040 Madrid, Spain Received 21 February 2007; received in revised form 11 July 2007; accepted 13 July 2007

Abstract Studies on the effect of NOx on zinc corrosion are scarce and their results are variable and at times seemingly contradictory. This paper reports laboratory tests involving the dry deposition on zinc surfaces of 800 mg m3 NO2, alone and in combination with 800 mg m3 SO2, at temperatures of 35 and 25 1C and relative humidities of 90% and 70%. From the gravimetric results obtained and from the characterisation of the corrosion products by optical microscopy, scanning electron microscopy (SEM/EDX), grazing incidence X-ray diffraction (GIXD) and X-ray photoelectron spectroscopy (XPS), it has been verified that the corrosive action of NO2 alone is negligible compared with SO2. However, an accelerating effect has been observed when NO2 acts in conjunction with SO2 at 25 1C and 90% RH. At 35 1C and 90% RH, the accelerating effect is smaller, and at low relative humidities (70%), the synergistic effect is only slight, which suggests it may be favoured by the presence of moisture. In those cases where an accelerating effect has been observed, a greater proportion of sulphate ions has been found in the corrosion products, and nitrogen compounds have not been detected, indicating that NO2 participates indirectly as a catalyst of the oxidation of SO2 to sulphate. r 2007 Elsevier Ltd. All rights reserved. Keywords: NO2; SO2; Atmospheric corrosion; Zinc

1. Introduction Because of their highly corrosive effects, chlorides and sulphur dioxide have hitherto been the most thoroughly studied pollutants in atmospheric corrosion. Chlorides have a great influence in marine atmospheres and very special environments, while SO2 is associated with atmospheric corrosion in Corresponding author. Tel.: +34 915538900; fax: +34 915347425. E-mail addresses: [email protected] (J.G. Castan˜o), [email protected] (D. de la Fuente), [email protected] (M. Morcillo).

1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.07.022

many instances, as has been reported elsewhere (Vernon, 1935; Hudson and Stanners, 1953). The growing demand for environmental protective actions has led to lower atmospheric sulphur levels, at least in developed countries, as a result of the use of cleaner coals and sophisticated gas emission controls (Graedel and Frankental, 1990; Ross and Shaw, 1982; Henriksen and Arnesen, 2001; EPA, 2005). This has raised the relative significance of other atmospheric gases and the interest in studying their effects on exposed materials. In this respect, while the essential electrochemical processes involved in atmospheric corrosion are fairly well known, the synergistic or antagonistic

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role of several specific agents remains unclear (EPRI, 1988). Nitrogen oxide levels have remained constant or even increased (van Aardenne et al., 1999; EPA, 2005) somewhat as a result of the higher burning temperatures used (Graedel and Frankental, 1990) and the lack of specific control measures, and urban NOx levels are now higher than SO2 levels in most cities. NO2 levels can rise from 0.15 ppm in normal conditions to 1900 ppm in situations of temperature inversion (Mariaca et al., 1999). In 1982, a group of materials experts (UN/ECE, 1982) recommended in-depth studies on the influence of NOx, both alone and in combination with other pollutants, on atmospheric corrosion. From that time, a number of extensive studies on corrosion effects of NO2 and SO2 have been conducted. The mechanism by which atmospheric nitrogen compounds are deposited on exposed materials continues to be unclear. While wet deposition seems to prevail in areas remote from sources, dry deposition probably predominates near them. In the latter case, large amounts of precursors, which are less water soluble than the yet unformed HNO3, can be deposited. As regards their effects, on the basis of field work (Haynie and Upham, 1971, 1974) some authors believe nitrogen oxides to be of little influence (Holler et al., 1991) while others assign them an inhibitory action (Takazawa, 1985) and yet others consider that their behaviour is heavily dependent on other system conditions such as relative humidity, nature of the metal concerned, and NO2 and SO2 concentrations (Kucera, 1992). 1.1. Effect of nitrogen oxides on atmospheric corrosion of different metals In the case of copper, some authors have found in laboratory studies a notable synergistic effect of NO2 and SO2 (Eriksson et al., 1993). This is attributed to an increase in the sulphite to sulphate oxidation rate in the presence of NO2, which results in destruction of the protective oxide film. However, other authors have not encountered this effect in field studies (Tibdlad et al., 1991a). In laboratory tests in a climate cabinet, a mainly synergistic effect has been seen (Mariaca, 1997; Feliu et al., 2003, 2005; Kleber et al., 2002), although depending on the testing conditions even an inhibitive effect has been detected (Mariaca, 1997). The results of studies on other materials also suggest a deleterious effect of atmospheric nitrogen

dioxide. This is the case with aluminium; while the nature of the attack leads to disperse results, the SO2–NO2 system has been shown to considerably reduce the effect of SO2 (Henriksen and Rode, 1986; Takazawa, 1985). Conversely, other authors have reported significantly increased corrosion rates on the addition of NO2 to SO2 at high relative humidity (Eriksson and Johansson, 1986). Correlations between environmental data and field corrosion rate values have revealed no synergistic effect of NO2 on nickel corrosion (Kucera, 1992; Tidblad et al., 1991b). In laboratory tests, Rice et al. (1980) observed a very slight increase in the corrosion rate on the addition of NO2, while Zaquipour and Leygraf (1986a, b) reported a significant effect of NO2 alone and a marked increase in the corrosion rate arising from the joint presence of SO2 and NO2. In the case of steel, more studies have been performed than in the preceding cases, but here too there is a lack of agreement. While some authors have not been able to establish a synergistic relation (Haynie et al., 1976, 1978) others have observed this at 50% and 90% RH, noting that the most probable cause is the catalysing effect of NO2 on sulphate oxidation (Eriksson and Johansson, 1986). On the other hand, a substantial inhibiting effect has also been observed at 95% RH, attributed to the action of HNO2 and the NO 3 that is formed (Henriksen and Rode, 1986). In tests in a climate cabinet at several RH, it has been found that while no accelerating or inhibiting effect on steel is perceived at high RH, at low RH an accelerating effect is detected (Arroyave, 1995; Arroyave and Morcillo, 1996). In the case of zinc, despite this being one of the most widely used metals, especially as a protective coating for steel in structures and parts exposed to the atmosphere, given its good resistance to atmospheric corrosion and its relatively low cost, studies on the effect of NOx are scarce and their results are variable and at times seemingly contradictory. In field tests, no correlation has been found between NO2 and zinc corrosion (Kucera, 2000; Tidblad et al., 1999). In contrast, in laboratory tests the researchers studying the possible accelerating effect of NO2 in combination with SO2 can be split into two groups: on the one hand those who find a synergistic effect (Eriksson and Johansson, 1986; Mansfeld et al., 1986; Svensson and Johansson, 1993a, b); and on the other hand those who consider such an effect to be non-existent or insignificant

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compared to what SO2 can do by itself (Henriksen and Rode, 1986; Haynie et al., 1976; Oesch and Faller, 1997; Strandberg et al., 1997). With regard to the detractors, Henriksen and Rode (1986), Strandberg et al. (1997) and Oesch and Faller (1997) found no effect of NO2 on zinc corrosion at 22–25 1C and 90–95% RH. Among the proponents of the accelerating effect of NO2, especially at high RH, Svensson and Johansson (1993a, b) suggest that this effect is due to the fact that NO2 catalyses the oxidation of SO2 to sulphate on the metal surface and discourages the formation of zinc sulphite. The reactions proposed by these authors for the process of SO2 adsorption and the formation of sulphite and sulphate ions are as follows (Svensson and Johansson, 1993b): SO2ðgÞ 2SO2ðadsÞ ;

(1)

þ SO2ðadsÞ þH2 O2HSO 3ðadsÞ þHðadsÞ ;

(2)

2 þ HSO 3ðadsÞ 2SO3ðadsÞ þ HðadsÞ ,

(3)

2 1 SO2 3ðadsÞ þ 2O2 2SO4ðadsÞ ,

(4)

2 þ 1 HSO 3ðadsÞ þ 2O2 2SO4ðadsÞ þ HðadsÞ .

(5)

The acidity of the aqueous film promotes the dissolution of the previously formed corrosion products: ZnOðsÞ þ 2Hþ ðadsÞ 2Zn2þ ðadsÞ þ H2 O;

(6)

ZnðOHÞ2ðsÞ þ 2Hþ ðadsÞ 2Zn2þ ðadsÞ þ 2H2 O:

(7)

These authors observed that in the presence of NO2 the relationship between the corrosion product mass and the corroded zinc mass increased, indicating that the corrosion products are sulphate richer than those formed in tests with only SO2. They also found that SO2 deposition was greater in atmospheres with both pollutants, which implies an increase in the S4+ to S6+ oxidation rate (reactions (1)–(5)), and that gaseous NO was not produced, which means that the increase in the oxidation rate was not due to a direct relationship between NO2 and SO2 to form NO and H2SO4. The indirect intervention of NO2 was also verified by the absence of nitrites and nitrates on the zinc surface. There seem to be certain characteristics of the effect of NO2 on metals that could explain the lack of agreement on how it affects atmospheric corrosion, e.g. the absence or non-permanence of nitrogen compounds in the corrosion products;

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the existence of a critical pollutant concentration to trigger a synergistic NO2–SO2 effect; and the possible occurrence of photolytic reactions in field tests which may involve ozone or organic compounds and which obviously do not occur in laboratory tests (Leygraf, 1995). In field tests, the large number of variables and overlapping effects makes it extremely difficult to determine the role played by NO2. In contrast, in laboratory tests in carefully controlled conditions close to those existing in real atmospheres it is possible to study the separate effect of this compound and to investigate the effect of its combination with SO2 (Castan˜o, 2001). The present work analyses the effect of atmospheric NO2 on zinc corrosion, comparing it with similar concentrations of SO2, and studies the simultaneous effect of both pollutants at different temperatures and RH levels. The study has been carried out by means of laboratory tests at different environmental parameters: temperature, relative humidity and pollutants concentration, the latter faithfully representing theoretic real situation concentration peaks in a period of high contamination in an industrial atmosphere. 2. Experimental procedure For the performance of the tests, zinc specimens of commercial purity (Pbo0.005%, Ti ¼ 0.075%, Zn balance) and dimensions 100 mm  50 mm  0.6 mm were prepared. The specimens were subjected to a surface preparation process consisting of dry polishing with silicon carbide abrasive papers (240, 320, 400 and 600), degreasing with absolute grade acetone and washing with detergent. They were finally subjected to ultrasonic stirring in ethanol for 15 min and dried in a hot air jet. Following their preparation, the specimens were stored for 24 h in a desiccator over a silica gel. Immediately before testing, they were weighed on an analytical balance of 10 mg of sensitivity. The pollutant gas deposition tests were carried out in a HERAEUS HC2033 climate cabinet equipped with a pollutant gas dosing system. The selected concentrations of 800 mg m3 faithfully represent theoretic real situation concentration peaks in a period of high contamination in an industrial atmosphere. The air flow was 1 m3 h1, permitting the atmosphere inside the cabinet be renewed eight times every hour, in completely laminar regime conditions. The tests were performed at both 2571 1C and at

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3571 1C. With regard to RH, two values were considered: 7075% and 9075%. The exposure time was 7 days, although in some cases the exposure time was extended up to 15 days in order to obtain a better and more complete characterisation of the corrosion products. After each exposure period, the specimens were removed from the cabinet and allowed to reach the laboratory room temperature before being stored in a desiccator. Their weights were then recorded in order to determine the mass gain during exposure. The mass gain was determined with five specimens for each exposure period and condition. The standard deviation was generally o10%. In order for the determination of mass loss, three of the five specimens were subjected to chemical etching according to ISO standard 8407 (ISO, 1985). In accordance with this standard, etching was carried out by immersing the specimens for 3 min in a solution prepared by adding 100 g ammonium acetate to 1 l of distilled water heated to 70 1C. The remaining two specimens for each series were used for the analysis of corrosion products, being stored in a desiccator in hermetically sealed polythene bags previously de-aerated with N2. Characterisation of the corrosion products formed was performed by detailed visual analysis, in an OLYMPUS SZ-CT optical microscope at up to 63  magnification with photographic recording, and by scanning electron microscopy (SEM) in a JEOL JXA 840 unit with a LINK SYSTEM electron microprobe for energy dispersive X-ray (EDX) analysis. The characterisation was complemented by X-ray diffraction analysis (XRD) and X-ray photoelectron spectroscopy (XPS). XRD analysis was performed in a SIEMENS D500 two-circle diffractometer with copper anode radiation (Cu Ka, l ¼ 1.5406 A˚), a flash detector and a SOLLER slit with a secondary monochromator,

operating at a voltage of 40 kV and a current of 40 mA. In view of the small thickness of the layers formed, the grazing incidence method (GIXD) was used, at an angle of 0.71 and a scanning rate of 1 s, from 2y ¼ 41 to 901. XPS spectra were obtained using a VG MICROTECH Series ESCA spectrometer, model MT 500, using magnesium radiation Ka ¼ 1253.6 keV, with pass energies of 10 and 50 eV and resolutions of 0.8 and 1.4 eV, operating at a voltage of 15 kV and an emission current of 20 mA. The vacuum in the analysis chamber was approximately 5  108 Torr. A general spectrum was obtained for each specimen, along with a high-resolution spectrum for the elements of greatest interest. 3. Results 3.1. Gravimetric measurements Tables 1 and 2 show the average mass gain and loss values, respectively, after 7 days of exposure in the different atmospheric conditions. Fig. 1 shows the corrosion rates obtained. 3.2. Characterisation of corrosion products In the tests at 25 1C and 70% RH, no significant variation in the appearance of the specimens is observed by optical microscopy. The only notable aspect is that the surfaces generally present a more opaque tone. At 35 1C and 70% RH in atmospheres with only NO2 no significant changes are observed, although in the presence of SO2 (alone or combined with NO2) a slight and irregular staining of the surfaces is seen. However, at higher relative humidities (90% RH) and in the presence of SO2 (alone or combined with

Table 1 Mass gain values obtained after 7 days of exposure in different atmospheric conditions SO2 (mg m3)

NO2 (mg m3)

Mass gain (mg cm2) T, 25 1C RH, 70%

T, 35 1C RH, 90%

RH, 70%

RH, 90%

0

0 800

0.50 1.05

0.33 2.60

0.50 2.66

1.12 1.20

800

0 800

2.26 2.47

27.60 62.40

2.38 12.04

19.98 30.86

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Table 2 Mass loss values obtained after 7 days of exposure in different atmospheric conditions SO2 (mg m3)

NO2 (mg m3)

Mass loss (mg cm2) T, 25 1C

T, 35 1C

RH, 70%

RH, 90%

RH, 70%

RH, 90%

0

0 800

o1.00 6.60

o1.00 o1.00

o1.00 o1.00

o1.00 1.30

800

0 800

3.60 10.40

27.63 114.50

1.10 14.00

40.85 97.30

-2

-1

Corrosion rate (µ g·cm ·d )

7 days 18 16 14 12 10 8 6 4 2

SO2-NO2

0 70% 90% 25ºC

SO2 70% 35ºC

90%

NO2 No gases

Fig. 1. Zinc corrosion rates after 7 days of exposure in different atmospheric conditions.

NO2) a uniform staining is observed at both testing temperatures (25 and 35 1C) on all the surfaces, along with a series of randomly distributed white colour points which are more numerous in the combined atmosphere (SO2+NO2). In contrast, the appearance of the specimens exposed in atmospheres with only NO2 presents very little variation after the test. Fig. 2 displays photographs obtained by optical microscopy of the surfaces exposed at 25 1C and 90% RH after 15 days of exposure in the different atmospheres. Scanning electron microscopy (SEM) of the specimens exposed in atmospheres without contamination (Fig. 3a) and in atmospheres with only NO2 (Fig. 3b) shows incipient corrosion at isolated points on the surface. EDX spectra reveal the presence of O and Zn and the absence of N. Fig. 4 shows the appearance presented after 15 days of exposure at 35 1C and 90% RH in atmo-

spheres with only SO2 (Fig. 4a) or SO2+NO2 combined (Figs. 4b and c). In both cases, the morphology observed is very similar, with an accumulation of white colour corrosion products in the form of well-defined isles. Fig. 4c also shows a detail of one of these corrosion product islands where two types of crystals are detected. At deeper levels, hexagonal plates are observed which reflect a high degree of crystallisation. On these plates, other smaller and irregular crystals are observed. The EDX spectra reveal the presence of sulphur and oxygen in all cases. The distribution and morphology of the corrosion products obtained at 25 1C and 90% RH (Fig. 5) is very similar, with the only difference that the isles where these accumulate are of a larger size than those observed at 35 1C and 90% RH. The crystals have a laminar shape and grow irregularly. The EDX spectra also show the presence of sulphur and oxygen. By means of the grazing incidence X-ray diffraction technique, in the absence of contaminants or in atmospheres with only NO2, only metallic zinc is detected after 15 days of exposure. In atmospheres with SO2 or SO2+NO2, trihydrated zinc hydroxysulphate, Zn4SO4(OH)6  3H2O, is also detected. Fig. 6 shows the GIXD spectra obtained in these atmospheres. The presence of trihydrated zinc hydroxysulphate is much more evident at 25 1C in atmospheres with SO2+NO2. Figs. 7–9 show the high-resolution spectra for oxygen (O1s), zinc (Zn2p3/2) and nitrogen (N1s), respectively, obtained after exposure during 15 days to the different considered atmospheres at 25 1C and 90% RH. In all cases, the O1s spectra may be fitted to three components: the first at 530.2 eV may be attributed to the presence of ZnO, the second one at 531.8 eV is the most intense and includes the 2 contribution of OH ions and SO2 in the 3 , SO4

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Fig. 2. Surface appearance observed by optical microscopy after 15 days of exposure in the following atmospheres: (a) 25 1C, 90% RH and 800 mg m3 NO2 (10  magnification), (b) 25 1C, 90% RH and 800 mg m3 SO2 (10  magnification), (c) 25 1C, 90% RH and 800 mg m3 SO2+800 mg m3 NO2 (10  magnification) and (d) 25 1C, 90% RH and 800 mg m3 SO2+800 mg m3 NO2 (63  magnification).

Fig. 3. SEM/EDX results obtained on the surface of a zinc specimen exposed for 15 days in the following atmospheres: (a) 35 1C, 90% RH, without contaminants and (b) 35 1C, 90% RH and 800 mg m3 NO2.

case of sulphur-containing atmospheres and finally a third component at 533.2 eV associated to the presence of H2O. High-resolution Zn2p spectra shows in all cases a single component at 1021.8 eV that includes metallic zinc (Zn) and zinc oxide (Zn2+). The high-resolution N1s spectrum does not reveal the presence of any nitrogen compound. No significant differences have been observed at other temperature and RH considered.

Fig. 10 shows the high-resolution spectra for sulphur (S2p) obtained after exposure in the different atmospheres. Two components are differentiated in all cases, each one of which is in turn fitted to two peaks, since this is a sublevel 2p spectrum in which a doublet is generated. The first of these components, at lower binding energies, corresponds to the sulphite ion (SO2 3 ) while the second corresponds to the sulphate ion (SO2 4 ). An

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Fig. 4. SEM/EDX results obtained on the surface of a zinc specimen exposed for 15 days in the following atmospheres: (a) 35 1C, 90% RH and 800 mg m3 SO2, (b) and (c) 35 1C, 90% RH and 800 mg m3 SO2+800 mg m3 NO2.

increase may be observed in the component corresponding to SO2 compared to SO2 in the 4 3 case of the mixed atmospheres (SO2+NO2), especially at 25 1C and 90% RH. Table 3 shows a summary of the corrosion products detected by GIXD and XPS as a function of the composition of the atmosphere. 4. Discussion 4.1. Behaviour in uncontaminated atmospheres In natural atmospheres, zinc surfaces in contact with clean air, at room temperature, are instantaneously coated by a very thin zinc oxide (ZnO) film formed by a chemical oxidation mechanism, which does not affect subsequent corrosion (Zhang, 1996). Once the moisture layer has been established, zinc

hydroxide rapidly forms on this film, in this case due to an electrochemical mechanism. The formation of a moisture layer of sufficient thickness, together with the action of atmospheric CO2, leads to the formation of basic zinc carbonates from the initially formed hydroxide (Zhang, 1996; Kucera and Mattsson, 1987). Both the hydroxide and the carbonates are very stable and have a protective character, and they therefore tend to inhibit zinc corrosion in atmospheres without contamination. However, if the access of air is limited and the CO2 insufficient, as occurs in this laboratory study, a large amount of ZnO may be produced, which is of a less protective character (Zhang, 1996), and therefore the corrosion process may increase. However, the gravimetric data (Tables 1 and 2; Fig. 1) shows that the corrosion in atmospheres without contaminants is very low in all cases, and at

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Fig. 5. SEM/EDX results obtained on the surface of a zinc specimen exposed for 15 days in the following atmospheres: (a) 25 1C, 90% RH and 800 mg m3 SO2 and (b) 25 1C, 90% RH and 800 mg m3 SO2+800 mg m3 NO2.

times the mass losses are even smaller than the sensitivity of the weighting procedure followed. These results are in agreement with the findings of other authors (Zhang, 1996; Kucera and Mattsson, 1987; Morcillo, 1998; Slunder and Boyd, 1983). With regard to the corrosion products, SEM analysis reveals an incipient accumulation of corrosion products at isolated sites, as was reported by Zhang (1996). Only with XPS has it been possible to establish the presence of ZnO (in a lesser proportion) and OH ions (in a greater proportion), corresponding in part to the Zn(OH)2 and in part to the adsorbed water on the metallic surface. 4.2. Behaviour in atmospheres contaminated with NO2 What all researchers seem to agree on is the small effect of NO2 on zinc corrosion when it acts alone, compared with the effect of other atmospheric agents (Henriksen and Rode, 1986; Eriksson and Johansson, 1986; Svensson and Johansson, 1993b; Oesch and Faller, 1997). In concordance with results published in the literature, the present study has witnessed very little zinc corrosion in the presence of NO2 for the two RH and temperature conditions considered. The mass gain and mass loss data are of the same order, or only slightly higher, than that obtained in uncontaminated atmospheres. Observations made with both optical microscopy and SEM confirm the

incipient state of the corrosion process. For this reason, GIXD analysis has only detected metallic zinc while XPS analysis has detected a greater amount of Zn(OH)2 than in uncontaminated atmospheres, especially at 35 1C and 90% RH, confirming the findings made by gravimetric methods. Another important aspect is that nitrites, nitrates or any other nitrogen compound are not detected in the corrosion products by any of the characterisation techniques used. This may be due either to the fact that they have not yet formed due to the incipient nature of the process, that they have been leached out due to their low concentration and high solubility or because, as noted by Friel (1986), zinc nitrates are highly unstable in acid conditions such as those commonly found in atmospheric exposure. Although some authors in laboratory studies at 25 1C and 90% RH have detected the presence of basic zinc nitrates in the corrosion products (Oesch and Faller, 1997), it should be taken into account that the NO2 concentration used was fairly high (18,800 mg m3) compared to that used in the present study (800 mg m3). Zinc carbonates were also not detected, for the reasons explained in the preceding section. 4.3. Behaviour in atmospheres contaminated with SO2 In numerous studies, it is noted that the atmospheric corrosion of zinc is strongly dependent on

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Fig. 6. GIXD spectra corresponding to samples exposed to different gases concentration atmospheres at 25 1C, 90% RH.

the SO2 concentration (Oesch and Faller, 1997; Slunder and Boyd, 1983; Knotkova-Cermakova et al., 1974; Morcillo and Feliu, 1983) and that the increase in relative humidity in the presence of SO2 intensifies this effect even more (Zhang, 1996; Sydberger and Vannerberg, 1972). As has already been mentioned, the corrosion product layer that forms in atmospheres with low contamination is highly stable and protective. However, its stability is restricted to a narrow pH range (Graedel, 1989). For this reason, the deposition of significant amounts of SO2 on the moisture layer causes the dissolution of the protective layer by producing an important reduction in the pH value. On the other hand, the formation of highly water soluble corrosion products also contributes to the fact that in this type of environments the corrosion rate remains practically constant with exposure time

(Odnevall and Leygraf, 1994; Costa and Vilarasa, 1993). In laboratory studies in humid environments and with similar contaminant concentrations to the average concentrations in real atmospheres it has been found that SO2 is deposited on the zinc surface to form low solubility zinc sulphites, which are subsequently oxidised to water-soluble zinc sulphates (Svensson and Johansson, 1993b; Strandberg et al., 1997). It has also been found that at high relative humidity values, the corrosion kinetics is high due to the formation of an aqueous film of zinc sulphate. The corrosion products formed, mainly zinc hydroxysulphate and zincite, would provide new sites for the adsorption of SO2, which would result in its rapid deposition at high humidities. However, at low relative humidities, both the zinc corrosion rate and the SO2 deposition

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Fig. 7. High-resolution O1s XPS spectra obtained on zinc specimens after 15 days of exposure in the different atmospheres at 25 1C and 90% RH.

rate are much slower due to the less amount of electrolyte. Similarly, in the present study, it has been found that the kinetics of the corrosion process in the presence of SO2 is significantly greater than in the presence of NO2 at high relative humidity (90%), while at a relative humidity of 70% no appreciable differences are observed (Tables 1 and 2). With regard to the temperature, this may play a dual and antagonistic role in atmospheric corrosion, because besides having a positive effect on the kinetics of the chemical reactions and the diffusion of the agents involved in the corrosion phenomenon, it also exerts a negative effect by speeding up the drying of the moisture film and thus reducing the time of wetness. In the present case, a slight negative effect of the temperature on the corrosion kinetics has been observed, which is somewhat clearer with

longer exposure times and a high relative humidity. At low relative humidities, it has not been possible to establish a tendency due to the low corrosion values encountered. In the case of exposure in atmospheres of high relative humidity, XPS reveals that the main component in the first week is already zinc hydroxysulphate and that there are important amounts of ZnSO3 and smaller amounts of Zn(OH)2. After 15 days, a reduction is seen in the proportion of hydroxysulphate, an increase in the sulphite, and the appearance of ZnO. Grazing incidence X-ray diffraction (GIXD) after 15 days of exposure confirms the presence of zinc hydroxysulphate, with the chemical formula (Zn4SO4(OH)6  3H2O). The presence of this compound has also been verified in field tests (Odnevall, 1994). Furthermore, in optical microscopy observations, it was possible to see the slipping of the moisture

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Fig. 8. High-resolution Zn2p3/2 XPS spectra obtained on zinc specimens after 15 days of exposure in the different atmospheres at 25 1C and 90% RH.

layer across the specimen surfaces (positioned vertically) exposed at 25 1C and 90% RH. In accordance with what has previously been mentioned, with the degree of attack observed and the corrosion rates measured, it is inferred that these conditions are highly propitious for the consolidation of a moisture layer of an appreciable thickness, which together with the high SO2 concentration causes greater corrosion of the zinc. The tendency for the corrosion rate to increase with time may be associated to the formation of soluble sulphates. Thus, the mechanism proposed by Svensson and Johansson (1993b) would be confirmed, since important amounts of zinc sulphites of poor solubility are detected, which would subsequently be oxidised to soluble zinc sulphates and which at high relative humidities would slip across the specimens dissolved in water. Agreement has also been found with these authors in relation with the

corrosion products found, zinc hydroxysulphate as the main component and zincite, which would provide new sites for the adsorption of SO2, accelerating its deposition at high humidities. At 70% RH, the less amount of electrolyte notably reduces the kinetics of the process, despite detecting the same components, although obviously these are found in discreet amounts. 4.4. Behaviour in atmospheres contaminated with SO2 and NO2 The gravimetric data (Tables 1 and 2; Fig. 1) show greater zinc corrosion after its exposure to atmospheres with both gases compared to that found in atmospheres with only SO2. It should be taken into account that in this case, the concentration is 800 mg m3 SO2+800 mg m3 NO2, which is thus 4800 mg m3 SO2 of the tests with only SO2.

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Fig. 9. High-resolution N1s XPS spectra obtained on zinc specimens after 15 days of exposure in the different atmospheres at 25 1C and 90% RH.

However, bearing in mind that the corrosion rates found in atmospheres with only NO2 are practically negligible, it may be stated that the presence of NO2 in combination with SO2 has a synergistic accelerating effect. The clearest accelerating effect takes place at 25 1C and 90% RH, while at 35 1C and the same RH, a less severe synergistic effect is seen. At low relative humidities (70% RH), the corrosion rates are very low and only a slight synergistic effect has been observed. The characterisation of the corrosion products carried out by SEM/EDX has made it possible to observe their accumulation in the form of isles (Fig. 4) and a clearly defined hexagonal morphology, which may perhaps correspond to a well crystallised hydroxysulphate or to zincite, which is known to have a hexagonal structure. XPS analysis confirms the presence of zinc hydroxysulphate as the major component, along with zinc sulphite, zinc hydroxide and zincite. The GIXD study confirms that this is trihydrated zinc hydroxysulphate, Zn4SO4(OH)6  3H2O (Fig. 6). It should be noted that none of the characterisation techniques used have detected nitrogen compounds, and for this reason it may be inferred that the accelerating effect of NO2 is an indirect effect. As may be seen in Fig. 11, the increase in the proportion of sulphate ion in atmospheres where a accelerating effect of NO2 has been verified seems to indicate that its effect is related with the increase in the rate of SO2 oxidation to sulphate (reactions (1)–(5)), in accordance with the findings of Svensson and Johansson (1993b), and is not related to a direct

reaction between NO2 and SO2. In this way, the increase in sulphate formation may give rise to a very acid electrolyte which would accelerate zinc corrosion, in turn increasing the capacity of the surface to adsorb SO2. This phenomenon would be favoured in low temperature and high relative humidity conditions, which are the most propitious for the consolidation of a moisture layer on the metallic surface. The fact that the accelerating effect is of little significance at 35 1C is related with the difficulty to maintain the moisture layer in high temperature conditions. For the same reason, at low relative humidities, very low corrosion rates have been observed, as was to be expected and has previously been noted by other authors (Svensson and Johansson, 1993a, b).

5. Conclusions Zinc corrosion in atmospheres contaminated with 800 mg m3 NO2 was of the same or only marginally greater than that found in uncontaminated atmospheres, and as in these atmospheres only ZnO and Zn(OH)2 were detected. In atmospheres contaminated with 800 mg m3 SO2 and high relative humidity (90%), zinc corrosion was significantly greater than in uncontaminated atmospheres or those with 800 mg m3 NO2 due to the consolidation of a moisture layer which allows the dissolution of SO2. At low relative humidity (70%), corrosion was of the same order as that found in atmospheres with

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Fig. 10. High-resolution S2p XPS spectra obtained on zinc specimens after 15 days of exposure in the different sulphur-containing atmospheres.

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Table 3 Corrosion products detected by XPS and GIXD SO2 (mg m3)

NO2 (mg m3)

0 800 a

Corrosion products XPS

GIXD

0 800

Zn(OH)2, ZnO Zn(OH)2, ZnO

a

0 800

Zinc hydroxysulphate, ZnSO3, Zn(OH)2, ZnO Zinc hydroxysulphate, ZnSO3, Zn(OH)2, ZnO

Zn4SO4(OH)6  3H2O Zn4SO4(OH)6  3H2O

a

Only metallic Zn was detected.

study has confirmed that this is trihydrated zinc hydroxysulphate, Zn4SO4(OH)6  3H2O. The results of this laboratory study are of special interest for a better understanding of the effect of NO2 on zinc corrosion, synergy, the role played by temperature and RH, as well as to suppress controversy of previous research studies.

Sulphate Sulfate

Sulfite Sulphite

%

100 90 80 70 60 50 40 30 20 10 0

References

25ºC 35ºC 25ºC 35ºC 25ºC 35ºC 25ºC 35ºC 70% RH

90% RH

SO2

70% RH

90% RH

SO2 + NO2

Fig. 11. Percentage composition obtained from the fitting of the two components of the high-resolution XPS spectra for S2p obtained on zinc specimens after 15 days of exposure in the different sulphur-containing atmospheres.

NO2 due to the difficulty for the formation of the moisture layer. In mixed atmospheres (800 mg m3 SO2+800 mg m3 NO2), an accelerating effect of NO2 has been observed, especially at 25 1C and 90% RH. Higher temperatures or lower RH make difficult the formation of a moisture layer and less amount of electrolyte is deposited on the zinc surface. The synergistic effect is less severe in the case of 35 1C and 90% RH and only slight in the case of 70% RH. The effect of NO2 is indirect given that nitrogen compounds have not been detected among the corrosion products and its influence is related with the increase in the rate of SO2 oxidation to sulphate. XPS analysis has confirmed the presence of zinc hydroxysulphate as the major component, along with ZnSO3, Zn(OH)2 and ZnO. The GIXD

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