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Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions
Electrochimica Acta 50 (2004) 43–49
Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions M.A. Ameer∗ , A.M. Fekry, F. El-Taib Heakal Chemistry Department, Faculty of Science, Cairo University, Egypt Received 26 April 2004; received in revised form 13 July 2004; accepted 13 July 2004 Available online 13 September 2004
Abstract Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na2 SO4 solutions of different concentrations at 25 ± 0.2 ◦ C between −1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that icorr and ic increases with increasing either Cl− or SO4 2− concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl− or SO4 2− concentration. © 2004 Elsevier Ltd. All rights reserved. Keywords: Corrosion; Electrochemical impedance spectroscopy; Molybdenum; Polarization; Stainless steel alloys
1. Introduction Recently, Betova et al. [1] found that the rate of transpassive dissolution of highly alloyed stainless steels is the lowest in chloride solutions and the highest in sulphate electrolytes. Olefjord et al. [2] reported that chloride ions are incorporated into the passive film when a Mo-containing stainless steel was exposed in hydrochloric acid at various potentials in the active and passive ranges of the alloys. The composition of the metal phase changes during active dissolution. Thus alloying elements are enriched on the surface and thereby control the dissolution rate, control overpotentials and provoke passivation of the alloy. Another evidence for the beneficial effects of Mo in stainless steel is furthermore reported by Olefjord and co-workers [3]. The authors showed that oxide particles are formed during the initial stage of the passivation process of high-alloyed stainless steels anodized in acidic chloride solution, by deprotonation of hydroxide. Chloride was found to be present in both the oxide and hydroxide layers of the ∗
Corresponding author. Tel.: +202 2729335; fax: +202 2756533. E-mail address: mameer [email protected] (M.A. Ameer).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.07.011
film. It is suggested that Mo can form strong soluble oxochlorocomplexes, which thereby will decrease the free Cl− ion concentration close to surface during passivation. Sugimoto and Sawada [4] reported that the presence of an adequate amount of Cr is indispensable for the improvement of pitting resistance by the Mo addition. The passive films of the Mo-containing steels were found to be composed of a complex oxyhydroxide containing Cr3+ , Fe3+ , Ni2+ , Mo6+ and Cl− , and showed a rather higher dc resistance in HCl solution than in H2 SO4 solution. The thickness of the passive film increased with increase in Mo content. This paper describes a study of the electrochemical behaviour of Mo-containing stainless steel in Na2 SO4 and NaCl solutions.
2. Expermintal procedures Three samples of molybdenum-containing stainless steels were tested in the present study. The alloys were supplied in the form of plates by Avesta AB, (Research and Development Physical Metallurgy, Sweden) with its cross-sectional area of
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M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
Table 1 Designation and compositions wt.% of molybdenum-containing stainless steels Alloys
Designation
C
I II III
2205 17-14-4LN 654SMo
0.019 21.9 0.022 17.3 0.020 23.9
Cr
Ni
Mo
N
Cu
Mn
5.4 13.3 22.2
3.0 4.4 7.0
0.148 − 0.150 − 0.452 0.4
− − 2.0
The balance is the wt.% of Fe in each steel.
0.16 cm2 . The designation and chemical composition of the alloys, as given by the supplier, are listed in Table 1. The test solutions were NaCl (Aldrich) and Na2 SO4 (BDH) analytical reagents of different concentrations. Triple distilled water was used for preparing all solutions. In all measurements, mechanically polished electrodes were used. Polishing was affected using successively finer grade of emery papers (600–1200 grade). Polarization and electrochemical impedance spectroscopy (EIS) measurements were carried out using the electrochemical workstation IM6e Zahnerelectrik GmbH, Metechnik, Kronach, Germany. The excitation ac signal has amplitude of 10 mV peak to peak in a frequency domain from 0.1 to 100 kHz. The EIS was recorded after reading a steady state open-circuit potential.
3. Results and discussion 3.1. Potentiodynamic polarization measurements In this part the potentiodynamic polarization behavior of three nominated alloys, viz., steels I, II and III, having varying content of Mo (namely, 3.0, 4.4 and 7.0%) were studied in relation to anion nature and concentration of the aqueous solutions. Fig. 1 shows a typical linear sweep potentiodynamic traces for the three steels in 0.1 M Na2 SO4 solutions. The scanning was carried out at a rate of 30 mV/min over the potential range from −1000 to +1500 mV versus saturated calomel electrode (SCE). Prior to the potential sweep,
Fig. 1. Potentiodynamic polarization curves of steels I, II, III in 0.1 M Na2 SO4 solution.
each electrode was left under open-circuit in the respective solution for ∼3 h until a steady free corrosion potential was recorded. The anodic polarization curves display a large passive region of 1000–1200 mV which extends to ∼1500 mV (SCE), at which point the anodic current beginning to increase, reaches a maximum, and in most solution a second passivation region may appear, depending on the solution composition, before oxygen evolution at ∼1200 mV. For some steels, in chloride medium the current continue to increase, albeit at a slower rate in this region. For those cases, no peak current and secondary passivation features are observed. The primary passivation potential (Epp ) is the potential at which the anodic dissolution current is a maximum. The critical current (ic ) is the maximum anodic dissolution current. The active region, which is the region between the corrosion potential and the primary passivation potential. The passive region, i.e. the region where the anodic dissolution current density is very small, starting from the flade or the critical potential, and the passive current (ip ), which is the current density in the passive region. There are a number of theories on passivity, of these; the Evan’s oxide film theory is the more prominent one [5]. For the three tested steels the estimated active dissolution parameters are given in Tables 2 and 3 as a function of the concentration for both Na2 SO4 and NaCl solutions, respectively. The results indicate clearly that those parameters are dependent on the anion type and concentration of the medium, as well as on the alloy composition. The corrosion current (icorr ), which is equivalent to the corrosion rate, is given by the intersection of the Tafel lines extrapolation. Because of the presence of a degree of nonlinearity in the Tafel slope part of the obtained polarization curves, the Tafel constants were calculated as a slope of the points after Ecorr by ±50 mV using a computer least-squares analysis. Corrosion Table 2 Electrochemical parameters of molybdenum-containing stainless steels in naturally aerated Na2 SO4 solution at 25 ± 0.2 ◦ C. Electrode No.
C (M)
−Ecorr (mV)
icorr (A cm2 )
Epp (mV)
I II III
0.005
348 68 400
1l.59 0.87 0.60
151.00 149.25 278.50
2.60 2.30 2.10
I II III
0.025
399 411 447
2.24 1.02 0.98
307.25 153.75 392.63
4.23 4.06 4.00
I II III
0.05
446 421 426
3.02 1.58 1.23
309.00 313.75 332.88
4.93 4.89 4.83
I II III
0.25
398 553 460
3.80 2.69 1.78
440.00 163.00 294.00
12.84 6.03 5.62
I II III
0.5
533 454 533
6.76 3.55 2.51
395.25 372.63 449.38
17.78 7.30 6.21
ic (A cm2 )
M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
45
Table 3 Electrochemical parameters of molybdenum-containing stainless steels in naturally aerated NaCl solution at 25 ± 0.2 ◦ C. Electrode No.
C (M)
−Ecorr (mV)
icorr (A cm−2 )
Epp (mV)
ic (A cm−2 )
I II III
0.001
377 378 296
0.67 0.44 0.28
201.38 260.13 214.38
2.58 1.98 1.78
I II III
0.05
451 439 475
1.21 1.02 0.96
339.25 330.13 391.50
3.40 3.22 3.00
I II III
0.1
425 443 442
1.96 1.29 1.05
303.88 357.88 361.50
3.92 3.62 3.41
I II III
0.5
425 456 365
2.72 1.59 1.32
289.88 380.38 251.13
4.53 4.33 4.07
I II III
1
412 440 442
2.00 2.26 1.62
315.75 355.38 377.13
4.57 4.52 4.30
currents were determined by the intersection of the cathodic Tafel line with the open-circuit potential. For any steel over the concentration range studied, each of the two current densities icorr and ic increases linearly with increasing the molar concentration of the anion in accordance to the following equation: log i = γ + β log C
Fig. 2. Variation of log icorr with log [Cl− ] for steels I, II, III in NaCl solution.
(1)
where γ is the value of log icorr or log ic at unit concentration and β is the slope of the linear plot. Eq. (1) was tested experimentally for the variation of log icorr Fig. 2 as an example in NaCl solution. Generally, the results can imply the following: (I) At a fixed anion level from both solutions, the two parameters icorr and ic have always higher values in the sulphate electrolyte compared with the chloride. (II) For any steel the slope β value of the log icorr −log C or log ic −log C plots was found to be higher in the sulphate than in the chloride solutions, where it amounts to ca. 0.30 and 0.22 for the first relationship, while it is ca. 0.25 and 0.11 for the second relationship in both Na2 SO4 and NaCl solutions, respectively. (III) For each medium at almost all dilutions, raising the level of Mo content in the steels was found to lower significantly the values of the two current densities (icorr and ic ). The corrosion rate was found to be higher in the sulphate medium as compared with the chloride solution at comparable concentration. The significant difference in the icorr and ic values between SO4 2− and Cl− media may possibly be due to the difference in the extent of incorporation of the oxide film on the steel surface by the electrolyte species during the anodization process. A fact that can enhance the partial dissolution rate of this film. As indicated by the present results, the extent of contamination in the passive film by the foreign
anions from the electrolyte appears to be higher in sulphate. Evidence for these impurities in the anodic passive films has been previously reported on some valve metals [6,7]. The role of Mo in the steel during anodization in sulphate solution is quite different since, there is meager information concerning the possible formation of any soluble salt for Mo complexes in this medium. The extent of free SO4 2− ion incorporation built into the passive film will be greater here, and consequently the rate of anion attack will be the highest in sulphate solution. The relative decrease in the icorr or ic values by increasing the Mo content in the steel, may be attributed in this case to the ability of Mo to eliminate the active surface sites through the formation of Mo oxyhydroxide or molybdate on these sites, on which it is difficult to form the stable passive film [8]. This leads to the appearance of a homogenous steel surface and to the formation of a homogenous passive film. 3.2. Impedance measurements Fig. 3 shows the EIS data for steels I, II and III traced at the rest potential in NaCl solutions varying from 0.05 M till 1.00 M. The corresponding data for the same three alloys in Na2 SO4 solutions ranging between 0.025 and 0.50 M are depicted in Fig. 4. In all cases the impedance Bode data display only one maximum phase lag, as could be deduced from the present results of no additional time constants corresponding to different corrosion or passivation steps. It is
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M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
Fig. 3. EIS plots of steel III in different concentrations of NaCl solution: (a) Bode plot, (b) Nyquist plot.
also clear from the figures that both log Z−log f relations deviate from the value of −1 and θ max is far removed from 90◦ ; which are required in case of pure capacitive behavior. Further, the Nyquist plots are all arcs of semicircles with centers depressed below the x-axis. Based on the constant phase element (CPE) concept, the results were analyzed using the proposed model for the metal/passive film system shown in Fig. 5. Detailed analysis of the impedance spectra for the passive films formed on the steel surfaces under open-circuit conditions shows that a simple equivalent circuit cannot represent the electrode/electrolyte interface. The Nyquist plots were no longer ideal capacitive semicircles. Also, the Bode plots showed phase angle maxima smaller than 90◦ and absolute values of the impedance slope lower than minus unity (≤0.92). The impedance behavior of such systems can be interpreted as a deviation of the double layer from the ideal capacitive behavior due to many reasons such as surface roughness, local inhomogenities in the dielectric material, porosity, mass transport effects and relaxation effects [9]. The deviation represents also an indication of the presence of a defective passive barrier film and no continuous layer can be present. To account for such behavior a model was proposed in which a constant phase element is used instead of a pure double layer capacitance (Cdl ), as shown by the EEC for the passive
Fig. 5. Equivalent circuit model representing the electrode/electrolyte interface.
film/electrolyte system which is depicted in Fig. 5 and consisting of a parallel combination of a CPE and a resistor (RT ) together with an additional series resistance corresponding to the solution resistance (R ). The total cell impedance Z (j) of such systems can be represented by the following transfer function [10], Z(jω) =
RΩ + RT 1 + (QRT (jω)α )
(2)
where ω = 2f is the angular frequency in rad/s, RT is the total resistance of the oxide film, j = (−1)1/2 , Q is the total capacitance of the constant phase angle element, and ␣ is a fit parameter which is an empirical exponent varies between 1 for a perfect capacitor and 0 for a perfect resistor, and is cor-
Fig. 4. EIS plots of steel III in different concentrations of Na2 SO4 solution: (a) Bode plot, (b) Nyquist plot.
M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
47
related to the angle of rotation of the center of the semicircle Φ below the real axis in the Nyquist format through, Φ = (1 − α)
π 2
(3)
A value of α less than unity would represent a somewhat imperfect capacitor and is generally thought to arise from a distribution of the relaxation times as a result of heterogeneities present on the microscopic level under the oxide phase and at the oxide/electrolyte interface [11]. The slope of the linear part of the Bode plots gives the value of α. The impedance of the CPE (ZCPE ) is described mathematically as; 1 = Y = Qo (jω)α ZCPE
(4)
This relation indicates that Q◦ is not dimensionally an ideal capacitance and has units of −1 sα . Q◦ is the admittance (1/|Z|) at ω = 1 rad/s. A consequence of this simple equation is that the phase angle of the CPE impedance is independent of the frequency and has a value of −(α × π/2) degrees. This gives the CPE its name. The simulated response of the representative circuit compares well with the experimental results both in admittance and impedance planes. This indicates that the suggested model is suitable for explaining the behavior of the austenitic stainless steel passive films in chloride as well as in sulphate solutions over the concentration range studied. The theoretical simulated impedance parameters for the three tested steels at each concentration were computed in the two forming media and summarized in Table 4. The variation of the electrode impedance for the various steels with the composition of the forming solution gives a suitable measure for film stability [12]. Figs. 6 and 7 show the variation of the frequencyindependent total resistive component (RT ) of the electrode
Fig. 6. Variation of RT value for steels I, II and III with Na2 SO4 concentration.
impedance for the various steels as a function of solution composition in chloride and sulphate media, respectively. It is well established that the properties of austenitic stainless steel favor the spontaneous formation of an oxide passive film, which confers great resistance to corrosion in aqueous solutions. However, the susceptibility of this passive film to corrosion (general or localized) depends not only on the na-
Table 4 Equivalent circuit parameters in NaCl and Na2 SO4 solutions of different concentrations at 25 ± 0.2 ◦ C. NaCl (M)
R ()
Q (F)
RT (k)
α
Na2 SO4 (M)
R ()
Q (F)
RT (k)
α
(a): 0.05 0.1 0.25 0.5 1
144.6 67.5 32.6 24.1 16.6
2.084 4.436 4.830 3.700 2.259
176.1 163.2 124.5 94 308.2
0.776 0.852 0.792 0.721 0.843
0.025 0.05 0.25 0.5 –
169.7 102.2 24.8 16.1 –
0.740 2.660 3.878 6.956 –
1512.0 613.1 318.9 169.4 –
0.794 0.811 0.807 0.799 –
(b): 0.05 0.1 0.25 0.5 1
127.6 67.7 32.1 21.6 15.5
1.622 4.325 4.42 4.644 3.393
250.1 232.4 166.0 110.3 200.1
0.781 0.851 0.833 0.764 0.871
0.025 0.05 0.25 0.5 –
176.3 94.9 28.4 18.9 –
1.614 2.480 4.447 6.642 –
1223.0 929.4 392.1 318.8 –
0.848 0.824 0.867 0.848 –
(c): 0.05 0.1 0.25 0.5 1
101.8 51.8 28.1 19.4 13.2
2.278 3.413 4.351 3.532 5.380
144.2 139.8 103.3 76.1 160.0
0.764 0.858 0.813 0.794 0.851
0.025 0.05 0.25 0.5 –
140.3 86.3 20.3 13.0 –
2.086 3.628 4.127 6.271 –
1080.3 775.4 615.2 448.5 –
0.857 0.845 0.864 0.882 –
(a) Electrode I [3.0 Mo–5.4 Ni–0.148 N]. (b) Electrode II [4.0 Mo–13.3 Ni–0.150 N]. (c) Electrode III [7.0 Mo–22.2 Ni–0.452 N–0.4 Cu–2.0 Mn].
48
M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
Fig. 7. Variation of RT value for steels I, II and III with NaCl concentration.
ture of solution but also on the nature and amount of the alloy additives. In neutral chloride solutions, Fig. 6 indicates that RT value for any steel decreases sharply, at first, with Cl− concentration, reaching a minimum at [Cl− ] of 0.5 M, then increases systematically to higher values with increasing the Cl− concentration. On the other hand, the RT value for steel II always found to surpasses those for the other two steels (I or III) at Cl− concentration less than or equal to 0.5 M, whereas in 1.0 M NaCl solution, the decrease in the RT value matches the increase in the Mo content in the alloy i.e. follows the sequence. 3.3. Steel I > II > III As far as the present results are concerned, it is possible to assume that Cl− can participate in enhancing the formation of soluble oxochlorocomplexes encompassing Mo [3] which initiate pit nucleation at the active inclusion sites leading to an increase in the corrosion rate or RT decrease. Increasing the Cl− concentration (>0.5 M) suppresses the solubility of these species. A possible concomitant hydrolysis reaction cannot be also excluded which generates insoluble salts or hydroxides that hinder contact between the metallic surface and the electrolyte solution in the pit with a consequent decrease in the corrosion rate associated with RT value increase. These results are consistent with those reported by Ogura et al. [13] for the effect of Mo content in acidic NaCl solu-
tions on the corrosion rate of 18Cr–9Ni–0.01Cu–Mo series, where they observed a threshold maximum corrosion rate at a concentration of ca. 0.2 M Cl− , and the corrosion rate generally increases with the gradual increase of Mo content (from 0.13 to 1.03 wt.%) for all Cl− concentrations. For Cl− at concentration 1.0 M, the present results agree well with Sawada et al. observations. However, at any Cl− concentration ≤0.5 M the anomalous behavior between steels II (4.0 Mo–13.3 Ni–0.15N) and III (3.0 Mo–5.5 Ni–0.15N) may be attributed to the high Ni content of the former as compared with that of the later which can overshadow the effect due to Mo content increase at low Cl− ion concentrations. As to the passive properties of the same three stainless steels (I, II and III) in neutral sulphate solution, Fig. 7 shows clearly that for each steel the RT parameter of the passive film decreases sharply at first and then smoothly with increasing Na2 SO4 concentration from 0.25 to 0.5 M. Generally, its value also decreases with increasing Mo content, especially at high SO4 2− concentrations (>0.2 M), in a fashion contrary to the trend in the Cl− solution. The improvement due to Mo addition is attributed to the ability of Mo to better stabilize a passive film, which is formed in and subjected to high SO4 2− solution. All these results suggested that high Mo content in austenitic stainless steel appears to be beneficial in neutral sulphate solution as it inhibits their corrosion due to an increase in the polarization resistance of the cathodic reaction; and being detrimental in neutral chloride solution as it accelerates the corrosion, probably due to a decrease in the cathodic polarization [14]. For open circuit oxide film formation [15], it was suggested that the driving force of surface oxide film formation on the metal is the free energy of the reaction between the metal and the test solution. This reaction is assumed to proceed by migration of positive metal cations and/or oxygen vacancies from the metal towards the electrolyte, or possibly by migration of negative oxygen anions in the opposite direction [16]. For Mo bearing stainless steel, when its surface is subjected to corrosive environment (Cl− or SO4 2− ) the behavior of the passive film is ascribed to the model suggested for both solutions based on the determined composition of the film and the properties of the compounds formed on the surface. The cations in the barrier layer are mainly the trivalent species Cr3+ and Fe3+ , which give an oxide of the type (Cr, Fe)2 O3 . Ferrous ions (Fe2+ ) also present in the film will be accompanied by point defects. It has been suggested that these defects are canceled by the presence of some oxidized Mo species [2]. Hence, lower defect concentration in the film decrease the penetration of the aggressive anions and thereby improve the resistance to pitting corrosion. Furthermore, the distribution of the prevailing anion (Cl− or SO4 2− ) of the forming solution in the passive film is also an important factor as well. It has been reported that the faradaic impedance due to metal dissolution at the oxide film/electrolyte interface depends strongly on the anion type together with the local pH at the passive film/solution interface. Both pH and faradaic
M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
impedance determine the chemical stability of the film [17]. In this respect, it has been shown that Cl− ions are incorporated in the passive film when the passivation is performed in chloride-containing electrolyte. Analysis [18,19] shows also that the Cl− ions are mainly present in the outer part of the passive film. Other studies [20,21] indicate that the Cl− ions are almost uniformly distributed throughout the film. The thickness of the passive film formed on high-alloyed stainless steels in acidic Cl− solutions is not influenced by the Mo content of the alloy [19,22]. However, Mischler et al. [19] found that the thickness of the chloride-containing region is thinner for the Mo alloyed steels compared to steels without Mo. No studies on the incorporation of SO4 2− ions in the passive film from salt solution seem to be found in the literatures. However, the results in Figs. 6 and 7 and of Table 4 show clearly that RT value in sulphate medium is much higher than in chloride for steels I, II and III. This behavior supports the stabilization of the formed oxide film on these alloys due to the possible incorporation of SO4 2− anion in the oxide film during its growth with a much higher extent than in chloride medium. Anion incorporation in the oxide films greatly reduces their porosity [23], which leads to an increase in the RT value.
4. Conclusions From the results of the two-electrochemical techniques used, namely potentiodynamic polarization and electrochemical impedance spectroscopy EIS at 25 ± 0.2 ◦ C, the following are the main points found in this study: • In both NaCl and Na2 SO4 solutions Mo alloying element greatly improves the resistance of austenitic steels to corrosion attack. Generally for any steel, icorr and ic are found to increase linearly with raising the molar concentration of the prevailing anion (SO4 2− or Cl− ) in the test solution in accordance with the relation: log i = γ + β log C. • At a fixed anion level from both sulphate and chloride solutions, the two parameters icorr and ic have always higher values in the sulphate electrolyte compared with the chloride. Additionally, the slope β value of the log icorr −log C or log ic −log C plots was found to be higher in the sulphate than in the chloride solutions. This behavior may possibly be due to the difference in the extent of incorporation of the oxide film by the electrolyte species during the anodization process, being higher in the sulphate solutions. • Quantitative analysis based on the CPE concept gives a better agreement between the experimental results and the theoretical data indicating the validity of the proposed model in explaining the experimental data. • The total resistance RT values decrease sharply with increasing in NaCl concentrations lower than 0.5 M then
49
increase systematically until reaching 1 M concentration for each steel. It is possible to assume that Cl− can participate in enhancing the formation of soluble oxochloro complexes encompassing Mo, which initiate pits nucleation at the active inclusion sites leading to an increase in the corrosion rate or lower value of the RT . At high Cl− concentration (>0.5 M) the solubility of these species is suppressed. A possible concomitant hydrolysis reaction cannot be also excluded which generates insoluble salts or hydroxides that hinder contact between the metallic surface and the electrolyte solution in the pit with a consequent decrease in the corrosion rate associated with RT value increase • The total resistance values also decrease sharply at first with increasing sulphate anion concentrations then decreases smoothly. The improvement due to Mo addition is attributed to the ability of Mo to better stabilize a passive film.
References [1] I. Betova, M. Bojinov, T. Laitinen, K. Makela, Corros. Sci. 44 (2002) 2699. [2] I. Olefjord, B. Brox, U. Jelvestam, J. Electrochem. Soc. 132 (1985) 2854. [3] L. Wegrelius, F. Falkenberg, I. Olefjord, J. Electrochem. Soc. 146 (1999) 397. [4] K. Sugimoto, Y. Sawada, Corros. Sci. 17 (1977) 425. [5] U.R. Evans, Metallic Corrosion Passivity and Protection, Edward Arnold and Co., London, 1946. [6] F. El-Taib Heakal, A.A. Mazhar, M.A. Ameer, Br. Corros. J. 23 (1988) 41. [7] F. El-Taib Heakal, A.S. Mogoda, A.A. Mazhar, A.A. Ghoneim, Corrosion 46 (1990) 247. [8] K. Hashimoto, K. Asami, K. Teramoto, Corros. Sci. 19 (1979) 3. [9] K. Sugimoto, Y. Sawada, Corros. Sci. 17 (1977) 425. [10] M.S. El-Basiouny, A.A. Mazhar, F. El-Taib Heakal, M.A. Ameer, J. Electroanal. Chem. 147 (1983) 181. [11] M.F. Montemor, A.M.P. Sim˜oes, M.G.S. Ferreira, M. Da Cunha, M.D.C. Belo, Corros. Sci 41 (1999) 17. [12] S.S. El-Egamy, W.A. Badawy, H. Shehata, Mat. -wiss. u. Werkstofftech 31 (2001) 737. [13] S. Ogura, K. Sugimoto, Y. Sawada, Corros. Sci. 16 (1971) 323. [14] M. Asawa, J. Japan Inst. Met. 34 (1970) 871. [15] F.E.-T. Heakal, M.A. Ameer, A.M. Fekry, J. Mater. Sci. in press. [16] L. Young, Anodic oxide films, Academic Press, London/New York, 1961. [17] J.B. Bessone, D.R. Salinas, C.E. Mayer, M. Ebert, W.J. Lorenz, Electrochim. Acta 37 (1992) 2283. [18] W.P. Yang, D. Cota, P. Marcus, J. Electrochem. Soc. 141 (1994) 2669. [19] S. Mischler, A. Vogel, H.J. Mathieu, D. Landolt, Corros. Sci. 32 (1991) 925. [20] I. Olefjord, L. Wegrelius, Corros. Sci. 31 (1990) 89. [21] L. Wegrelius, I. Olefjord, Mater. Sci. Forum 185–188 (1995) 347. [22] D. Landolt, S. Mischler, A. Vogel, H.J. Mathieu, Corros. Sci. 31 (1990) 431. [23] G.T. Rogers, P.H.G. Draper, S.S. Wood, Electrochim. Acta 13 (1968) 251.
Electrochemical behaviour of passive films on molybdenum-containing austenitic stainless steels in aqueous solutions M.A. Ameer∗ , A.M. Fekry, F. El-Taib Heakal Chemistry Department, Faculty of Science, Cairo University, Egypt Received 26 April 2004; received in revised form 13 July 2004; accepted 13 July 2004 Available online 13 September 2004
Abstract Passivation and its breakdown reactions have been studied on Mo-containing stainless steel specimens using different electrochemical techniques. Mo-containing stainless steel specimens were polarized in both naturally aerated NaCl and Na2 SO4 solutions of different concentrations at 25 ± 0.2 ◦ C between −1000 and 1500 mV versus saturated calomel electrode (SCE). The results of potentiodynamic polarization showed that icorr and ic increases with increasing either Cl− or SO4 2− concentration indicating the decrease in passivity of the formed film. EIS measurements under open circuit conditions confirmed that the passivity of the film decrease with increase in either Cl− or SO4 2− concentration. © 2004 Elsevier Ltd. All rights reserved. Keywords: Corrosion; Electrochemical impedance spectroscopy; Molybdenum; Polarization; Stainless steel alloys
1. Introduction Recently, Betova et al. [1] found that the rate of transpassive dissolution of highly alloyed stainless steels is the lowest in chloride solutions and the highest in sulphate electrolytes. Olefjord et al. [2] reported that chloride ions are incorporated into the passive film when a Mo-containing stainless steel was exposed in hydrochloric acid at various potentials in the active and passive ranges of the alloys. The composition of the metal phase changes during active dissolution. Thus alloying elements are enriched on the surface and thereby control the dissolution rate, control overpotentials and provoke passivation of the alloy. Another evidence for the beneficial effects of Mo in stainless steel is furthermore reported by Olefjord and co-workers [3]. The authors showed that oxide particles are formed during the initial stage of the passivation process of high-alloyed stainless steels anodized in acidic chloride solution, by deprotonation of hydroxide. Chloride was found to be present in both the oxide and hydroxide layers of the ∗
Corresponding author. Tel.: +202 2729335; fax: +202 2756533. E-mail address: mameer [email protected] (M.A. Ameer).
0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.07.011
film. It is suggested that Mo can form strong soluble oxochlorocomplexes, which thereby will decrease the free Cl− ion concentration close to surface during passivation. Sugimoto and Sawada [4] reported that the presence of an adequate amount of Cr is indispensable for the improvement of pitting resistance by the Mo addition. The passive films of the Mo-containing steels were found to be composed of a complex oxyhydroxide containing Cr3+ , Fe3+ , Ni2+ , Mo6+ and Cl− , and showed a rather higher dc resistance in HCl solution than in H2 SO4 solution. The thickness of the passive film increased with increase in Mo content. This paper describes a study of the electrochemical behaviour of Mo-containing stainless steel in Na2 SO4 and NaCl solutions.
2. Expermintal procedures Three samples of molybdenum-containing stainless steels were tested in the present study. The alloys were supplied in the form of plates by Avesta AB, (Research and Development Physical Metallurgy, Sweden) with its cross-sectional area of
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M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
Table 1 Designation and compositions wt.% of molybdenum-containing stainless steels Alloys
Designation
C
I II III
2205 17-14-4LN 654SMo
0.019 21.9 0.022 17.3 0.020 23.9
Cr
Ni
Mo
N
Cu
Mn
5.4 13.3 22.2
3.0 4.4 7.0
0.148 − 0.150 − 0.452 0.4
− − 2.0
The balance is the wt.% of Fe in each steel.
0.16 cm2 . The designation and chemical composition of the alloys, as given by the supplier, are listed in Table 1. The test solutions were NaCl (Aldrich) and Na2 SO4 (BDH) analytical reagents of different concentrations. Triple distilled water was used for preparing all solutions. In all measurements, mechanically polished electrodes were used. Polishing was affected using successively finer grade of emery papers (600–1200 grade). Polarization and electrochemical impedance spectroscopy (EIS) measurements were carried out using the electrochemical workstation IM6e Zahnerelectrik GmbH, Metechnik, Kronach, Germany. The excitation ac signal has amplitude of 10 mV peak to peak in a frequency domain from 0.1 to 100 kHz. The EIS was recorded after reading a steady state open-circuit potential.
3. Results and discussion 3.1. Potentiodynamic polarization measurements In this part the potentiodynamic polarization behavior of three nominated alloys, viz., steels I, II and III, having varying content of Mo (namely, 3.0, 4.4 and 7.0%) were studied in relation to anion nature and concentration of the aqueous solutions. Fig. 1 shows a typical linear sweep potentiodynamic traces for the three steels in 0.1 M Na2 SO4 solutions. The scanning was carried out at a rate of 30 mV/min over the potential range from −1000 to +1500 mV versus saturated calomel electrode (SCE). Prior to the potential sweep,
Fig. 1. Potentiodynamic polarization curves of steels I, II, III in 0.1 M Na2 SO4 solution.
each electrode was left under open-circuit in the respective solution for ∼3 h until a steady free corrosion potential was recorded. The anodic polarization curves display a large passive region of 1000–1200 mV which extends to ∼1500 mV (SCE), at which point the anodic current beginning to increase, reaches a maximum, and in most solution a second passivation region may appear, depending on the solution composition, before oxygen evolution at ∼1200 mV. For some steels, in chloride medium the current continue to increase, albeit at a slower rate in this region. For those cases, no peak current and secondary passivation features are observed. The primary passivation potential (Epp ) is the potential at which the anodic dissolution current is a maximum. The critical current (ic ) is the maximum anodic dissolution current. The active region, which is the region between the corrosion potential and the primary passivation potential. The passive region, i.e. the region where the anodic dissolution current density is very small, starting from the flade or the critical potential, and the passive current (ip ), which is the current density in the passive region. There are a number of theories on passivity, of these; the Evan’s oxide film theory is the more prominent one [5]. For the three tested steels the estimated active dissolution parameters are given in Tables 2 and 3 as a function of the concentration for both Na2 SO4 and NaCl solutions, respectively. The results indicate clearly that those parameters are dependent on the anion type and concentration of the medium, as well as on the alloy composition. The corrosion current (icorr ), which is equivalent to the corrosion rate, is given by the intersection of the Tafel lines extrapolation. Because of the presence of a degree of nonlinearity in the Tafel slope part of the obtained polarization curves, the Tafel constants were calculated as a slope of the points after Ecorr by ±50 mV using a computer least-squares analysis. Corrosion Table 2 Electrochemical parameters of molybdenum-containing stainless steels in naturally aerated Na2 SO4 solution at 25 ± 0.2 ◦ C. Electrode No.
C (M)
−Ecorr (mV)
icorr (A cm2 )
Epp (mV)
I II III
0.005
348 68 400
1l.59 0.87 0.60
151.00 149.25 278.50
2.60 2.30 2.10
I II III
0.025
399 411 447
2.24 1.02 0.98
307.25 153.75 392.63
4.23 4.06 4.00
I II III
0.05
446 421 426
3.02 1.58 1.23
309.00 313.75 332.88
4.93 4.89 4.83
I II III
0.25
398 553 460
3.80 2.69 1.78
440.00 163.00 294.00
12.84 6.03 5.62
I II III
0.5
533 454 533
6.76 3.55 2.51
395.25 372.63 449.38
17.78 7.30 6.21
ic (A cm2 )
M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
45
Table 3 Electrochemical parameters of molybdenum-containing stainless steels in naturally aerated NaCl solution at 25 ± 0.2 ◦ C. Electrode No.
C (M)
−Ecorr (mV)
icorr (A cm−2 )
Epp (mV)
ic (A cm−2 )
I II III
0.001
377 378 296
0.67 0.44 0.28
201.38 260.13 214.38
2.58 1.98 1.78
I II III
0.05
451 439 475
1.21 1.02 0.96
339.25 330.13 391.50
3.40 3.22 3.00
I II III
0.1
425 443 442
1.96 1.29 1.05
303.88 357.88 361.50
3.92 3.62 3.41
I II III
0.5
425 456 365
2.72 1.59 1.32
289.88 380.38 251.13
4.53 4.33 4.07
I II III
1
412 440 442
2.00 2.26 1.62
315.75 355.38 377.13
4.57 4.52 4.30
currents were determined by the intersection of the cathodic Tafel line with the open-circuit potential. For any steel over the concentration range studied, each of the two current densities icorr and ic increases linearly with increasing the molar concentration of the anion in accordance to the following equation: log i = γ + β log C
Fig. 2. Variation of log icorr with log [Cl− ] for steels I, II, III in NaCl solution.
(1)
where γ is the value of log icorr or log ic at unit concentration and β is the slope of the linear plot. Eq. (1) was tested experimentally for the variation of log icorr Fig. 2 as an example in NaCl solution. Generally, the results can imply the following: (I) At a fixed anion level from both solutions, the two parameters icorr and ic have always higher values in the sulphate electrolyte compared with the chloride. (II) For any steel the slope β value of the log icorr −log C or log ic −log C plots was found to be higher in the sulphate than in the chloride solutions, where it amounts to ca. 0.30 and 0.22 for the first relationship, while it is ca. 0.25 and 0.11 for the second relationship in both Na2 SO4 and NaCl solutions, respectively. (III) For each medium at almost all dilutions, raising the level of Mo content in the steels was found to lower significantly the values of the two current densities (icorr and ic ). The corrosion rate was found to be higher in the sulphate medium as compared with the chloride solution at comparable concentration. The significant difference in the icorr and ic values between SO4 2− and Cl− media may possibly be due to the difference in the extent of incorporation of the oxide film on the steel surface by the electrolyte species during the anodization process. A fact that can enhance the partial dissolution rate of this film. As indicated by the present results, the extent of contamination in the passive film by the foreign
anions from the electrolyte appears to be higher in sulphate. Evidence for these impurities in the anodic passive films has been previously reported on some valve metals [6,7]. The role of Mo in the steel during anodization in sulphate solution is quite different since, there is meager information concerning the possible formation of any soluble salt for Mo complexes in this medium. The extent of free SO4 2− ion incorporation built into the passive film will be greater here, and consequently the rate of anion attack will be the highest in sulphate solution. The relative decrease in the icorr or ic values by increasing the Mo content in the steel, may be attributed in this case to the ability of Mo to eliminate the active surface sites through the formation of Mo oxyhydroxide or molybdate on these sites, on which it is difficult to form the stable passive film [8]. This leads to the appearance of a homogenous steel surface and to the formation of a homogenous passive film. 3.2. Impedance measurements Fig. 3 shows the EIS data for steels I, II and III traced at the rest potential in NaCl solutions varying from 0.05 M till 1.00 M. The corresponding data for the same three alloys in Na2 SO4 solutions ranging between 0.025 and 0.50 M are depicted in Fig. 4. In all cases the impedance Bode data display only one maximum phase lag, as could be deduced from the present results of no additional time constants corresponding to different corrosion or passivation steps. It is
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M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
Fig. 3. EIS plots of steel III in different concentrations of NaCl solution: (a) Bode plot, (b) Nyquist plot.
also clear from the figures that both log Z−log f relations deviate from the value of −1 and θ max is far removed from 90◦ ; which are required in case of pure capacitive behavior. Further, the Nyquist plots are all arcs of semicircles with centers depressed below the x-axis. Based on the constant phase element (CPE) concept, the results were analyzed using the proposed model for the metal/passive film system shown in Fig. 5. Detailed analysis of the impedance spectra for the passive films formed on the steel surfaces under open-circuit conditions shows that a simple equivalent circuit cannot represent the electrode/electrolyte interface. The Nyquist plots were no longer ideal capacitive semicircles. Also, the Bode plots showed phase angle maxima smaller than 90◦ and absolute values of the impedance slope lower than minus unity (≤0.92). The impedance behavior of such systems can be interpreted as a deviation of the double layer from the ideal capacitive behavior due to many reasons such as surface roughness, local inhomogenities in the dielectric material, porosity, mass transport effects and relaxation effects [9]. The deviation represents also an indication of the presence of a defective passive barrier film and no continuous layer can be present. To account for such behavior a model was proposed in which a constant phase element is used instead of a pure double layer capacitance (Cdl ), as shown by the EEC for the passive
Fig. 5. Equivalent circuit model representing the electrode/electrolyte interface.
film/electrolyte system which is depicted in Fig. 5 and consisting of a parallel combination of a CPE and a resistor (RT ) together with an additional series resistance corresponding to the solution resistance (R ). The total cell impedance Z (j) of such systems can be represented by the following transfer function [10], Z(jω) =
RΩ + RT 1 + (QRT (jω)α )
(2)
where ω = 2f is the angular frequency in rad/s, RT is the total resistance of the oxide film, j = (−1)1/2 , Q is the total capacitance of the constant phase angle element, and ␣ is a fit parameter which is an empirical exponent varies between 1 for a perfect capacitor and 0 for a perfect resistor, and is cor-
Fig. 4. EIS plots of steel III in different concentrations of Na2 SO4 solution: (a) Bode plot, (b) Nyquist plot.
M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
47
related to the angle of rotation of the center of the semicircle Φ below the real axis in the Nyquist format through, Φ = (1 − α)
π 2
(3)
A value of α less than unity would represent a somewhat imperfect capacitor and is generally thought to arise from a distribution of the relaxation times as a result of heterogeneities present on the microscopic level under the oxide phase and at the oxide/electrolyte interface [11]. The slope of the linear part of the Bode plots gives the value of α. The impedance of the CPE (ZCPE ) is described mathematically as; 1 = Y = Qo (jω)α ZCPE
(4)
This relation indicates that Q◦ is not dimensionally an ideal capacitance and has units of −1 sα . Q◦ is the admittance (1/|Z|) at ω = 1 rad/s. A consequence of this simple equation is that the phase angle of the CPE impedance is independent of the frequency and has a value of −(α × π/2) degrees. This gives the CPE its name. The simulated response of the representative circuit compares well with the experimental results both in admittance and impedance planes. This indicates that the suggested model is suitable for explaining the behavior of the austenitic stainless steel passive films in chloride as well as in sulphate solutions over the concentration range studied. The theoretical simulated impedance parameters for the three tested steels at each concentration were computed in the two forming media and summarized in Table 4. The variation of the electrode impedance for the various steels with the composition of the forming solution gives a suitable measure for film stability [12]. Figs. 6 and 7 show the variation of the frequencyindependent total resistive component (RT ) of the electrode
Fig. 6. Variation of RT value for steels I, II and III with Na2 SO4 concentration.
impedance for the various steels as a function of solution composition in chloride and sulphate media, respectively. It is well established that the properties of austenitic stainless steel favor the spontaneous formation of an oxide passive film, which confers great resistance to corrosion in aqueous solutions. However, the susceptibility of this passive film to corrosion (general or localized) depends not only on the na-
Table 4 Equivalent circuit parameters in NaCl and Na2 SO4 solutions of different concentrations at 25 ± 0.2 ◦ C. NaCl (M)
R ()
Q (F)
RT (k)
α
Na2 SO4 (M)
R ()
Q (F)
RT (k)
α
(a): 0.05 0.1 0.25 0.5 1
144.6 67.5 32.6 24.1 16.6
2.084 4.436 4.830 3.700 2.259
176.1 163.2 124.5 94 308.2
0.776 0.852 0.792 0.721 0.843
0.025 0.05 0.25 0.5 –
169.7 102.2 24.8 16.1 –
0.740 2.660 3.878 6.956 –
1512.0 613.1 318.9 169.4 –
0.794 0.811 0.807 0.799 –
(b): 0.05 0.1 0.25 0.5 1
127.6 67.7 32.1 21.6 15.5
1.622 4.325 4.42 4.644 3.393
250.1 232.4 166.0 110.3 200.1
0.781 0.851 0.833 0.764 0.871
0.025 0.05 0.25 0.5 –
176.3 94.9 28.4 18.9 –
1.614 2.480 4.447 6.642 –
1223.0 929.4 392.1 318.8 –
0.848 0.824 0.867 0.848 –
(c): 0.05 0.1 0.25 0.5 1
101.8 51.8 28.1 19.4 13.2
2.278 3.413 4.351 3.532 5.380
144.2 139.8 103.3 76.1 160.0
0.764 0.858 0.813 0.794 0.851
0.025 0.05 0.25 0.5 –
140.3 86.3 20.3 13.0 –
2.086 3.628 4.127 6.271 –
1080.3 775.4 615.2 448.5 –
0.857 0.845 0.864 0.882 –
(a) Electrode I [3.0 Mo–5.4 Ni–0.148 N]. (b) Electrode II [4.0 Mo–13.3 Ni–0.150 N]. (c) Electrode III [7.0 Mo–22.2 Ni–0.452 N–0.4 Cu–2.0 Mn].
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M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
Fig. 7. Variation of RT value for steels I, II and III with NaCl concentration.
ture of solution but also on the nature and amount of the alloy additives. In neutral chloride solutions, Fig. 6 indicates that RT value for any steel decreases sharply, at first, with Cl− concentration, reaching a minimum at [Cl− ] of 0.5 M, then increases systematically to higher values with increasing the Cl− concentration. On the other hand, the RT value for steel II always found to surpasses those for the other two steels (I or III) at Cl− concentration less than or equal to 0.5 M, whereas in 1.0 M NaCl solution, the decrease in the RT value matches the increase in the Mo content in the alloy i.e. follows the sequence. 3.3. Steel I > II > III As far as the present results are concerned, it is possible to assume that Cl− can participate in enhancing the formation of soluble oxochlorocomplexes encompassing Mo [3] which initiate pit nucleation at the active inclusion sites leading to an increase in the corrosion rate or RT decrease. Increasing the Cl− concentration (>0.5 M) suppresses the solubility of these species. A possible concomitant hydrolysis reaction cannot be also excluded which generates insoluble salts or hydroxides that hinder contact between the metallic surface and the electrolyte solution in the pit with a consequent decrease in the corrosion rate associated with RT value increase. These results are consistent with those reported by Ogura et al. [13] for the effect of Mo content in acidic NaCl solu-
tions on the corrosion rate of 18Cr–9Ni–0.01Cu–Mo series, where they observed a threshold maximum corrosion rate at a concentration of ca. 0.2 M Cl− , and the corrosion rate generally increases with the gradual increase of Mo content (from 0.13 to 1.03 wt.%) for all Cl− concentrations. For Cl− at concentration 1.0 M, the present results agree well with Sawada et al. observations. However, at any Cl− concentration ≤0.5 M the anomalous behavior between steels II (4.0 Mo–13.3 Ni–0.15N) and III (3.0 Mo–5.5 Ni–0.15N) may be attributed to the high Ni content of the former as compared with that of the later which can overshadow the effect due to Mo content increase at low Cl− ion concentrations. As to the passive properties of the same three stainless steels (I, II and III) in neutral sulphate solution, Fig. 7 shows clearly that for each steel the RT parameter of the passive film decreases sharply at first and then smoothly with increasing Na2 SO4 concentration from 0.25 to 0.5 M. Generally, its value also decreases with increasing Mo content, especially at high SO4 2− concentrations (>0.2 M), in a fashion contrary to the trend in the Cl− solution. The improvement due to Mo addition is attributed to the ability of Mo to better stabilize a passive film, which is formed in and subjected to high SO4 2− solution. All these results suggested that high Mo content in austenitic stainless steel appears to be beneficial in neutral sulphate solution as it inhibits their corrosion due to an increase in the polarization resistance of the cathodic reaction; and being detrimental in neutral chloride solution as it accelerates the corrosion, probably due to a decrease in the cathodic polarization [14]. For open circuit oxide film formation [15], it was suggested that the driving force of surface oxide film formation on the metal is the free energy of the reaction between the metal and the test solution. This reaction is assumed to proceed by migration of positive metal cations and/or oxygen vacancies from the metal towards the electrolyte, or possibly by migration of negative oxygen anions in the opposite direction [16]. For Mo bearing stainless steel, when its surface is subjected to corrosive environment (Cl− or SO4 2− ) the behavior of the passive film is ascribed to the model suggested for both solutions based on the determined composition of the film and the properties of the compounds formed on the surface. The cations in the barrier layer are mainly the trivalent species Cr3+ and Fe3+ , which give an oxide of the type (Cr, Fe)2 O3 . Ferrous ions (Fe2+ ) also present in the film will be accompanied by point defects. It has been suggested that these defects are canceled by the presence of some oxidized Mo species [2]. Hence, lower defect concentration in the film decrease the penetration of the aggressive anions and thereby improve the resistance to pitting corrosion. Furthermore, the distribution of the prevailing anion (Cl− or SO4 2− ) of the forming solution in the passive film is also an important factor as well. It has been reported that the faradaic impedance due to metal dissolution at the oxide film/electrolyte interface depends strongly on the anion type together with the local pH at the passive film/solution interface. Both pH and faradaic
M.A. Ameer et al. / Electrochimica Acta 50 (2004) 43–49
impedance determine the chemical stability of the film [17]. In this respect, it has been shown that Cl− ions are incorporated in the passive film when the passivation is performed in chloride-containing electrolyte. Analysis [18,19] shows also that the Cl− ions are mainly present in the outer part of the passive film. Other studies [20,21] indicate that the Cl− ions are almost uniformly distributed throughout the film. The thickness of the passive film formed on high-alloyed stainless steels in acidic Cl− solutions is not influenced by the Mo content of the alloy [19,22]. However, Mischler et al. [19] found that the thickness of the chloride-containing region is thinner for the Mo alloyed steels compared to steels without Mo. No studies on the incorporation of SO4 2− ions in the passive film from salt solution seem to be found in the literatures. However, the results in Figs. 6 and 7 and of Table 4 show clearly that RT value in sulphate medium is much higher than in chloride for steels I, II and III. This behavior supports the stabilization of the formed oxide film on these alloys due to the possible incorporation of SO4 2− anion in the oxide film during its growth with a much higher extent than in chloride medium. Anion incorporation in the oxide films greatly reduces their porosity [23], which leads to an increase in the RT value.
4. Conclusions From the results of the two-electrochemical techniques used, namely potentiodynamic polarization and electrochemical impedance spectroscopy EIS at 25 ± 0.2 ◦ C, the following are the main points found in this study: • In both NaCl and Na2 SO4 solutions Mo alloying element greatly improves the resistance of austenitic steels to corrosion attack. Generally for any steel, icorr and ic are found to increase linearly with raising the molar concentration of the prevailing anion (SO4 2− or Cl− ) in the test solution in accordance with the relation: log i = γ + β log C. • At a fixed anion level from both sulphate and chloride solutions, the two parameters icorr and ic have always higher values in the sulphate electrolyte compared with the chloride. Additionally, the slope β value of the log icorr −log C or log ic −log C plots was found to be higher in the sulphate than in the chloride solutions. This behavior may possibly be due to the difference in the extent of incorporation of the oxide film by the electrolyte species during the anodization process, being higher in the sulphate solutions. • Quantitative analysis based on the CPE concept gives a better agreement between the experimental results and the theoretical data indicating the validity of the proposed model in explaining the experimental data. • The total resistance RT values decrease sharply with increasing in NaCl concentrations lower than 0.5 M then
49
increase systematically until reaching 1 M concentration for each steel. It is possible to assume that Cl− can participate in enhancing the formation of soluble oxochloro complexes encompassing Mo, which initiate pits nucleation at the active inclusion sites leading to an increase in the corrosion rate or lower value of the RT . At high Cl− concentration (>0.5 M) the solubility of these species is suppressed. A possible concomitant hydrolysis reaction cannot be also excluded which generates insoluble salts or hydroxides that hinder contact between the metallic surface and the electrolyte solution in the pit with a consequent decrease in the corrosion rate associated with RT value increase • The total resistance values also decrease sharply at first with increasing sulphate anion concentrations then decreases smoothly. The improvement due to Mo addition is attributed to the ability of Mo to better stabilize a passive film.
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