Influence of Cu and Sn content in the corrosion of AISI 304 and 316 stainless steels in H2SO4

Corrosion Science 48 (2006) 1075–1092 www.elsevier.com/locate/corsci

Influence of Cu and Sn content in the corrosion of AISI 304 and 316 stainless steels in H2SO4 A. Pardo *, M.C. Merino, M. Carboneras, F. Viejo, R. Arrabal, J. Mun˜oz Departamento de Ciencia de Materiales, Facultad de Quı´mica, Universidad Complutense, 28040 Madrid, Spain Received 22 March 2004; accepted 13 May 2005 Available online 20 July 2005

Abstract This paper addresses the influence of Cu and Sn addition on the corrosion resistance of AISI 304 and 316 stainless steels in 30 wt% H2SO4 at 25 and 50 C. The corrosion process was evaluated by gravimetric tests, DC measurements and electrochemical impedance spectroscopy (EIS). The corrosion products were analysed by SEM, X-ray mapping and XPS before and after accelerated tests. The behaviour of both AISI 304 and 316 stainless steels in sulphuric acid solution was greatly improved by increasing Cu concentration and the synergic effect of Cu and Sn. Addition of Sn increased corrosion resistance, but less than addition of copper.  2005 Elsevier Ltd. All rights reserved. Keywords: Stainless steel; Acid corrosion; General corrosion

1. Introduction Austenitic stainless steel manufacturers have started to replace iron oxide ores with iron scrap as raw material. Usually, iron scraps are contaminated with Cu *

Corresponding author. Tel.: +34 1 3944348; fax: +34 1 3944357. E-mail address: [email protected] (A. Pardo).

0010-938X/$ - see front matter  2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2005.05.002

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and Sn, which can significantly alter the corrosion behaviour of stainless steels. Moreover, Cu and Sn contents above 3 wt% and 0.12 wt% respectively can negatively affect hot workability during manufacturing. With this in mind, if Cu and Sn additions were not too much detrimental in behaviour of stainless steels, these alloys could be fabricated from recycled iron scrap without the need to reduce copper and tin to below the limits established by standards in the steelmaking process. The use of copper as an alloying element in austenitic stainless steels can be justified as follows: (a) copper stabilizes austenite [1], making it possible to reduce the nickel content in the alloy. This entails a significant economic saving since nickel content is high, and nickel is expensive; (b) copper is a good stabilizer against martensitic transformation [2,3], giving rise to more stable austenite; (c) Copper increases general corrosion resistance and helps to improve steel corrosion resistance in sulphuric acid [4–7]. The behaviour of stainless steels in sulphuric acid is complex, since these are neither strong oxidizing nor strong reducing solutions. In this sense, small amounts of metal salts or organic substances in solution are enough to transform stainless steels from the passive to the active state [8]. Easily reducible cations such as Fe3+, Cu2+, Sn4+ and Ce4+ are oxidizing agents, capable of inhibiting attack on stainless steels in H2SO4 [9]. Therefore, in addressing stainless steel resistance in sulphuric acid solutions, we need to specify the exact composition of the corrosive medium. Vernau et al. [10] have reported that copper always increases stainless steel corrosion resistance in acid media, although its influence depends on the oxidizing strength of the solution. Many other researchers have studied the beneficial effect of copper addition on stainless steels corrosion in acid mediums [11–14]. Regarding the mechanism of the beneficial effect of copper, several authors [15,16] have pointed out that this is based on the suppression of anodic dissolution by elemental copper deposition on the steel surface immersed in the corrosive medium. The role of copper in stainless steel passivation is complex and leads to apparently incongruent results, which could be related to the synergetic effect between different elements in the alloy. For instance, Wilde and Greene [17–19] noted that copper, due to its low hydrogen overpotential, has a beneficial effect on stainless steel passivation in non-oxidizing acid mediums. Similarly, Ramchandran et al. [20] showed the positive effect of copper on steel passivation in sulphuric acid. However, Seo et al. [5] established that copper reduces the stability of the passive layer. Again, Lizlovs [4] found that copper reduces the stability of the passive layer in stainless steels with less than 1% Mo, but such stability increases for higher levels of Mo up to 3%. There are only a limited number of references in the literature to the effect of Sn addition and the synergy of Sn and Cu. Osozawa [21] and Takizawa et al. [22] observed positive synergy of Cu and Sn in austenitic stainless steel corrosion resistance in both diluted sulphuric and organic-chlorinated acid mediums. One possible mechanism suggested by these authors is preferential dissolution of Cu and Sn, favouring the formation of a stable film of metallic copper and tin oxides on the steel surface. This paper attempts to evaluate the effect of Cu and Sn on the corrosion resistance of two austenitic stainless steels (AISI 304 and 316) in 30 wt% H2SO4.

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2. Experimental procedure The test materials were AISI 304 and 316 austenitic stainless steels with different concentrations of Cu and Sn. Chemical compositions of these materials are given in Tables 1 and 2. The alloys chosen were fabricated in ingots of 40 kg in a Pfeiffer VSG030 vacuum induction furnace. Vacuum was applied during the first part of the melting process in order to remove oxygen. No vacuum was applied in fine-tuning additions and casting operations at the end of the process, which were carried out in a 1 bar argon atmosphere. The ingots were hot forged into 4 mm plates and cold rolled into 2.5 mm sheets. Rectangular samples (50 · 25 · 2.5 mm) were used for the corrosion tests. Before the general corrosion test, specimens were prepared by pickling in HNO3 15 wt%– HF 2 wt% at 60 C for 2 min and water cleaning, followed by passivation in HNO3 65 wt% at 60 C for 1 min. Gravimetric tests were carried out in 30 wt% H2SO4 at 25 and 50 C open to air. The acid solutions were renewed every three days. Before the experiment, the sample area was measured and weighed to a precision of 0.01 mg. Upon completion of the experiment, the sample was extracted, cleaned with water, dried at 105 C for 30 min in a furnace and then weighed again at room temperature. The subsequent loss of mass per unit of surface area was then calculated for the different test times. The tests were performed in duplicate to guarantee the reliability of the results. DC electrochemical measurements were performed using rectangular samples with a surface area of approximately 14 cm2 exposed to the test medium. A threeelectrode cell was used for electrochemical measurements. The working electrode was the test material. The counter and reference electrodes were graphite and Ag/ AgCl respectively. Polarization measurements were carried out at a scan rate of 0.1 mV/s, from 100 mV to +100 mV with respect to the corrosion potential (Ecorr).

Table 1 Chemical composition of AISI 304 stainless steels Material

Elements (wt%) Cu

Sn

C

Si

Mn

Ni

Cr

P

S

Mo

N

290 291 292 293

0.500 1.010 1.980 3.100

0.009 0.008 0.009 0.008

0.060 0.052 0.053 0.050

0.410 0.390 0.410 0.420

1.070 1.730 1.420 1.670

8.130 8.020 8.020 7.980

18.210 18.070 17.980 17.630

0.028 0.027 0.027 0.029

0.002 0.001 0.001 0.001

0.100 0.100 0.100 0.100

0.0288 0.0400 0.0337 0.0423

294 295 296

0.560 0.991 1.997

0.023 0.073 0.113

0.053 0.062 0.043

0.430 0.411 0.335

1.520 1.750 1.560

8.050 8.012 7.800

18.110 17.932 17.648

0.028 0.028 0.030

0.001 0.001 0.001

0.100 0.102 0.311

0.0404 0.0421 0.0360

297 298 299 300 301

0.280 0.267 0.268 0.267 0.262

0.012 0.049 0.100 0.142 0.188

0.046 0.042 0.048 0.045 0.044

0.344 0.308 0.330 0.355 0.354

1.599 1.668 1.719 1.64 1.487

8.056 8.035 8.075 8.071 8.094

18.101 18.022 17.986 18.091 17.978

0.030 0.030 0.030 0.029 0.030

0.001 0.001 0.001 0.002 0.002

0.320 0.323 0.323 0.328 0.329

0.0393 0.0424 0.0482 0.0402 0.0381

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Material

Elements (wt%) Cu

Sn

C

Si

Mn

Ni

Cr

P

S

Mo

Ti

Nb

Co

N

302 303 304 305 306

0.500 0.980 1.474 1.988 2.479

0.012 0.012 0.011 0.011 0.011

0.022 0.034 0.041 0.037 0.041

0.380 0.402 0.343 0.366 0.369

1.562 1.438 1.463 1.381 1.505

10.326 10.228 10.036 10.170 10.091

17.345 17.325 17.105 17.148 17.017

0.033 0.032 0.032 0.034 0.034

0.001 0.001 0.001 0.001 0.001

2.151 2.128 2.117 2.107 2.083

0.010 0.007 0.006 0.005 0.006

0.009 0.009 0.007 0.009 0.009

0.119 0.120 0.123 0.119 0.116

0.0365 0.0345 0.0352 0.0367 0.0392

307 308 309 310

0.404 0.401 0.401 0.402

0.041 0.091 0.132 0.185

0.042 0.038 0.033 0.039

0.375 0.384 0.368 0.378

1.593 1.538 1.555 1.555

10.317 10.207 10.214 10.164

17.348 17.187 17.152 17.154

0.032 0.033 0.033 0.031

0.001 0.003 0.001 0.001

2.149 2.157 2.140 2.147

0.004 0.004 0.004 0.004

0.009 0.008 0.008 0.007

0.117 0.123 0.125 0.123

0.0382 0.0362 0.0366 0.0370

311 312 313 314

0.734 0.988 1.483 1.990

0.057 0.078 0.090 0.114

0.048 0.034 0.045 0.028

0.378 0.376 0.376 0.376

1.632 1.482 1.531 1.532

10.103 10.075 9.987 9.905

17.194 17.171 16.958 16.956

0.033 0.034 0.032 0.032

0.001 0.001 0.001 0.001

2.137 2.140 2.120 2.111

0.004 0.004 0.003 0.003

0.007 0.007 0.007 0.007

0.123 0.125 0.123 0.125

0.0538 0.0330 0.0420 0.0327

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Table 2 Chemical composition of AISI 316 stainless steels

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Electrochemical impedance measurements were performed using an AUTOLAB model PGSTAT12 potentiostat with frequency response analyzer (FRA) software. The frequency ranged from 100 kHz to 1 mHz with five points/decade. In each case, the target material was immersed in 30 wt% H2SO4 at 25 C. The immersed area was 2 cm2. The morphology of the corrosion products was analysed by SEM. Stainless steels were examined by X-ray mapping and X-ray photoelectron spectroscopy (XPS) before and after gravimetric tests. XPS spectra were taken in an ultra high vacuum chamber (UHV) equipped with an energy electron analyser (VG 100 AX) (pressure around 109 Torr, Mg Ka radiation, 15 kV and 20 mA). Before analysis, the samples were degassed overnight (107 Torr) in the pre-treatment chamber and then placed in the analysis chamber. After subtraction of a Shirley-type non-linear baseline, the spectra were decomposed using a commercial fitting program (VGX 900) with a Gaussian/Lorentzian ratio of 85/15. Binding energies are referenced to the C–(C, H) component of the C(1s) adventitious carbon fixed at 284.6 eV. Atomic ratios were calculated from relative intensities corrected by the elemental sensitivity factor of each atom [23].

3. Results and discussion 3.1. Gravimetric tests Fig. 1a shows the variation of the corrosion rate with increasing Cu, Sn and Cu + Sn contents in 30 wt% H2SO4 at 25 and 50 C for AISI 304 stainless steels. Addition of Cu drastically reduced the corrosion rate of AISI 304 material in diluted H2SO4. This effect was more significant when the test temperature was reduced from 50 to 25 C. Addition of Sn reduced the corrosion rate of AISI 304 stainless steels in sulphuric acid. Nevertheless, the corrosion rate was high at all Sn concentrations tested, indicating that Sn provided less protection than Cu. Additionally, the corrosion rate drastically decreased with temperature. The synergic effect of Cu and Sn

Fig. 1. Variation of the corrosion rate with Cu and Sn concentration: (a) AISI 304 and (b) AISI 316.

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significantly increased the corrosion resistance of AISI 304. The corrosion resistance due to the synergic effect of Cu and Sn was similar to that of materials with only Cu addition. Fig. 1b shows the variation of the corrosion rate of AISI 316 stainless steels with increasing Cu, Sn and Cu + Sn contents in 30 wt% H2SO4 at 25 and 50 C. The corrosion rates were lower in AISI 316 than in AISI 304 stainless steels. The effect of Cu, and Cu + Sn addition on corrosion behaviour in AISI 316 stainless steels was similar to the effect in AISI 304 stainless steels, but less intense. Addition of Sn did not affect general corrosion of AISI 316 in sulphuric acid, but the corrosion rate was high at all Sn concentrations tested. Sn provided less protection than Cu. The corrosion rate increased drastically with temperature. Tables 3 and 4 show the kinetic laws calculated for all tests from the experimental data. In each case the kinetics were calculated adjusting to a linear equation y = a + bt, where ‘‘y’’ coordinate represents the mass loss in units of mg/cm2, ‘‘t’’ is the immersion time in days, and ‘‘a’’ and ‘‘b’’ are the parameters of the linear regression, being ‘‘b’’ the corrosion rate in mg/cm2 d. In all cases, the fit regression parameters (r2) were close to unity. For AISI 304, copper addition up to 3 wt% reduced the corrosion rate from 54.89 to 2.51 mg/cm2 d at 25 C and from 350.73 to 23.94 mg/cm2 d at 50 C. The synergic effect of Cu and Sn presented a similar pattern. The influence of tin addition was slighter, and the corrosion rate decreased from 58.43 to 50.06 mg/ cm2 d at 25 C and from 600.13 to 210.21 mg/cm2 d at 50 C. Corrosion rates in AISI 316 were lower. Addition of 2.479 wt%Cu reduced the corrosion rate from 0.26 to 0.05 mg/cm2 d at 25 C and from 59.60 to 9.00 mg/cm2 d at 50 C. Addition of 0.185 wt%Sn reduced the corrosion rate from 6.30 to 4.96 mg/cm2 d at 25 C and from 56.80 to 47.96 mg/cm2 d at 50 C. In the AISI 316 stainless steel, copper addition practically inhibited corrosion in stainless steel immersed in 30 wt% H2SO4 at room temperature and open to air.

Table 3 Kinetic laws of gravimetric tests of AISI 304 stainless steels in 30 wt% H2SO4 at 25 and 50 C for 6 days Material

Kinetic law [y (mg/cm2), t (d)] H2SO4 30 wt%, 25 C

H2SO4 30 wt%, 50 C

290 291 292

y = 54.89t  67.96 y = 30.05t  53.10 y = 6.27t  9.78

26t66 26t66 26t66

(r2 = 0.995) (r2 = 0.994) (r2 = 0.986)

y = 350.73t + 22.41 y = 214.24t  5.27 y = 67.31t  2.03

06t62 06t63 06t66

(r2 = 0.988) (r2 = 0.999) (r2 = 1)

293 294 295 296

y = 2.51t  2.99 y = 49.25t  56.90 y = 19.91t  22.80 y = 3.44t  4.10

26t66 16t66 16t66 16t66

(r2 = 0.985) (r2 = 0.994) (r2 = 0.993) (r2 = 0.993)

y = 23.94t  0.43 y = 304.33t  7.71 y = 135.81t  4.53 y = 23.91t  1.89

06t66 06t62 06t64 06t66

(r2 = 0.997) (r2 = 0.998) (r2 = 0.999) (r2 = 0.999)

297 298 299 300 301

y = 58.43t  57.70 y = 55.22t  60.03 y = 56.70t  77.83 y = 49.25t  81.76 y = 50.06t  82.15

16t66 16t66 26t66 26t66 26t66

(r2 = 0.999) (r2 = 0.999) (r2 = 0.988) (r2 = 0.972) (r2 = 0.988)

y = 600.13t  1778.8 y = 478.76t  751.30 y = 281.96t + 21.56 y = 226.31t  0.24 y = 210.21t  229.31

36t64 26t63 06t62 06t62 26t64

(r2 = 1) (r2 = 1) (r2 = 0.983) (r2 = 1) (r2 = 0.996)

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Table 4 Kinetic laws of gravimetric tests of AISI 316 stainless steels in 30 wt% H2SO4 at 25 and 50 C for 6 days Material

Kinetic law [y (mg/cm2), t (d)] H2SO4 30 wt%, 25 C

H2SO4 30 wt%, 50 C

302 303 304 305 306

y = 0.26t  0.54 y = 0.18t  0.41 y = 0.18t  0.34 y = 0.10t  0.18 y = 0.05t  0.04

26t66 26t66 26t66 26t66 36t66

(r2 = 0.974) (r2 = 0.978) (r2 = 0.999) (r2 = 0.998) (r2 = 0.987)

y = 59.60t + 0.82 y = 20.47t  3.05 y = 15.79t  0.06 y = 11.11t  1.17 y = 9.00t  0.99

06t66 06t66 06t66 06t66 06t66

(r2 = 0.998) (r2 = 0.996) (r2 = 0.999) (r2 = 0.997) (r2 = 0.999)

307 308 309 310

y = 6.30t  13.08 y = 5.06t  10.75 y = 5.83t  12.74 y = 4.96t  9.36

26t66 26t66 26t66 26t66

(r2 = 0.990) (r2 = 0.985) (r2 = 0.982) (r2 = 0.996)

y = 56.80t + 6.17 y = 56.78t + 6.39 y = 47.26t + 1.42 y = 47.96t  0.08

06t66 06t66 06t66 06t66

(r2 = 0.999) (r2 = 0.999) (r2 = 1) (r2 = 1)

311 312 313 314

y = 3.48t  7.17 y = 2.53t  5.52 y = 1.89t  2.46 y = 0.86t  2.45

26t66 26t66 26t66 36t66

(r2 = 0.994) (r2 = 0.988) (r2 = 0.986) (r2 = 0.992)

y = 28.33t + 3.81 y = 17.33t  1.11 y = 11.41t  3.48 y = 8.94t  0.26

06t66 06t66 06t66 06t66

(r2 = 0.997) (r2 = 0.998) (r2 = 0.992) (r2 = 1)

3.2. DC electrochemical results The repassivation effect of Cu and the synergic effect of Cu and Sn were checked by carrying out consecutive polarization tests on the same sample at different immersion times in 30 wt% H2SO4. Between measurements the system evolved freely, so that each polarization measurement indicates the degree of repassivation due to the presence of Cu on surface material. Fig. 2a shows the polarization curves of the AISI 304 stainless steel without Cu (material 290). After 6 days of immersion the material showed a marked tendency to dissolve. With the addition of 3.1 wt%Cu (material 293) the corrosion rate decreased sharply. After the first day of immersion this material tended to revert to its original behaviour, which suggests that the copper dissolved was deposited on the surface, causing repassivation of the material (Fig. 2b). However, AISI 316 presented high corrosion resistance when immersed in 30 wt% H2SO4 at 25 C in all cases (Fig. 2c and d), probably due to the synergic effect of Mo and Cu. The synergic effect of Cu and Sn was comparable to the effect of adding 3 wt%Cu in AISI 304 (Fig. 2e and f). The synergic effect of Cu and Sn was less positive in AISI 316, probably due to synergism between Mo, Cu and Sn (Fig. 2g and h). The reduction in the corrosion rate produced by Sn addition was smaller in both AISI 304 and AISI 316 stainless steels in 30 wt% H2SO4. 3.3. AC Electrochemical results The effect of Cu addition on the corrosion resistance of AISI 304 and AISI 316 stainless steels in 30 wt% H2SO4 at 25 C was evaluated by electrochemical impedance spectroscopy (EIS). Fig. 3a and b shows the Nyquist diagrams as a function of the immersion time for materials 290 and 293. A first arc was observed at high and intermediate frequencies followed by a second arc or ill-defined tail. The first

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Fig. 2. Polarization curves for stainless steels in 30 wt% H2SO4at 25 C. Materials: (a) 290; (b) 293; (c) 302; (d) 306; (e) 294; (f) 296; (g) 311 and (h) 314.

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Fig. 3. Nyquist diagrams of AISI 304 stainless steels in 30 wt% H2SO4 at 25 C for 14 days. Materials: (a) 290; (b) 293; (c) 294 and (d) 296.

arc is attributed to charge transfer, associated with the effect of ionic double layer capacity. Fig. 3c and d shows the Nyquist diagrams as a function of the immersion time for materials 294 and 296. The Nyquist diagram was similar for both 294 and 290 stainless steels. However, the synergic effect of Cu and Sn produced changes in the diagrams of stainless steels with higher percentages of these elements, generating arcs that did not intersect with the x-axis at any point in the frequency interval. Fig. 4a and b shows the Nyquist diagrams as a function of immersion time for materials 302 and 306 (AISI 316). At a low Cu concentration the behaviour was similar to material 290. When Cu concentration increased to 2.479 wt% the arc widened to low frequencies after 3 days of immersion, without intersecting with the x-axis. After 3 days of immersion the corrosion behaviour was similar to material 302 but lower in intensity. Fig. 4c and d shows the Nyquist diagrams as a function of the immersion time for materials 311 and 314. The synergic effect of Cu and Sn produced changes in the diagrams. Both 311 and 314 stainless steels presented arcs that did not intersect with the x-axis and which widened into low frequencies. These arcs became smaller as immersion time increased. Tables 5 and 6 show the values of Rct (charge transfer resistance) and Cdl (double ionic layer capacity), deduced from the arcs at high and intermediate frequencies.

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Fig. 4. Nyquist diagrams of AISI 316 stainless steels in 30 wt% H2SO4 at 25 C for 14 days. Materials: (a) 302; (b) 306; (c) 311 and (d) 314.

Table 5 Charge transfer resistance (Rct) values for tested stainless steels in 30 wt% H2SO4 at 25 C Time (d)

1 3 6 14

Rct (X cm2) 290

293

302

306

294

296

311

314

9 6 5 3

4.5 · 103 1.7 · 103 6.7 · 102 3.9 · 102

23 8 6 5

6.9 · 105 5.2 · 105 5.9 · 103 3.7 · 103

8 4 6 3

1.2 · 106 1.3 · 106 1.5 · 106 5 · 105

2.3 · 106 1 · 106 6.5 · 105 2.2 · 105

2.1 · 106 1.6 · 106 6 · 105 2.1 · 105

296

311

314

31 28 24 71

22 49 77 2.3 · 102

13 17 47 1.3 · 102

Table 6 Capacitance (Cdl) values for tested stainless steels in 30 wt% H2SO4 at 25 C Time (d)

Cdl (lF/cm2) 290

1 3 6 14

293 4

1.3 · 10 4.7 · 104 1.3 · 105 1.1 · 105

302 2

1.9 · 10 2.7 · 102 1.3 · 103 3.8 · 103

306 4

1.6 · 10 4.9 · 104 7.8 · 104 9.3 · 104

3 4 3.6 · 102 5.7 · 102

294 4

1.1 · 10 8.6 · 104 2.5 · 105 1.4 · 105

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Stainless steels with low Cu and Sn contents showed small values of Rct that did not change significantly with immersion time. However, addition of higher percentages of Cu and Sn produced a sharp increase of the Rct value. The enormous capacitance values measured in stainless steels with low Cu content stand out from all the other values measured for higher Cu additions. This is logical if the electrode is porous, in which case the active surface could be 100 to 1000 times greater than the apparent surface. The porous morphology was probably due to an irregular corrosion attack or to a spongy copper deposit with a dendritic morphology; this would have formed on the corroded stainless steel surface (Fig. 5), giving rise to the formation of a porous electrode. Nevertheless, the Rct values were substantially increased by both Cu addition and the synergic effect of Cu and Sn. Stern-GearyÕs B constant was calculated from the Tafel slopes of the polarization curves. The icorr was obtained from Rct values for different immersion times from the expression icorr = B/Rct. Current density was converted to mass loss (W), expressed in mg/cm2 d, by applying FaradayÕs law and integrating the graphic of icorr versus time Z W ¼ KEW icorr dt; where K = 8.95 · 104 mg cm2/lA cm2 d, EW = alloy equivalent weight (considered dimensionless in these calculations) and icorr = corrosion current density in lA/cm2. Mass loss data after 1, 3, 6 and 14 days, evaluated by electrochemical impedance spectroscopy, reveal a clear tendency for corrosion rates to increase with immersion time after an initial induction period (Table 7). The equivalent circuit changed as a function of Cu and Sn concentration in the materials tested. Therefore, stainless steels with a high corrosion rate (materials 290, 294, 302) presented Rct values in the region of 5–25 X cm2. Besides the characteristic elements of the circuit -charge transfer resistance (Rct), double ionic layer

Fig. 5. Dendritic morphology of Cu deposit on the surface of AISI 304 (material 293) after 6 days immersed in 30 wt% H2SO4 at 50 C.

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Table 7 Mass loss versus immersion time for tested stainless steels in 30 wt% H2SO4 at 25 C Material

Mass loss (mg/cm2) 3 days

6 days

14 days

290 293 302 306

1 day 3.21 0.02 4.15 0.0002

144.85 0.37 64.11 0.001

625.43 1.31 161.34 0.135

2083.97 6.27 400.93 0.461

294 296 311 314

17.27 0.0001 0.0001 0.0001

140.61 0.0002 0.0003 0.0005

264.02 0.0008 0.0008 0.0028

645.40 0.0082 0.0037 0.0054

capacity (Cdl), ohmic resistance of electrolyte (Rs), the inductive contribution must be incorporated by including the L element (inductance) and the resistance R2 connected in series (Fig. 6a). Addition of higher percentages of Cu and Sn significantly reduced the corrosion rate because of the synergic effect Cu–Sn and Cu–Sn–Mo. In fact, Rct was very high and modified the equivalent circuit represented in Fig. 6b. In all cases, Rs was very small (0.5–1 X). 3.4. Morphology of corrosion products Fig. 7 shows an SEM analysis of AISI 316 stainless steel (material 306) surface morphology when immersed in 30 wt% H2SO4 at 50 C for 6 days. The material surface showed that the corrosion rate was related to a major general corrosion attack. X-ray mapping revealed the presence of Cu on the material surface. Fig. 8 shows XPS spectra of Cu 2p3/2 before and after the sulphuric immersion test for both AISI 304 and 316 stainless steels. The Cu signal in both 293 and 306 passivated spectra was weak, since copper concentration on the surface was less than 1 at%. Both 293 and 306 spectra presented two superimposed peaks, one at 932.2 eV corresponding to Cu0 and another at 934.1 eV corresponding to Cu2+; however, there was little Cu2+ on the surface of the material before the test, and its surface

Fig. 6. Equivalent circuit models for tested materials: (a) in active state and (b) in less active state.

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Fig. 7. Material 306 (AISI 316) after 6 days in 30 wt% H2SO4 at 50 C: (a) SEM morphology of corrosion attack and (b) Cu X-ray mapping.

Fig. 8. XPS spectra of Cu 2p3/2 for 293 and 306 materials before and after exposure to 30 wt% H2SO4 at 50 C.

concentration after the test was in the range 20–28 at%. Fig. 9 shows the Fe 2p3/2 spectra for materials 293 and 306 before and after aggressive immersion, with two overlapping peaks corresponding to Fe3+, one at 710.2 eV and another at

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Fig. 9. XPS spectra of Fe 2p3/2 for 293 and 306 materials before and after exposure to 30 wt% H2SO4 at 50 C.

712.5 eV. These peaks reflect oxides and sulphates of Fe3+. Fe0 was not detected on the surface of the material after immersion in sulphuric acid. This indicates that the corrosion layer was thicker than the initial passivated layer. Table 8 shows the XPS results of Cu and Fe elements for materials 290, 293, 302 and 306 after immersion tests. Table 9 shows the surface chemical composition of the same materials before and after exposure to sulphuric medium expressed as atomic percentages calculated from the XPS results without taking carbon and oxygen into account. Note the high Cu concentration on the material surface exposed to sulphuric acid. SEM, X-ray mapping and XPS studies confirmed the presence of copper metal and ferric oxide and sulphate on the material surfaces after the immersion tests. Both AISI 304 and AISI 316 stainless steels in contact with H2SO4 can present any of three kinds of electrochemical behaviour: active, passive or active–passive. Without Cu, these steels presented active behaviour and a high corrosion rate when immersed in 30 wt% H2SO4 at 50 C. Since copper reduced the hydrogen overpotential, addition of up to 3.1 wt%Cu in AISI 304 and up to 2.479 wt%Cu in AISI 316 favoured active–passive behaviour and a significant reduction of the corrosion rate. Corrosion behaviour in this zone was not stable and produced erratic corrosion rates. Slight changes in the environment can cause a shift towards active behaviour.

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Table 8 Cu and Fe XPS adjusted results for tested stainless steels Material

Species

290

Cu0 + Cu+ Cu2+ Fe3þ joxide

82.4 17.6 77.9

Fe3þ jsulphate

22.1

421.4

712.5

115.9

3.3

Cu0 + Cu+ Cu2+ Fe3þ joxide

54.5 47.5 58.6

3451.9 3125.7 973.8

932.3 934.0 710.2

1573.5 1035.1 268.1

2.0 2.8 3.3

293

Concentration (at.%)

Fe3þ jsulphate 302

Position (eV)

Height (a.u.)

Width (eV)

8001.1 1708.3 1489.0

932.2 934.2 710.2

3661.5 769.8 409.6

2.0 2.0 3.3

41.4

689.0

712.5

189.1

3.3

100 82.7

3668.7 5071.9

932.3 710.2

1639.2 1484.7

2.1 3.1

17.3

1058.5

712.5

291.8

3.3

Cu + Cu Cu2+ Fe3þ joxide

51.6 48.4 85.3

3092.5 2899.7 2678.7

932.1 934.2 710.4

1366.8 988.7 789.4

2.1 2.7 3.1

Fe3þ jsulphate

14.7

463.2

712.5

131.0

3.3

Cu0 + Cu+ Fe3þ joxide Fe3þ jsulphate

306

Area (a.u.)

0

+

Table 9 Surface chemical composition of tested stainless steels before and after immersion in 30 wt% H2SO4 at 50 C Material

Cu

Si

Ni

Cr

Before immersion tests

290 293 302 306

0.56 1.43 0.41 0.84

10.61 15.11 11.85 10.25

1.14 1.33 1.71 0.80

66.44 56.64 62.01 58.56

After immersion tests

290 293 302 306

23.64 13.73 15.61 28.8

53.89 48.97 15.77 9.81

0.41 0.29 6.78 3

5.62 2.15 3.61 2.9

S 0 0 0 0 12.69 30.51 20.2 23.41

Mo

Fe

0 0 4.80 7.10

21.25 25.49 19.22 22.45

0 0 16.61 19.54

3.75 4.35 21.42 12.54

In fact, there was a regular oscillation between active and passive behaviour. Behaviour at 25 C was preferentially passive [24]. Tin increased the hydrogen overpotential. Addition of Sn shifted the cathodic polarization curve to lower current densities, thus reducing the corrosion rate. The reduction in the corrosion rate was lower than with addition of Cu. Additions of Cu and Sn at higher concentrations than considered in this paper seem likely to increase corrosion resistance. That increase will depend on the whether Cu and Sn affect the process of material manufacturing negatively or positively (such levels of Cu and Sn can affect the hot ductility of stainless steels during fabrication). The authors propose a new mechanism: the material in contact with 30 wt% H2SO4 loses its passive layer, producing mainly Cr3+ ions (Fig. 10a). Once the passive layer

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Fig. 10. Corrosion mechanism proposed: (a) passive layer dissolution; (b) metal cation dissolution and (c) Cu reduction and formation of an oxide semi-protective layer.

has been dissolved, other elements of the metallic matrix can be incorporated into the solution in the form of cations, such as Cu2+ and Fe3+ (Fig. 10b). These cations have high reduction potentials (Fe3+ + 1 e ! Fe2+, E = 0.771 V, and Cu2+ + 2 e ! Cu0, E = 0.34 V) and are strong oxidizing agents. In this way, Cu and Fe are reduced, plating the surface as Cu0, CuSO4, Fe2O3 and FeSO4. Copper plating considerably reduces hydrogen overpotential and favours partial regeneration of the passive layer. The layer of oxidized products is composed mainly of Cr2O3, Cu0, CuSO4, Fe2O3 and FeSO4 (Fig. 10c). The deposition of these elements and compounds drastically reduces the corrosion rate of these materials when immersed in 30 wt% H2SO4 at 50 C and tends to inhibit the process at room temperature.

4. Conclusions 1. The corrosion resistance of AISI 304 stainless steels in 30 wt% H2SO4 at 50 C increased sharply with the addition of Cu as an alloy element. This effect was more significant in AISI 316 stainless steels. However, the corrosion rate was still

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high. When the test temperature was reduced to 25 C, corrosion resistance was very high and the corrosion rate was not a significant damage factor. 2. Sn addition reduced the corrosion rate of AISI 304 stainless steels, but less than Cu. The effect of Sn was insignificant in AISI 316 stainless steels. 3. The synergic effect of Cu and Sn was similar to the effect of Cu added solely in AISI 304 stainless steels, and also, but to a lesser extent, in AISI 316 stainless steels. 4. Both AISI 304 and AISI 316 stainless steels in contact with H2SO4 can present any of three kinds of electrochemical behaviour: active, passive or active–passive. Additions of Cu to AISI 304 and AISI 316 favoured active–passive behaviour and a significant reduction of the corrosion rate. Additions of Sn reduced the corrosion rate in lower magnitude than Cu addition. The synergic effect of Cu and Sn seems likely to increase corrosion resistance as Cu does. Acknowledgements The authors wish to thank both ACERINOX S:A for the supply of stainless steels and the MCYT for the financial support given to this work (Project MAT200304931-C02-01). References [1] I. LeMay, L.Mc.d. Schetky, Copper in Iron and Steel, John Wiley and Sons Inc., New York, 1982. [2] B.M. Gonzalez, C.S.B. Castro, V.T.L. Buono, J.M.C. Vilela, M.S. Andrade, J.M.D. Moraes, M.J. Mantel, Mater. Sci. Eng. A 343 (1-2) (2003) 51. [3] A. Kanni Raj, K.A. Padmanabhan, Trans. Indian Inst. Metals 51 (1998) 201. [4] E.A. Lizovs, Corrosion 22 (1966) 279. [5] M. Seo, G. Hultquist, C. Leygraf, N. Sato, Corros. Sci. 26 (11) (1986) 949. [6] Y. Jiangnan, W. Lichang, S. Wenhao, Corros. Sci. 33 (6) (1992) 851. [7] K. Takizawa, Y. Nakayama, K. Kurokawa, H. Imai, Corros. Eng. (Japan) 37 (12) (1988) 657. [8] E. Otero Huerta, Corrosio´n y degradacio´n de materials, Ed. Sı´ntesis, 1997. [9] Metals Handbook. ASM, Ninth edition, vol. 13, 1994. [10] M. Verneau, J.P. Audovard, J. Charles, in: Proceedings of the International Congress Stainless Steels Õ96, Du¨sseldorf-Neuss, 1996, pp. 163–170. [11] S. El Hajjaj, L. Aries, P. Audouard, F. Dabosi, Corros. Sci. 37 (6) (1995) 927. [12] Y. Fujiwara, T. Tohge, R. Nemoto, in: Proceeding of International Conference on Stainless Steels, Chiba, Japan, ISIJ, 1991, pp. 53–57. [13] A.A. Hermas, K. Ogura, T. Adachi, Electrochem. Acta 40 (7) (1995) 837. [14] A. Belfrouh, C. Masson, D. Vouagner, A.M. De Becdelievre, N.S. Prakash, J.P. Audouard, Corros. Sci. 38 (10) (1996) 1639. [15] A. Yamamoto, T. Ashiura, E. Kamisaka, Boshoku Gijutsu 35 (1986) 448. [16] T. Moroishi, Y. Tarutani, J. Murayama, T. Usuki, in: Proceeding of the 28th Corrosion Discussion Meeting, Japan Soc. Corros. Eng., 1981, p. 133. [17] N.D. Greene, C.R. Bishop, M. Stern, J. Electron. Soc. 108 (1961) 836. [18] B.E. Wilde, N.D. Greene, Corrosion 25 (1969) 300. [19] N.D. Greene, B.E. Wilde, Corrosion 26 (1970) 533. [20] T. Ramchandran, K. Roesch, H.J. Engell, Arch. Eisenhuttenwsen 32 (1961) 173. [21] K. Osozawa, Boshoku Gijutsu 20 (5) (1971) 221.

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