Chromium–palladium films on 316L stainless steel by pulse electrodeposition and their corrosion resistance in hot sulfuric acid solutions

Corrosion Science 53 (2011) 3788–3795

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Chromium–palladium films on 316L stainless steel by pulse electrodeposition and their corrosion resistance in hot sulfuric acid solutions Liang Xu, Yu Zuo ⇑, Junlei Tang, Yuming Tang, Pengfei Ju School of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 6 May 2011 Accepted 16 July 2011 Available online 23 July 2011 Keywords: A. Metal coatings A. Stainless steel B. Weight loss B. XPS C. Acid corrosion C. Passivity

a b s t r a c t Chromium–palladium alloy films with good adhesive strength and higher micro-hardness have been deposited on 316L stainless steel by pulse electroplating. The films are composed mainly of chromium and palladium crystallites in the metallic state, with grain sizes less than 100 nm. On the film surface Cr(OH)3 and Cr2O3 are present. The co-deposited Cr and Pd in the films show a synergetic effect on passivation. In boiling 20 wt.% H2SO4 solution, boiling acetic–formic acid mixture, and simulated PEM fuel cells environment, the Cr–Pd-plated 316L steel shows excellent corrosion resistance. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction It is well known that the corrosion resistance of stainless steels is due to the passive film on the surface. In non-oxidizing environments such as boiling dilute sulfuric acid solutions or boiling formic–acetic acid media where the passive film cannot be stably established on the surface, stainless steels would be severely corroded. A recent example is the application of stainless steels as bipolar plates in proton exchange membrane (PEM) fuel cells [1]. The environments are similar to a dilute sulfuric acid solution at or above 70 °C. The corrosion products on the surface would increase the interfacial contact resistance and poison the catalysts in the polymeric membrane, thus decreasing the power output of the fuel cell [2]. A small amount of noble metals in stainless steels may effectively increase the potential and promote passivation. The effect of additions of 0.14%, 0.22%, and 0.28% of ruthenium on the passivation of 22% Cr–9% Ni–3% Mo stainless steel in 3.5% NaCl solution has been studied by Sherif et al. [3]. The presence of Ru passivates the alloy against general and pitting corrosion through reducing the anodic and cathodic currents as well as shifting the corrosion and pitting potentials in the positive direction. Palladium plating on porous stainless steels has been studied by many authors [4–8]. However, most of the studies were aimed to prepare catalytic membrane reactors, since palladium film is permeable to hydrogen and catalytically active to many hydrogen-involved

⇑ Corresponding author. Tel./fax: +86 10 64423795. E-mail address: [email protected] (Y. Zuo). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.07.029

reactions. In our previous studies [9,10], palladium deposition was carried out on stainless steels by electroplating, electroless plating or brush plating, and the corrosion resistance of the steels was significantly improved in non-oxidizing corrosive environments, such as boiling 20 wt.% H2SO4 solution and acetic acid–formic acid mixtures because of the promoted passivation of stainless steels by the palladium film. However, it is obvious that the industrial applications of palladium plating on stainless steels would be limited by the high price of palladium. Wen et al. [11] prepared a Cr–Pd alloy by mixing metallic chromium powders with palladium powders in a vessel at high temperatures above 600 °C. The material exhibited excellent corrosion resistance to organic and inorganic acids, even under high temperature and high acid concentration conditions. A Cr–Pd composite film was reported [12] by electroplating a Pd film on the surface of a Cr film. The corrosion potential of the film in H2SO4 solution increased because of the presence of Pd element, which promoted the stability of the passive film. However, these methods are relatively expensive and difficult to apply in industry. It is known that the co-deposition of Pd and Cr is difficult because of the large difference between the standard electrode potentials of the two metals [13]. In order to co-deposit Cr–Pd alloy films effectively, appropriate complexing agents are needed to bring the deposition potentials of the alloying metals closer. In this study, based on a trivalent chromium electroplating bath, Cr–Pd alloy films are deposited on 316L stainless steel by pulse plating. With the Cr–Pd films, the corrosion resistance of the stainless steel is significantly improved, at a much lower cost than that of pure Pd plating. The composition, morphology, and properties of the deposited Cr–Pd alloy films are studied.

L. Xu et al. / Corrosion Science 53 (2011) 3788–3795

2. Experimental methods The test material was rolled commercial 316L stainless steel (produced by Shanxi Taigang Stainless Steel Co., Ltd.) which was solution annealed and quenched, with a thickness of 2 mm. The steel composition (wt.%) is as follows: Cr 16.80%, Ni 13.50%, C 0.02%, Mn 1.40%, Si 0.32%, P 0.017%, S 0.014%, Mo 2.30%, and the remainder is Fe. The stainless steel plates (50 mm  20 mm) were abraded on both sides with 1000 # SiC abrasive papers, degreased first in alcohol then in a basic solution (Na2CO3: 40–60 g L1, NaOH: 20–30 g L1, Na3PO412H2O: 40–60 g L1, and C8H17C6H4O(CH2CH2O)10H (alkylphenol ethoxylate) as a surfactant: 1–5 ml L1) at 70–80 °C for 30 min, followed by electro-etching in an acidic solution (H2SO4: 100–200 ml L1, (NH4)2SO4: 50–100 ml L1) at 40 °C for about 5 min, and finally rinsed in deionized water. A formate–acetate compound was selected as the complexing agent for Cr3+. Formates may increase the cathodic polarization for Cr3+ deposition, and decrease the deposited grain size, while acetates impede the formation of precipitation in solution and stabilize the plating bath [14]. For the complex of Pd ions, ammonium is usually used, which reacts with PdCl2 to form Pd(NH3)4Cl2. However, this complex is stable only in neutral and basic environments. The best pH range for Cr3+ deposition is 1–4. In order to obtain co-deposition of Pd and Cr, NH4Cl and H2C2O42H2O were used as the complexing agents to react with Pd(CH3COO)2, forming a palladium–ammonium oxalate complex as follows:

PdðCH3 COOÞ2 þ 2NHþ4 þ H2 C2 O4  2H2 O ! PdðNH3 Þ2 C2 O4 þ 2CH3 COOH þ 2Hþ þ 2H2 O

ð1Þ

The pulse current deposition was applied in the electroplating process, and the optimal electroplating condition was determined by an orthogonal L16 (45) test design. The tested parameters include Pd2+ concentration (0.5, 4, 8 and 12 g L1), Cr3+ concentration (50, 100, 150 and 200 g L1), pH (1, 2, 3 and 4), frequency (0.2, 1, 5 and 10 Hz), and current density (6, 8, 10 and 12 A dm2). The results were evaluated by the homogeneity of the deposited film and the adhesion of the film to the substrate. Based on the orthogonal test results, the optimal electroplating conditions were as follows: concentration of Pd(CH3COO)2 was 14 g L1, concentration of CrCl36H2O was 100 g L1, concentration of H2C2O42H2O was 5–10 g L1, concentration of HCOONa was 50–100 g L1, concentration of CH3COONa was 10–50 g L1, concentration of NH4Cl was 50–100 g L1, H3BO3 and NaF were used as additives, pH was 1–3, and temperature was 35 °C. The pulse frequency was 1 Hz, and the current density in each pulse cycle was 1 A dm2 at the low level and 10 A dm2 at the high level with the same duration period. The potential response during the pulse cycle was about 0.6 VSCE and 0.9 VSCE, respectively. Therefore, the high pulse current density is in favor of Cr deposition and the low current density is beneficial to Pd deposition. Also the pulse plating promotes the deposition efficiency and results in small grains. The microstructure of the deposited films was investigated with a Cambridge LEO-1450 scanning electron microscope (SEM). The chemical composition of the films was analyzed with energy dispersive X-ray spectroscopy (EDX). Although EDX is not a precise method for quantitative analysis, it is extensively used as an in-situ method for film analysis. X-ray photoelectron spectroscopy (XPS) measurements were performed in an ESCALAB250 system using Mg Ka radiation to obtain the chemical composition on the film surface. The pressure in the chamber was about 3  107 Pa. The crystallographic structure of the deposited films was investigated with an X-ray diffractometer at 25 kV, with Cu Ka radiation and Ni filter. The crystallite size was estimated by using Scherrer’s formula according to the XRD peak broadening, which gives a lower

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limit of the crystallite size since the XRD peaks may also be broadened by stress. The adhesive strength of the film to stainless steel was measured according to ASTM D 3359-90 (ASTM Standard Test Method for Film Adhesion by Tape Test) TEST METHOD B Cross-Cut Tape Test. The micro-hardness of the film was measured with a Fisher H 2000 micro-hardness tester. The applied load was 20 mN and the time was 20 s. The film thickness was measured according to the weight gain of the sample before and after electroplating. The corrosion behavior of the stainless steel samples with Cr–Pd alloy films in boiling 20 wt.% sulfuric acid solution was studied with polarization tests and electrochemical impedance tests. The samples for electrochemical studies were sealed with phenolic resin, leaving an area of 0.5 cm2 exposed to the solution. Electrochemical tests were carried out in a typical three electrodes system with an EG&G Model 273A potentiostat. The counter electrode was a platinum foil and the reference electrode was a saturated calomel electrode (SCE). During measurement the saturated calomel electrode remained at room temperature and was connected to the cell through a salt bridge and a solution bridge. The sample was polarized first from the open circuit potential to a cathodic potential 100 mV negative to the open circuit potential and then in the anodic direction at a potential scanning rate of 0.66 mV s1. Electrochemical impedance spectroscopy (EIS) measurements were performed with a Model 5210 lock-in amplifier connected to the cell via the Model 273A potentiostat at the open circuit potential with a 10 mV perturbation, and the measuring frequency was from 100 kHz to 10 mHz. The corrosion rates of the Cr–Pd plated 316L stainless steel samples in different corrosive media were measured by weight loss tests. Three parallel samples were used for each condition and the average values were taken.

3. Results and discussion 3.1. Features of the chromium–palladium alloy films The electroplated films show a bright color and smooth surface. Fig. 1 shows the surface morphology of the deposited films obtained in the electrolytes with different Pd2+ concentrations. The films are continuous and compact, and cover the surface well. With the increase of Pd2+ concentration, the grain size of the deposited Cr–Pd films decreases. In the electrolytes charged with 2 and 4 g L1 Pd2+, the deposited films show nano-crystalline structures with a grain size of about 70–80 nm. EDX was used to determine the composition of the Cr–Pd alloy films. Fig. 2 shows the EDX spectrum of the film deposited in 0.5 g L1 Pd2+ solution. The electroplated film is composed mainly of chromium, and the palladium content was 2.7 wt.%. As the Pd2+ concentration in the electroplating solution increases from 0.5 to 4 g L1, the palladium content in the deposited Cr–Pd film increases from 2.7 to 83 wt.%, which indicates that the depositing speed of Pd is much higher than that of Cr. Hence the Pd content in the Cr–Pd alloy films may be controlled by changing the Pd2+ concentration in the electroplating solution. The adhesive strength of the film to the substrate was examined by the cross-cut tape test according to ASTM-D3359-90 (Standard Test Method for Film Adhesion by Cross-Cut Tape Test). For the Cr– Pd films with different Pd contents, the adhesion of the films to stainless steel reaches the 4B level, which means good adhesion. According to the weight change of the sample before and after electroplating, the calculated film thickness is about 1–1.5 lm. Table 1 shows the results of micro-hardness test for the Cr–Pd plated 316L stainless steel samples. It is obvious that the Cr–Pd films show higher micro-hardness than both the stainless steel substrate and the pure Pd film. With the increase of Cr content in

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Fig. 1. Surface views of Cr–Pd alloy films deposited in electrolytes with different Pd2+ concentrations: (a) 0.5 g L1, (b) 1 g L1, (c) 2 g L1, (d) 4 g L1.

Cr K

Intensity / a.u.

3.0

Element wt.% at.% Pd 2.67 1.32 Cr 91.45 81.03 O 05.88 17.65

2.4 1.8 1.2

Cr L Ok

0.6

chromium because of the very negative reducing potential of Cr. Since hydrogen is prone to enter the lattice, the lattice constants of Pd and Cr in the film would increase. The presence of hydrogen in the film may slightly decrease the potential of the plated sample, but the protective property of the film may be recovered after removal of hydrogen by heating [16]. The crystallite size of the deposited Cr–Pd films may be estimated using Scherrer’s formula:

Pd L

0.0

0

1

2

3

4

5

6

7

D¼

8

Energy / keV Fig. 2. EDX spectrum of Cr–Pd alloy film deposited in 0.5 g L1 Pd2+ solution.

the Cr–Pd alloy films, the micro-hardness of Cr–Pd alloy films increases. Both the Cr deposition and the fine grain structure made contributions to the increased film hardness, increasing the wear and erosion resistances of the films. Fig. 3 shows the XRD patterns of the Cr–Pd alloy films with different Pd contents. The diffraction peaks of (1 1 1) and (3 1 1) for Pd and (2 1 1) for Cr are seen in the XRD patterns, indicating that Cr and Pd crystals were separately deposited during the deposition. The peaks in the 2h range of 43–53° are due to the stainless steel substrate since the film is very thin. It can be seen in Fig. 3 that with the increase of palladium content in the deposited Cr–Pd alloy film, both the (1 1 1) peak of Pd and the (2 1 1) peak of Cr move in the lower angle direction, indicating an increase of plane distance. During electrodeposition, hydrogen co-deposition with palladium could happen [15,16]. Hydrogen is also apt to co-deposit with

0:9k b cos h

ð2Þ

where D is the crystallite size, k is the wavelength of the X-ray used, b is the broadening of the diffraction peak measured at half of the maximum intensity, and h is the angle of the diffraction peak [17]. The calculated data according to the Cr (2 1 1) peaks on the patterns are shown in Table 2. The results indicate that the crystallite size of the deposited chromium decreases with the increase of Pd content in the Cr–Pd alloy films, consistent with the SEM observation given in Fig. 1. Fig. 4 shows the wide-scan XPS spectra of the surface of the Cr–Pd alloy films. The binding energy (BE) values and the atomic percentages of the elements are shown in Table 3. It is seen that the electroplated film surface is composed mainly of chromium, palladium, oxygen, and carbon (carbon was added for calibration of the spectrum). The chemical states of the elements were identified by comparing the photoelectron binding energy values (BE) with literature values. Fig. 5 shows the high-resolution spectra of Cr 2p, O 1s, and Pd 3d. The BE of the Cr 2p band varies from 572 to 578 eV, which

Table 1 The micro-hardness testing results for Cr–Pd films with different Pd contents. Sample

316L stainless steel (SS)

Pd plated SS

Cr–Pd plated SS (2.7 wt.% Pd)

Cr–Pd plated SS (10 wt.% Pd)

Cr–Pd plated SS (33 wt.% Pd)

Cr–Pd plated SS (83 wt.% Pd)

Hardness/HV

278

260

386

323

285

257

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(a)

Cr(211)

Cr 2p

Cr2 O3

3/2

Cr

Cr(OH)

3

83 wt.% Pd

2.7 wt.% Pd

Intensity / a. u.

Intensity / a.u.

Cr(211) Pd(111) Pd(311)

33 wt.% Pd

10 wt.% Pd

33 wt.% Pd 10 wt.% Pd

83 wt.% Pd 2.7 wt.%Pd 588

20

40

60

80

584

580

2 θ / degree Fig. 3. XRD patterns of Cr–Pd alloy films with different Pd contents.

Table 2 The calculated crystallite sizes for Cr in the Cr–Pd films. Alloy

2h/°

Crystallite size/nm

Cr–83 wt.% Pd Cr–33 wt.% Pd Cr–10 wt.% Pd Cr–2.7 wt.% Pd

74.187 74.391 74.548 75.123

24.3 28.7 33.9 37.0

576

572

568

Binding energy / eV

100

(b)

O 1s

OH

-

O

2-

83 wt.% Pd

Intensity / a. u.

0

33 wt.% Pd

10 wt.% Pd 2.7 wt.% Pd

4

O1s

8.0x10

550

545

540

535

530

525

520

515

Binding energy / eV

Cr 2p

4

3/2

(c) Pd 3d 5/2

C1s

PdO

Pd

4

4.0x10

Pd 3d

5/2

Intensity / a. u.

Counts / s -1

6.0x10

4

2.0x10

0.0 0

400

800

83 wt.% Pd

33 wt.% Pd

1200

Binding energy / eV

10 wt.% Pd

Fig. 4. XPS survey of Cr–2.7 wt.% Pd film.

2.7 wt.% Pd 345

Table 3 Measured binding energy values and at.% in Cr–2.7 wt.% Pd film by XPS.

340

335

330

325

Binding energy / eV

Element

Cr 2p3/2

Pd 3d5/2

O 1s

Peak B.E./eV At.%

580.07 9.31

336.38 0.85

537.04 89.84

consists of three different chemical states: 574.3 eV for metallic chromium Cr0 [18], 577.3 eV for Cr(OH)3, and 576.3 eV for Cr2O3 [19]. As the Cr content in the film increases, the peak intensities of both metallic chromium and Cr3+ increase, indicating the enrichment of chromium on the surface. Chromium is easily oxidized on the surface, and the Cr3+ spectra in Fig. 5a suggest that both Cr(OH)3 and Cr2O3 are present on the surface, a result supported by the O spectra in Fig. 5b. Both the OH and O2 peaks can be seen

Fig. 5. XPS peaks of Cr–Pd alloy films with different Pd contents: (a) Cr 2p, (b) O 1s, (c) Pd 3d.

in Fig. 5b, which also shows that the intensity of the O2 peak increases with the increase of chromium content in the film. For metallic palladium, the reported binding energy values of the spin–orbit couple peaks of Pd 3d are 335.30 and 340.65 eV [20], which are close to the measured values given in Fig. 5c. When Pd is oxidized, the binding energy of Pd 3d5/2 may shift from 335.4 to 336.8 eV. Hence the palladium in the film is in the metallic state. All the above results show that Cr–Pd films composed of nano size chromium and palladium crystallites were uniformly deposited on 316L stainless steel by pulse plating. During the pulse

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Table 4 Weight loss testing results for 316L SS samples with Cr–Pd films. Sample

Testing time/h

Corrosion rate in boiling 20 wt.% H2SO4 solution/g m2 h1

Standard deviation

Corrosion phenomena

316L stainless steel (SS) Cr plated SS Pd plated SS Cr–Pd plated SS (2.7 wt.% Pd) Cr–Pd plated SS (10 wt.% Pd) Cr–Pd plated SS (33 wt.% Pd) Cr–Pd plated SS (83 wt.% Pd)

2 2 72 72 72 72 72

288.7 274.2 0.011 0.023 0.014 0.010 0.009

21.6292 21.1702 0.0016 0.0035 0.0022 0.0021 0.0001

Severe corrosion Severe corrosion Intact film Intact film Intact film Intact film Intact film

Note: Generally the testing time was 72 h. For original 316L SS and Cr plated 316L SS samples, the testing was stopped after 2 h due to severe corrosion.

plating cycle, the potential varies alternately between the high level and the low level, which are in favor of Cr deposition and Pd deposition, respectively, leading to deposited Cr and Pd crystallites. Because the standard electrode potential of Cr is much more negative than that of Pd, in the low current periods (high potential) only palladium is deposited, while in the higher current periods (more negative potential) both chromium and palladium may be deposited. Therefore, the Pd content in the deposited film increases quickly with the increase of palladium salt concentration in the plating bath. On the other hand, previous studies [21,22] have shown that the critical radius of the electrodeposited nucleus for stable growth is inversely proportional to the cathodic overpotential, and the nucleation rate increases with the cathodic overpotential. In the high current periods the cathodic overpotential for palladium deposition is high, and the nucleation of palladium crystallites is obviously promoted. As a result the grain size decreases with the increase of Pd content in the Cr–Pd film. Hence the pulse plating promotes nucleation of the crystallites and inhibits the grain growth, resulting in a uniform film structure with nano-crystallites. However, from the macroscopic view the Cr and Pd crystallites in the film are still uniformly distributed, ensuring uniform mechanical and chemical properties. 3.2. Corrosion resistance of chromium–palladium alloy films Table 4 shows the weight loss test results of the Cr–Pd plated stainless steel samples in boiling 20 wt.% H2SO4 solution. The testing time was 72 h. However, in this environment, severe corrosion occurred for the original 316L stainless steel and pure chromium plated 316L samples. A large amount of bubbles were observed on the sample surface, hence the testing was stopped after 2 h.

The pure chromium film did not provide protection for the stainless steel substrate. But for Cr–Pd plated samples, just 2.7 wt.% Pd in the Cr–Pd film significantly improves the corrosion resistance of the sample. The corrosion rates of the Cr–Pd plated samples remain very low over a wide range of Pd contents (from 10 to 83 wt.%), about four orders of magnitude lower than those of the original stainless steel or pure chromium plated samples. Table 5 shows the weight loss test results for Cr–Pd plated samples with 10 wt.% Pd in different corrosive media. In boiling 20 wt.% H2SO4 solution with 100 mg L1 NaCl, the corrosion rate of the Cr–Pd plated samples is also very low, and no pitting was observed after testing. With the increase of chloride concentration, the corrosion rate of the Cr–Pd plated samples increases, but still several orders of magnitude lower than that of the original 316L sample. In boiling acetic acid–formic acid mixtures, the Cr–Pd plated samples also show significantly increased corrosion resistance over that of the original 316L sample. It is worth noting that the last corrosion medium in Table 5 is a simulated environment for bipolar plates in proton exchange membrane (PEM) fuel cells [1]. Although the medium is not as corrosive as boiling 20 wt.% H2SO4 solution, the 316L stainless steel samples are obviously corroded. On the other hand, the corrosion rate of the Cr–Pd plated samples with 10 wt.% Pd is very low in this environment, about three orders of magnitude lower than that of the original 316L stainless steel. Fig. 6 shows the polarization curves of the 316L stainless steel, palladium plated stainless steel, and Cr–Pd plated stainless steel samples in boiling 20 wt.% H2SO4 solution (102 °C). For the 316 stainless steel sample, the measured corrosion potential is about 0.2 VSCE, and the sample is in an active state. For the palladium plated sample, the open circuit potential is obviously increased to about +0.4 VSCE, and stable passivation is obtained. For the

Table 5 Weight loss testing results for 316L SS samples with Cr–10 wt.% Pd film in different corrosive media. Medium

Temperature/ °C

Sample

20 wt.% H2SO4 + 100 mg L1 NaCl

102

316L SS Cr–Pd plated (10 wt.% Pd) Cr–Pd plated (10 wt.% Pd) Cr–Pd plated (10 wt.% Pd) 316L SS Cr–Pd plated (10 wt.% Pd) 316L SS

20 wt.% H2SO4 + 500 mg L1 NaCl

102

20 wt.% H2SO4 + 1000 mg L1 NaCl

102

90 vol.% acetic acid + 10 vol.% formic acid

102

90 vol.% acetic acid + 10 vol.% formic acid + 400 mg L1 Br

0.5 M H2SO4 + 2 mg L

1

HF

102

80

Time/ h

Corrosion rate/ g m2 h1

Standard deviation

Phenomenon Severe corrosion Intact film

316L SS

2 72

549.4 0.014

32.1364 0.0028

316L SS

72

0.022

0.0022

Intact film

316L SS

72

0.046

0.0029

Intact film

316L SS

72 72

1.328 0.021

0.0166 0.0059

Obviously corroded Intact film

72

1.463

0.0329

72

0.032

0.0036

Obviously corroded with pits Intact film

72 240

2.37 0.0025

0.1347 0.0006

Obviously corroded Intact film

Cr–Pd plated 316L SS (10 wt.% Pd) 316LSS Cr–Pd plated 316L SS (10 wt.% Pd)

Note: Generally the testing time was 72 h. But for original 316L SS samples, the testing time was 2 h due to severe corrosion, and for the last medium/material system the testing time was extended to 240 h because of the very low corrosion rate.

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(a)

1.2

1.SS 2.SS+(Cr-10 wt.% Pd) 3.SS+Pd

1.0

1. without NaCl -1 2. 100 mg L NaCl -1 3. 500 mg L NaCl -1 4. 1000 mg L NaCl

3

2

1

SCE

0.5

3 4

E/V

E / V SCE

0.8

2

0.4

1 0.0

10

-9

10

-7

-5

10

10

i / A cm

(b) 1.2

1. SS+(Cr-83 wt.% Pd) 2. SS+(Cr-33 wt.% Pd) 3. SS+(Cr-10 wt.% Pd) 4. SS+(Cr-2.7 wt.% Pd)

0.8

0.0

-3

-9

10

-2

sce

E/V

4

0.0 -6

-1

10

2

3

10

-3

10 -2

Fig. 7. Polarization curves of Cr–10 wt.% Pd plated 316L stainless steel samples in boiling 20 wt.% H2SO4 (102 °C) with different contents of NaCl.

0.4

-7

-5

10

i / A cm

1

10

-7

10

-5

-4

10

10

-3

10

-2

10

-2

i / A cm

Fig. 6. Polarization curves of 316L stainless steel, palladium plated, and Cr–Pd plated samples in boiling 20 wt.% H2SO4 (102 °C): (a) comparison of 316L, 316L with Pd film, and 316L with Cr–Pd film, (b) effect of Pd content in Cr–Pd films.

Cr–Pd plated samples, the open circuit potentials are between those of original 316L stainless steel and Pd plated samples (Fig. 6a). As the Pd content in the Cr–Pd film decreases, the open circuit potential moves in the negative direction because of the influence of Cr on the mixed potentials of the film (Fig. 6b). However, even for the Cr–Pd plated sample with just 2.7 wt.% Pd, spontaneous passivation is obtained in boiling 20 wt.% H2SO4 solution, and the anodic current density is significantly decreased, indicating that passivation is obviously promoted by the presence of Pd on the surface. It is noted in Fig. 6 that the polarization curves for the samples plated with pure Pd or low-Cr/Pd ratio films (Cr– 83 wt.% Pd) show a small current peak at a potential around 0.8 VSCE, which may be attributed to the oxidation of Pd to Pd2+ [23]. However, the peak can not be seen for the samples plated with higher Cr/Pd ratio films, indicating that the surface is covered by oxides of chromium, in agreement with the XPS results given in Fig. 5a. Fig. 7 shows the polarization curves of Cr–10 wt.% Pd plated 316L stainless steel samples in boiling 20% H2SO4 (102 °C) with different contents of NaCl. In the media with chloride ions, the corrosion potential of the samples decreases by less than 100 mV (SCE) in contrast to that in the same medium without chloride. As the chloride concentration increases, the passive current density of the Cr–Pd plated samples increases gradually. In boiling 20 wt.% H2SO4 solution with 1000 mg L1 NaCl, the passive current density increases by about one order of magnitude from that in the solution without chloride. However, the corrosion potential does not change and the sample remains in the passive state. This result is

in accordance with the weight loss measurements in Table 5. The improvement of the corrosion resistance to chlorides may be explained by the increased passivity as a result of Cr–Pd plating. Fig. 8 shows the measured electrochemical impedance spectra for 316L stainless steel, palladium plated stainless steel, and Cr– Pd plated stainless steel samples in boiling 20% H2SO4 solution (102 °C). 316L stainless steel in the solution shows a very small impedance, again revealing that the steel is in an active dissolution state. The palladium-plated sample shows much higher impedance values than the 316L sample, indicating better corrosion resistance. The impedance values of the Cr–Pd plated samples are even higher than that of the pure palladium-plated sample. From Fig. 8 it is seen that the sample with a Cr–33 wt.% Pd film shows the highest impedance. This result means that the co-deposition of Cr and Pd obviously increases the passive ability of the surface. In this testing environment, both for the original 316L stainless steel sample and the pure Cr-plated samples, the passivation is not stable. The introduction of palladium in the film effectively raises the open circuit potential of the samples and promotes the passivation. It is interesting to note that only 2.7 wt.% Pd in the Cr–Pd film may be sufficient for passivation. However, higher Pd contents in the film would further increase the potential and impedance and lead to stable passivation. The chromium in the Cr–Pd film also plays an important role in the corrosion resistance of the film because higher Cr content on the surface is beneficial to the formation of a passive film [24,25]. In Fe–Cr alloys the addition of Cr greatly influences the active to passive transition of Fe and significantly stabilizes the passive state of the alloys, and both the potential and the current for passivation decrease with increasing chromium content [26]. In addition, if the Pd content in the film is too high, the oxidation of palladium on the surface may reduce the protective ability of the film. Therefore, an appropriate Cr/Pd ratio in the film would give an optimum passivation effect. According to the experimental results in Fig. 8, the sample with a Cr–33% Pd film shows the highest impedance among the tested samples. In strongly corrosive media such as boiling 20 wt.% sulfuric acid solution, stainless steels cannot be steadily passivated. Palladium as a noble metal on the surface may raise the potential of the plated stainless steel and promote passivation of the surface. Also, on the palladium film surface there is a high exchanging current density for hydrogen evolution reaction [27], and the cathodic polarization is promoted, which in turn promotes spontaneous passivation of the surface. On the other hand, the enriched chromium on the surface would decrease the critical current density

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(a)

4. Conclusions 316L SS

20

(1) Chromium–palladium alloy films with good adhesive strength have been deposited on 316L stainless steel by pulse electroplating. The films are compact with a thickness of about 1–1.5 lm. By adjusting the Pd2+ content in the plating solution, the Cr/Pd ratio in the film may be changed over a wide range. (2) In the Cr–Pd films both Cr and Pd are present mainly in the metallic state. On the surface, Cr(OH)3 and Cr2O3 are present. The crystallite size decreases with the increase of Pd content in the Cr–Pd alloy films. For Cr–Pd films with Pd contents higher than 33 wt.%, a nano-crystalline structure is obtained with a grain size of about 70–80 nm. The Cr–Pd plated samples show higher micro-hardness than the Pd plated sample and the 316L stainless steel substrate. (3) The Cr–Pd films on stainless steel significantly promote the passivation of the substrate in hot sulfuric acid solutions. In boiling 20 wt.% H2SO4 solution, the corrosion rates of the Cr–Pd plated samples are about four orders of magnitude lower than that of the 316L stainless steel substrate. A synergetic effect of the co-deposited Cr and Pd on passivation is observed. The samples with Cr–33% Pd film show higher impedance than the pure Pd plated samples. In boiling acetic–formic acid mixture and simulated PEM fuel cell environment, the Cr–Pd plated 316L steel also shows excellent corrosion resistance.

SS+(Cr-2.7 wt.% Pd) SS+(Cr-10 wt.% Pd) SS+(Cr-33 wt.% Pd)

Z'' / kohm cm 2

15

SS+Pd 10

5

0 0

5

10

15

20

25

Z' / kohm cm2

(b) 10

316L SS

4

SS+(Cr-2.7 wt.% Pd) SS+(Cr-10 wt.% Pd)

|Z| / ohm cm 2

10

SS+(Cr-33 wt.% Pd)

3

SS+Pd 2

10

1

10

References

0

10

-3

10

-1

10

1

10

3

10

5

10

Frequency / Hz Fig. 8. Measured electrochemical impedance spectra for 316L stainless steel, palladium plated, and Cr–Pd plate samples in boiling 20 wt.% H2SO4 (102 °C): (a) Nyquist plots, (b) Bode plots.

for activation/passivation transformation and facilitate the passivation. This may explain why the Cr–Pd plated stainless steel samples show higher impedance than the samples plated with pure palladium. Therefore, the synergetic effect of Cr–Pd films to promote passivation is due to the high potential of palladium, the strengthened cathodic reaction on the palladium surface, and the enrichment of Cr in the film. The increased resistance of the Cr– Pd plated stainless steel to chloride ions corrosion is also due to the promoted passivation. Therefore, the Cr–Pd plating films may be used for the protection of stainless steels in corrosive media with a small amount of halogen ions (100 mg L1, for example, as shown in Table 5). However, in the media with higher halogen concentrations, the passivation may be damaged by the aggressive ions. A potential application of the Cr–Pd films is in PEM fuel cells in which the typical corrosion medium is 0.5 M H2SO4 + 2 mg L1 HF at 80 °C [28]. The results in Table 5 indicate that in this medium, Cr–10 wt.% Pd plated 316L stainless steel shows a very low corrosion rate because of the promoted passivation on the surface. On the other hand, although passive films on stainless steel surface would increase the interfacial contact resistance [1,2], palladium as a low resistance metal may more or less lessen the effect. An advantage of the Cr–Pd plating films is that the Cr/Pd ratio may be adjusted over a wide range, allowing the balance among corrosion resistance, contact resistance, hydrogen permeation, and mechanical properties.

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