Inﬂuence of Cu and Sn content in the corrosion of AISI 304 ...webs.ucm.es/info/biohidro/Publicaciones del Grupo/Corrosion Scienc… · DC electrochemical measurements were performed

Corrosion Science 48 (2006) 1075–1092

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

A. Pardo *, M.C. Merino, M. Carboneras, F. Viejo,R. Arrabal, J. Munoz

Departamento de Ciencia de Materiales, Facultad de Quımica, Universidad Complutense,

28040 Madrid, Spain

Received 22 March 2004; accepted 13 May 2005Available online 20 July 2005

Abstract

This paper addresses the influence of Cu and Sn addition on the corrosion resistance ofAISI 304 and 316 stainless steels in 30 wt% H2SO4 at 25 and 50 �C. The corrosion processwas evaluated by gravimetric tests, DC measurements and electrochemical impedance spec-troscopy (EIS). The corrosion products were analysed by SEM, X-ray mapping and XPSbefore and after accelerated tests. The behaviour of both AISI 304 and 316 stainless steelsin sulphuric acid solution was greatly improved by increasing Cu concentration and the syn-ergic effect of Cu and Sn. Addition of Sn increased corrosion resistance, but less than additionof 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 oreswith iron scrap as raw material. Usually, iron scraps are contaminated with Cu

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

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

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1076 A. Pardo et al. / Corrosion Science 48 (2006) 1075–1092

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 nega-tively affect hot workability during manufacturing.

With this in mind, if Cu and Sn additions were not too much detrimental inbehaviour of stainless steels, these alloys could be fabricated from recycled iron scrapwithout the need to reduce copper and tin to below the limits established by stan-dards in the steelmaking process.

The use of copper as an alloying element in austenitic stainless steels can be jus-tified as follows: (a) copper stabilizes austenite [1], making it possible to reduce thenickel content in the alloy. This entails a significant economic saving since nickelcontent is high, and nickel is expensive; (b) copper is a good stabilizer against mar-tensitic transformation [2,3], giving rise to more stable austenite; (c) Copper increasesgeneral corrosion resistance and helps to improve steel corrosion resistance in sul-phuric acid [4–7].

The behaviour of stainless steels in sulphuric acid is complex, since these are nei-ther strong oxidizing nor strong reducing solutions. In this sense, small amounts ofmetal salts or organic substances in solution are enough to transform stainless steelsfrom 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 inH2SO4 [9]. Therefore, in addressing stainless steel resistance in sulphuric acid solu-tions, we need to specify the exact composition of the corrosive medium.

Vernau et al. [10] have reported that copper always increases stainless steel corro-sion resistance in acid media, although its influence depends on the oxidizingstrength of the solution. Many other researchers have studied the beneficial effectof 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 ele-mental copper deposition on the steel surface immersed in the corrosive medium.The role of copper in stainless steel passivation is complex and leads to apparentlyincongruent results, which could be related to the synergetic effect between differentelements in the alloy. For instance, Wilde and Greene [17–19] noted that copper, dueto its low hydrogen overpotential, has a beneficial effect on stainless steel passivationin non-oxidizing acid mediums. Similarly, Ramchandran et al. [20] showed the posi-tive 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 lessthan 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 Snaddition and the synergy of Sn and Cu. Osozawa [21] and Takizawa et al. [22] ob-served positive synergy of Cu and Sn in austenitic stainless steel corrosion resistancein both diluted sulphuric and organic-chlorinated acid mediums. One possible mech-anism suggested by these authors is preferential dissolution of Cu and Sn, favouringthe 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 resistanceof two austenitic stainless steels (AISI 304 and 316) in 30 wt% H2SO4.

A. Pardo et al. / Corrosion Science 48 (2006) 1075–1092 1077

2. Experimental procedure

The test materials were AISI 304 and 316 austenitic stainless steels with differentconcentrations of Cu and Sn. Chemical compositions of these materials are given inTables 1 and 2. The alloys chosen were fabricated in ingots of 40 kg in a PfeifferVSG030 vacuum induction furnace. Vacuum was applied during the first part ofthe melting process in order to remove oxygen. No vacuum was applied in fine-tun-ing additions and casting operations at the end of the process, which were carried outin a 1 bar argon atmosphere. The ingots were hot forged into 4 mm plates and coldrolled into 2.5 mm sheets.

Rectangular samples (50 · 25 · 2.5 mm) were used for the corrosion tests. Beforethe 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 inHNO3 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 samplearea was measured and weighed to a precision of 0.01 mg. Upon completion of theexperiment, the sample was extracted, cleaned with water, dried at 105 �C for 30 minin a furnace and then weighed again at room temperature. The subsequent loss ofmass per unit of surface area was then calculated for the different test times. The testswere performed in duplicate to guarantee the reliability of the results.

DC electrochemical measurements were performed using rectangular sampleswith a surface area of approximately 14 cm2 exposed to the test medium. A three-electrode cell was used for electrochemical measurements. The working electrodewas the test material. The counter and reference electrodes were graphite and Ag/AgCl respectively. Polarization measurements were carried out at a scan rate of0.1 mV/s, from �100 mV to +100 mV with respect to the corrosion potential (Ecorr).

Table 1Chemical composition of AISI 304 stainless steels

Material Elements (wt%)

Cu Sn C Si Mn Ni Cr P S Mo N

290 0.500 0.009 0.060 0.410 1.070 8.130 18.210 0.028 0.002 0.100 0.0288291 1.010 0.008 0.052 0.390 1.730 8.020 18.070 0.027 0.001 0.100 0.0400292 1.980 0.009 0.053 0.410 1.420 8.020 17.980 0.027 0.001 0.100 0.0337293 3.100 0.008 0.050 0.420 1.670 7.980 17.630 0.029 0.001 0.100 0.0423

294 0.560 0.023 0.053 0.430 1.520 8.050 18.110 0.028 0.001 0.100 0.0404295 0.991 0.073 0.062 0.411 1.750 8.012 17.932 0.028 0.001 0.102 0.0421296 1.997 0.113 0.043 0.335 1.560 7.800 17.648 0.030 0.001 0.311 0.0360

297 0.280 0.012 0.046 0.344 1.599 8.056 18.101 0.030 0.001 0.320 0.0393298 0.267 0.049 0.042 0.308 1.668 8.035 18.022 0.030 0.001 0.323 0.0424299 0.268 0.100 0.048 0.330 1.719 8.075 17.986 0.030 0.001 0.323 0.0482300 0.267 0.142 0.045 0.355 1.64 8.071 18.091 0.029 0.002 0.328 0.0402301 0.262 0.188 0.044 0.354 1.487 8.094 17.978 0.030 0.002 0.329 0.0381

Table 2Chemical composition of AISI 316 stainless steels

Material Elements (wt%)

Cu Sn C Si Mn Ni Cr P S Mo Ti Nb Co N

302 0.500 0.012 0.022 0.380 1.562 10.326 17.345 0.033 0. 2.151 0.010 0.009 0.119 0.0365303 0.980 0.012 0.034 0.402 1.438 10.228 17.325 0.032 0. 2.128 0.007 0.009 0.120 0.0345304 1.474 0.011 0.041 0.343 1.463 10.036 17.105 0.032 0. 2.117 0.006 0.007 0.123 0.0352305 1.988 0.011 0.037 0.366 1.381 10.170 17.148 0.034 0. 2.107 0.005 0.009 0.119 0.0367306 2.479 0.011 0.041 0.369 1.505 10.091 17.017 0.034 0. 2.083 0.006 0.009 0.116 0.0392

307 0.404 0.041 0.042 0.375 1.593 10.317 17.348 0.032 0. 2.149 0.004 0.009 0.117 0.0382308 0.401 0.091 0.038 0.384 1.538 10.207 17.187 0.033 0. 2.157 0.004 0.008 0.123 0.0362309 0.401 0.132 0.033 0.368 1.555 10.214 17.152 0.033 0. 2.140 0.004 0.008 0.125 0.0366310 0.402 0.185 0.039 0.378 1.555 10.164 17.154 0.031 0. 2.147 0.004 0.007 0.123 0.0370

311 0.734 0.057 0.048 0.378 1.632 10.103 17.194 0.033 0. 2.137 0.004 0.007 0.123 0.0538312 0.988 0.078 0.034 0.376 1.482 10.075 17.171 0.034 0. 2.140 0.004 0.007 0.125 0.0330313 1.483 0.090 0.045 0.376 1.531 9.987 16.958 0.032 0. 2.120 0.003 0.007 0.123 0.0420314 1.990 0.114 0.028 0.376 1.532 9.905 16.956 0.032 0. 2.111 0.003 0.007 0.125 0.0327

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Electrochemical impedance measurements were performed using an AUTOLABmodel 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 was2 cm2.

The morphology of the corrosion products was analysed by SEM. Stainless steelswere examined by X-ray mapping and X-ray photoelectron spectroscopy (XPS) be-fore and after gravimetric tests. XPS spectra were taken in an ultra high vacuumchamber (UHV) equipped with an energy electron analyser (VG 100 AX) (pressurearound 10�9 Torr, Mg Ka radiation, 15 kV and 20 mA). Before analysis, the sampleswere degassed overnight (10�7 Torr) in the pre-treatment chamber and then placedin the analysis chamber. After subtraction of a Shirley-type non-linear baseline,the spectra were decomposed using a commercial fitting program (VGX 900) witha Gaussian/Lorentzian ratio of 85/15. Binding energies are referenced to theC–(C, H) component of the C(1s) adventitious carbon fixed at 284.6 eV. Atomicratios were calculated from relative intensities corrected by the elemental sensitivityfactor 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 andCu + 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 dilutedH2SO4. This effect was more significant when the test temperature was reduced from50 to 25 �C. Addition of Sn reduced the corrosion rate of AISI 304 stainless steels insulphuric acid. Nevertheless, the corrosion rate was high at all Sn concentrationstested, indicating that Sn provided less protection than Cu. Additionally, the corro-sion 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.


significantly increased the corrosion resistance of AISI 304. The corrosion resistancedue to the synergic effect of Cu and Sn was similar to that of materials with only Cuaddition.

Fig. 1b shows the variation of the corrosion rate of AISI 316 stainless steels withincreasing Cu, Sn and Cu + Sn contents in 30 wt% H2SO4 at 25 and 50 �C. The cor-rosion 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 similarto the effect in AISI 304 stainless steels, but less intense. Addition of Sn did not affectgeneral corrosion of AISI 316 in sulphuric acid, but the corrosion rate was high at allSn concentrations tested. Sn provided less protection than Cu. The corrosion rateincreased drastically with temperature.

Tables 3 and 4 show the kinetic laws calculated for all tests from the experimentaldata. 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 theimmersion 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 cor-rosion rate from 54.89 to 2.51 mg/cm2 d at 25 �C and from 350.73 to 23.94 mg/cm2 dat 50 �C. The synergic effect of Cu and Sn presented a similar pattern. The influenceof 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 inAISI 316 were lower. Addition of 2.479 wt%Cu reduced the corrosion rate from0.26 to 0.05 mg/cm2 d at 25 �C and from 59.60 to 9.00 mg/cm2 d at 50 �C. Additionof 0.185 wt%Sn reduced the corrosion rate from 6.30 to 4.96 mg/cm2 d at 25 �C andfrom 56.80 to 47.96 mg/cm2 d at 50 �C. In the AISI 316 stainless steel, copper addi-tion practically inhibited corrosion in stainless steel immersed in 30 wt% H2SO4 atroom temperature and open to air.

Table 3Kinetic 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 y = 54.89t � 67.96 2 6 t 6 6 (r2 = 0.995) y = 350.73t + 22.41 0 6 t 6 2 (r2 = 0.988)291 y = 30.05t � 53.10 2 6 t 6 6 (r2 = 0.994) y = 214.24t � 5.27 0 6 t 6 3 (r2 = 0.999)292 y = 6.27t � 9.78 2 6 t 6 6 (r2 = 0.986) y = 67.31t � 2.03 0 6 t 6 6 (r2 = 1)

293 y = 2.51t � 2.99 2 6 t 6 6 (r2 = 0.985) y = 23.94t � 0.43 0 6 t 6 6 (r2 = 0.997)294 y = 49.25t � 56.90 1 6 t 6 6 (r2 = 0.994) y = 304.33t � 7.71 0 6 t 6 2 (r2 = 0.998)295 y = 19.91t � 22.80 1 6 t 6 6 (r2 = 0.993) y = 135.81t � 4.53 0 6 t 6 4 (r2 = 0.999)296 y = 3.44t � 4.10 1 6 t 6 6 (r2 = 0.993) y = 23.91t � 1.89 0 6 t 6 6 (r2 = 0.999)

297 y = 58.43t � 57.70 1 6 t 6 6 (r2 = 0.999) y = 600.13t � 1778.8 3 6 t 6 4 (r2 = 1)298 y = 55.22t � 60.03 1 6 t 6 6 (r2 = 0.999) y = 478.76t � 751.30 2 6 t 6 3 (r2 = 1)299 y = 56.70t � 77.83 2 6 t 6 6 (r2 = 0.988) y = 281.96t + 21.56 0 6 t 6 2 (r2 = 0.983)300 y = 49.25t � 81.76 2 6 t 6 6 (r2 = 0.972) y = 226.31t � 0.24 0 6 t 6 2 (r2 = 1)301 y = 50.06t � 82.15 2 6 t 6 6 (r2 = 0.988) y = 210.21t � 229.31 2 6 t 6 4 (r2 = 0.996)

Table 4Kinetic 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 y = 0.26t � 0.54 2 6 t 6 6 (r2 = 0.974) y = 59.60t + 0.82 0 6 t 6 6 (r2 = 0.998)303 y = 0.18t � 0.41 2 6 t 6 6 (r2 = 0.978) y = 20.47t � 3.05 0 6 t 6 6 (r2 = 0.996)304 y = 0.18t � 0.34 2 6 t 6 6 (r2 = 0.999) y = 15.79t � 0.06 0 6 t 6 6 (r2 = 0.999)305 y = 0.10t � 0.18 2 6 t 6 6 (r2 = 0.998) y = 11.11t � 1.17 0 6 t 6 6 (r2 = 0.997)306 y = 0.05t � 0.04 3 6 t 6 6 (r2 = 0.987) y = 9.00t � 0.99 0 6 t 6 6 (r2 = 0.999)

307 y = 6.30t � 13.08 2 6 t 6 6 (r2 = 0.990) y = 56.80t + 6.17 0 6 t 6 6 (r2 = 0.999)308 y = 5.06t � 10.75 2 6 t 6 6 (r2 = 0.985) y = 56.78t + 6.39 0 6 t 6 6 (r2 = 0.999)309 y = 5.83t � 12.74 2 6 t 6 6 (r2 = 0.982) y = 47.26t + 1.42 0 6 t 6 6 (r2 = 1)310 y = 4.96t � 9.36 2 6 t 6 6 (r2 = 0.996) y = 47.96t � 0.08 0 6 t 6 6 (r2 = 1)

311 y = 3.48t � 7.17 2 6 t 6 6 (r2 = 0.994) y = 28.33t + 3.81 0 6 t 6 6 (r2 = 0.997)312 y = 2.53t � 5.52 2 6 t 6 6 (r2 = 0.988) y = 17.33t � 1.11 0 6 t 6 6 (r2 = 0.998)313 y = 1.89t � 2.46 2 6 t 6 6 (r2 = 0.986) y = 11.41t � 3.48 0 6 t 6 6 (r2 = 0.992)314 y = 0.86t � 2.45 3 6 t 6 6 (r2 = 0.992) y = 8.94t � 0.26 0 6 t 6 6 (r2 = 1)


3.2. DC electrochemical results

The repassivation effect of Cu and the synergic effect of Cu and Sn were checkedby carrying out consecutive polarization tests on the same sample at different immer-sion times in 30 wt% H2SO4. Between measurements the system evolved freely, sothat each polarization measurement indicates the degree of repassivation due tothe presence of Cu on surface material. Fig. 2a shows the polarization curvesof the AISI 304 stainless steel without Cu (material 290). After 6 days of immersionthe 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 immersionthis material tended to revert to its original behaviour, which suggests that the cop-per dissolved was deposited on the surface, causing repassivation of the material(Fig. 2b). However, AISI 316 presented high corrosion resistance when immersedin 30 wt% H2SO4 at 25 �C in all cases (Fig. 2c and d), probably due to the synergiceffect of Mo and Cu. The synergic effect of Cu and Sn was comparable to the effect ofadding 3 wt%Cu in AISI 304 (Fig. 2e and f). The synergic effect of Cu and Sn wasless 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 wassmaller 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 316stainless steels in 30 wt% H2SO4 at 25 �C was evaluated by electrochemical imped-ance spectroscopy (EIS). Fig. 3a and b shows the Nyquist diagrams as a functionof the immersion time for materials 290 and 293. A first arc was observed at highand intermediate frequencies followed by a second arc or ill-defined tail. The first

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.


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 layercapacity. Fig. 3c and d shows the Nyquist diagrams as a function of the immersiontime for materials 294 and 296. The Nyquist diagram was similar for both 294 and290 stainless steels. However, the synergic effect of Cu and Sn produced changes inthe diagrams of stainless steels with higher percentages of these elements, generatingarcs 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 formaterials 302 and 306 (AISI 316). At a low Cu concentration the behaviour was sim-ilar to material 290. When Cu concentration increased to 2.479 wt% the arc widenedto 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 302but lower in intensity. Fig. 4c and d shows the Nyquist diagrams as a function ofthe immersion time for materials 311 and 314. The synergic effect of Cu and Sn pro-duced changes in the diagrams. Both 311 and 314 stainless steels presented arcs thatdid not intersect with the x-axis and which widened into low frequencies. These arcsbecame smaller as immersion time increased.

Tables 5 and 6 show the values of Rct (charge transfer resistance) and Cdl (doubleionic layer capacity), deduced from the arcs at high and intermediate frequencies.

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

Time (d) Rct (X cm2)

290 293 302 306 294 296 311 314

1 9 4.5 · 103 23 6.9 · 105 8 1.2 · 106 2.3 · 106 2.1 · 106

3 6 1.7 · 103 8 5.2 · 105 4 1.3 · 106 1 · 106 1.6 · 106

6 5 6.7 · 102 6 5.9 · 103 6 1.5 · 106 6.5 · 105 6 · 105

14 3 3.9 · 102 5 3.7 · 103 3 5 · 105 2.2 · 105 2.1 · 105

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

Time (d) Cdl (lF/cm2)

290 293 302 306 294 296 311 314

1 1.3 · 104 1.9 · 102 1.6 · 104 3 1.1 · 104 31 22 133 4.7 · 104 2.7 · 102 4.9 · 104 4 8.6 · 104 28 49 176 1.3 · 105 1.3 · 103 7.8 · 104 3.6 · 102 2.5 · 105 24 77 4714 1.1 · 105 3.8 · 103 9.3 · 104 5.7 · 102 1.4 · 105 71 2.3 · 102 1.3 · 102



Stainless steels with low Cu and Sn contents showed small values of Rct that did notchange significantly with immersion time. However, addition of higher percentagesof Cu and Sn produced a sharp increase of the Rct value. The enormous capacitancevalues measured in stainless steels with low Cu content stand out from all the othervalues measured for higher Cu additions. This is logical if the electrode is porous, inwhich case the active surface could be 100 to 1000 times greater than the apparentsurface. The porous morphology was probably due to an irregular corrosion attackor to a spongy copper deposit with a dendritic morphology; this would have formedon the corroded stainless steel surface (Fig. 5), giving rise to the formation of a por-ous electrode. Nevertheless, the Rct values were substantially increased by both Cuaddition and the synergic effect of Cu and Sn.

Stern-Geary�s B constant was calculated from the Tafel slopes of the polarizationcurves. The icorr was obtained from Rct values for different immersion times from theexpression icorr = B/Rct. Current density was converted to mass loss (W), expressedin mg/cm2 d, by applying Faraday�s law and integrating the graphic of icorr versustime

W ¼ KEWZ

icorr dt;

where K = 8.95 · 10�4 mg cm2/lA cm2 d, EW = alloy equivalent weight (considereddimensionless in these calculations) and icorr = corrosion current density in lA/cm2.Mass loss data after 1, 3, 6 and 14 days, evaluated by electrochemical impedancespectroscopy, reveal a clear tendency for corrosion rates to increase with immersiontime after an initial induction period (Table 7).

The equivalent circuit changed as a function of Cu and Sn concentration in thematerials tested. Therefore, stainless steels with a high corrosion rate (materials290, 294, 302) presented Rct values in the region of 5–25 X cm2. Besides the charac-teristic 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 daysimmersed in 30 wt% H2SO4 at 50 �C.

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

Material Mass loss (mg/cm2)

1 day 3 days 6 days 14 days

290 3.21 144.85 625.43 2083.97293 0.02 0.37 1.31 6.27302 4.15 64.11 161.34 400.93306 0.0002 0.001 0.135 0.461

294 17.27 140.61 264.02 645.40296 0.0001 0.0002 0.0008 0.0082311 0.0001 0.0003 0.0008 0.0037314 0.0001 0.0005 0.0028 0.0054


capacity (Cdl), ohmic resistance of electrolyte (Rs), the inductive contribution mustbe incorporated by including the L element (inductance) and the resistance R2 con-nected in series (Fig. 6a). Addition of higher percentages of Cu and Sn significantlyreduced the corrosion rate because of the synergic effect Cu–Sn and Cu–Sn–Mo. Infact, Rct was very high and modified the equivalent circuit represented in Fig. 6b. Inall 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) surfacemorphology when immersed in 30 wt% H2SO4 at 50 �C for 6 days. The material sur-face 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 immersiontest for both AISI 304 and 316 stainless steels. The Cu signal in both 293 and 306passivated spectra was weak, since copper concentration on the surface was less than1 at%. Both 293 and 306 spectra presented two superimposed peaks, one at 932.2 eVcorresponding 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.

Fig. 7. Material 306 (AISI 316) after 6 days in 30 wt% H2SO4 at 50 �C: (a) SEM morphology of corrosionattack 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 at50 �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, withtwo overlapping peaks corresponding to Fe3+, one at 710.2 eV and another at

Fig. 9. XPS spectra of Fe 2p3/2 for 293 and 306 materials before and after exposure to 30 wt% H2SO4 at50 �C.


712.5 eV. These peaks reflect oxides and sulphates of Fe3+. Fe0 was not detected onthe surface of the material after immersion in sulphuric acid. This indicates that thecorrosion layer was thicker than the initial passivated layer. Table 8 shows the XPSresults of Cu and Fe elements for materials 290, 293, 302 and 306 after immersiontests. Table 9 shows the surface chemical composition of the same materials beforeand after exposure to sulphuric medium expressed as atomic percentages calculatedfrom the XPS results without taking carbon and oxygen into account. Note the highCu concentration on the material surface exposed to sulphuric acid.

SEM, X-ray mapping and XPS studies confirmed the presence of copper metaland 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 presentany of three kinds of electrochemical behaviour: active, passive or active–passive.Without Cu, these steels presented active behaviour and a high corrosion rate whenimmersed in 30 wt% H2SO4 at 50 �C. Since copper reduced the hydrogen overpoten-tial, addition of up to 3.1 wt%Cu in AISI 304 and up to 2.479 wt%Cu in AISI 316favoured active–passive behaviour and a significant reduction of the corrosion rate.Corrosion behaviour in this zone was not stable and produced erratic corrosionrates. Slight changes in the environment can cause a shift towards active behaviour.

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

Material Species Concentration(at.%)

Area(a.u.)

Position(eV)

Height(a.u.)

Width(eV)

290 Cu0 + Cu+ 82.4 8001.1 932.2 3661.5 2.0Cu2+ 17.6 1708.3 934.2 769.8 2.0Fe3þjoxide 77.9 1489.0 710.2 409.6 3.3

Fe3þjsulphate 22.1 421.4 712.5 115.9 3.3


Fe3þjsulphate 41.4 689.0 712.5 189.1 3.3

302 Cu0 + Cu+ 100 3668.7 932.3 1639.2 2.1Fe3þjoxide 82.7 5071.9 710.2 1484.7 3.1

Fe3þjsulphate 17.3 1058.5 712.5 291.8 3.3


Fe3þjsulphate 14.7 463.2 712.5 131.0 3.3

Table 9Surface chemical composition of tested stainless steels before and after immersion in 30 wt% H2SO4 at50 �C

Material Cu Si Ni Cr S Mo Fe

Before immersion tests 290 0.56 10.61 1.14 66.44 0 0 21.25293 1.43 15.11 1.33 56.64 0 0 25.49302 0.41 11.85 1.71 62.01 0 4.80 19.22306 0.84 10.25 0.80 58.56 0 7.10 22.45

After immersion tests 290 23.64 53.89 0.41 5.62 12.69 0 3.75293 13.73 48.97 0.29 2.15 30.51 0 4.35302 15.61 15.77 6.78 3.61 20.2 16.61 21.42306 28.8 9.81 3 2.9 23.41 19.54 12.54


In fact, there was a regular oscillation between active and passive behaviour. Behav-iour at 25 �C was preferentially passive [24].

Tin increased the hydrogen overpotential. Addition of Sn shifted the cathodicpolarization curve to lower current densities, thus reducing the corrosion rate. Thereduction in the corrosion rate was lower than with addition of Cu.

Additions of Cu and Sn at higher concentrations than considered in this paperseem likely to increase corrosion resistance. That increase will depend on the whetherCu and Sn affect the process of material manufacturing negatively or positively (suchlevels 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

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 thesolution in the form of cations, such as Cu2+ and Fe3+ (Fig. 10b). These cations havehigh 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 considerablyreduces hydrogen overpotential and favours partial regeneration of the passive layer.The layer of oxidized products is composed mainly of Cr2O3, Cu0, CuSO4, Fe2O3 andFeSO4 (Fig. 10c). The deposition of these elements and compounds drastically re-duces the corrosion rate of these materials when immersed in 30 wt% H2SO4 at50 �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 wasmore significant in AISI 316 stainless steels. However, the corrosion rate was still


high. When the test temperature was reduced to 25 �C, corrosion resistance wasvery 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 thanCu. 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 inAISI 304 stainless steels, and also, but to a lesser extent, in AISI 316 stainlesssteels.

4. Both AISI 304 and AISI 316 stainless steels in contact with H2SO4 can presentany of three kinds of electrochemical behaviour: active, passive or active–passive.Additions of Cu to AISI 304 and AISI 316 favoured active–passive behaviour anda significant reduction of the corrosion rate. Additions of Sn reduced the corro-sion rate in lower magnitude than Cu addition. The synergic effect of Cu andSn seems likely to increase corrosion resistance as Cu does.

Acknowledgements

The authors wish to thank both ACERINOX S:A for the supply of stainless steelsand the MCYT for the financial support given to this work (Project MAT2003-04931-C02-01).

References

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Inﬂuence of Cu and Sn content in the corrosion of AISI 304 ...webs.ucm.es/info/biohidro/Publicaciones del Grupo/Corrosion Scienc… · DC electrochemical measurements were performed