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Study on corrosion resistance of palladium films on 316L stainless steel by electroplating and electroless plating
Corrosion Science 50 (2008) 2873–2878
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Study on corrosion resistance of palladium films on 316L stainless steel by electroplating and electroless plating Junlei Tang, Yu Zuo * 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 27 March 2008 Accepted 18 July 2008 Available online 29 July 2008 Keywords: A. Stainless steel C. Acid corrosion C. Passivity
a b s t r a c t Palladium films with good adhesive strength were deposited on 316L stainless steel by electroless plating and electroplating. Scanning electronic microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, weight loss tests and electrochemical methods were used to study the properties of the films. The electroless plated palladium film mainly consisted of palladium, phosphorus and nitrogen, and the electroplated palladium film was almost pure palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples prepared by both methods showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling 20% dilute sulfuric acid solution, the corrosion rates of the palladium plated 316L stainless steel samples were four orders of magnitude lower than that of the original 316L stainless steel samples. In the solution with 0.01 M NaCl, the palladium plated samples also showed better corrosion resistance. In comparison, the electroplated samples showed slightly better corrosion resistance than electroless plated samples, which may be attributed to less impurities and thereby higher corrosion potential for the former. Ó 2008 Published by Elsevier Ltd.
1. Introduction It is well known that stainless steels owe their corrosion resistance largely to the formation of passive films on the alloy surface. Stainless steels show good corrosion resistance in oxidizing corrosive mediums where the passive films formed on the surface are stable. However, in many reductive corrosion mediums such as boiling dilute sulfuric acid solutions or boiling acetic plus formic acids, passivity can not be steadily established on the surface and active corrosion happens for stainless steels. For the alloys with passive ability such as titanium alloys or stainless steels, if corrosion potential of the alloy is raised from active potential into passive region by applied anodic current or by alloying with elements with higher oxidation/reduction potentials, corrosion resistance would be improved. Different techniques were reported for palladium deposition on titanium, including vacuum evaporation, ion beam mixing, etc. [1]. For example, palladium deposition on titanium surface by ion beam mixing may effectively improve corrosion resistance of titanium in reductive acidic solutions [2–5]. There were also some reports on deposition of palladium on stainless steels. However, almost all the studies were aimed to prepare catalytic membrane reactors [6–8], since palladium film is permeable for hydrogen and catalytically active to many hydrogen-involved reactions. Hughes and co-workers [9] studied the method to deposit a Pd composite membrane on porous stainless steel tube * Corresponding author. Tel.: +86 10 64423795; fax: +86 10 64434908. E-mail address: [email protected] (Y. Zuo). 0010-938X/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.corsci.2008.07.014
by electroless plating. The membrane possessed high hydrogen selectivity and thermal stability at high temperature. Matsumura and co-workers [10–12] also prepared Pd membrane on porous stainless steel tube by electroless plating. Shi et al. [13] studied the initial formation of palladium membrane on a porous stainless steel substrate by electroless method and the morphology of pores in the membrane was examined. The effects of surface activity, defects and mass transfer on hydrogen permeation in palladium-porous stainless steel membrane were studied by Guazzene et al. [14]. In this paper, palladium deposition on 316L stainless steel was carried out by both electroless plating and electroplating, and corrosion behaviors of the Pd films/stainless steel in boiling 20% H2SO4 solution were studied. The palladium films on the surface of 316L stainless steel improved passive ability and corrosion resistance of the stainless steel, which were due to a positive shift of corrosion potential. The difference in composition and corrosion resistance for the palladium films produced by electroless plating and electroplating were investigated. 2. Experimental methods The testing material was rolled 316L stainless steel which was solution annealed and quenched, with a thickness of 3 mm. The steel composition was 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%, Fe% Bal. The samples were cut to the size of 20 10 mm, finished with abrasive papers up to 1000#, then were decreased first in alcohol then in a basic solution (Na2CO3: 30 g/L, Na3PO4: 30 g/L, NaOH:
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50 g/L) at 70 °C for 10 min, followed by acidic etching in 20% H2SO4 solution at 80 °C for about 30 s (electroless plating) or at 65 °C for about 5 min (electroplating), and rinsed in deionized water. The electroless plating condition was as follows: PdCl2: 2 g/L, NH3 H2O (28%): 150 ml/L, HCl: 4 ml/L, NH4Cl: 27 g/L, NaH2PO2: 12 g/L, temperature: 50 °C, pH 9.5–10, time: 30–60 min. The thickness of the electroless plated palladium film was about 0.5–1 lm. The electroplating condition was as follows: PdCl2: 15 g/L, NH3 H2O (28%): 15 ml/L, NH4Cl: 50 g/L, (NH4)2PO4: 50 g/L, current density: 1 A/dm2, temperature: 40 °C, pH 7–8, time: 3 min. The thickness of the electroplated palladium film was about 2–3 lm. Energy dispersive X-ray spectroscopy (EDX) and X-rays photoelectron spectroscopy (XPS) were used to analyze the composition of the films. The film surface was observed by using scanning electronic microscope (SEM). The porosity of the film was measured by following method: A filter paper (area 10 mm 10 mm) was wetted by the solution of K3Fe(CN)6 (10 g/L) and NH4Cl (30 g/L), adsorbed on the film surface and kept for 15 min, then was dried on a clean glass. The blue spots on the dried filter paper were counted. For each sample five tests in different areas were conducted then the average value was taken. Micro-hardness of the film was measured by a HXS-1000A digital micro-hardness tester. The film thickness was measured with SEM according to the crosssection images of the samples. The adhesive strength of the film to stainless steel matrix 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. Corrosion behaviors of the palladium plated samples in 20% sulfuric acid solutions were studied by polarization tests, electrochemical impedance tests and weight loss tests at boiling temperature. The samples were sealed with phenolic resin, leaving an area of 0.5 cm2 exposed to the solution. Polarization measurements were performed with an EG & G model 273A potentiostat. The sample was first polarized from the open circuit potential to a cathodic potential which was 100 mV negative to the open circuit potential, then was polarized in the positive direction with a potential scanning rate of 1 mV/s. A saturated calomel electrode (SCE) was used as the reference electrode, and the counter electrode was platinum. Because the testing temperature was relatively high, the potential was measured by an external calomel electrode coupled to the cell through a Luggin capillary. Electrochemical impedance spectroscopy (EIS) measurements were performed with a Model 5210 lock in amplifier connected to the cell via a Model 273A potentiostat, at the open circuit potential with a 10 mV perturbation. The frequency was ranged from 100 kHz to 10 mHz. All the reagents used were analytic grade, and the solutions were open to the air.
3. Results and discussion 3.1. Features of the palladium films deposited by electroless plating and electroplating Smooth surface was observed for the palladium films on stainless steel. The electroless plated film showed black color and the electroplated film showed gray color. For the plating time of 1 h, the obtained electroless plated palladium film was about 1 lm in thickness. For plating time of 2 min, the obtained electroplating palladium film was about 2 lm. Fig. 1 shows surface morphology of the two films under SEM. Because the plated palladium films were very thin, the traces left by surface finishing were still discernible. Both the films were continuous, compact and covered the surface well. The measured results for porosity of the two films (Table 1) indicate that the porosity of the film by electroless plating was lower.
Fig. 1. The surface views of the Pd films under SEM: (a) electroless plating, (b) electroplating.
Table 1 Porosity testing results for the two films Electroless plated film
Electroplated film
0.75 sopt/cm2
2 sopt/cm2
Table 2 shows the measured micro-hardness values of the palladium plated samples. Because the films were very thin, the measured values could not reflect the hardness of the palladium films exactly. However, the result indicated that the electroplated film had higher hardness than electroless plated film. The adhesive strength was examined by cross–cut tape test according to ASTM-D3359-90 (Standard Test Method for Film Adhesion by Cross–Cut Tape Test), and the adhesion of both the films to stainless steel reached 4B level, which means the adhesion was quite good. The compositions of the plated palladium films were measured with EDX. For electroless plated film, Pd, P, Fe, Cr and Ni were clearly observed on the spectrum plot as shown in Fig. 2. However, the existence of Fe, Cr and Ni were due to the stainless steel matrix since the film thickness is less than 1 lm. The electroplated film was mainly composed of palladium. XPS analysis was carried out to further understand the states of the elements in the films. Fig. 3a shows the wide scan XPS spectra on surface of the palladium plated films. The binding energy (BE) values and the atomic percentages for the elements are shown in Table 3. It is seen that there is some difference between compositions of electroless plated film and electroplated film. The electroplated film surface was mainly composed of palladium, oxygen, and carbon which
J. Tang, Y. Zuo / Corrosion Science 50 (2008) 2873–2878
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Table 2 Micro-hardness testing result 316L SS matrix
Electroless plating
Electroplating
245 HV
219 HV
270 HV
Fig. 2. The composition analysis of Pd plated stainless steel by EDX: (a) electroless plating, (b) electroplating.
was added for calibration of the spectrum. It was reported [15] that for metallic palladium, binding energy values of the spin-orbit couple peaks of Pd3d are 335.30 eV and 340.65 eV, respectively, which are very close to the measured values in Table 3. When Pd is oxidized, binding energy of Pd3d5/2 may shift from 335.4 eV to 336.8 eV [16]. Hence the results in Table 3 suggest that palladium in the surface film was present in metallic state. In addition, binding energy of Pd3p3/2 (532.4 eV) [17] is close to that of O1s which was reported near 531.8 eV when O2 adsorbed on Pd–Ag film [18]. Thus the peak around 532 eV in Fig. 3a should be the overlapped result of Pd3p3/2 peak and O1s peak. Fig. 3b shows the deconvolution of the peaks for the electroplated palladium film, where oxygen should come from the adsorbed oxygen on the surface because the BE values for O1s in PdxOy was reported as 529.65– 530.38 eV [19], obviously different from the measured values. The electroless plated film was composed of Pd, O, P, N and C. It is seen in Table 3 and Fig. 3c that the binding energy values of both Pd and O are almost the same as the values for electroplated film, indicating that palladium was present in the same state in the films prepared by electroplating or electroless plating. Phosphorus and nitrogen in electroless plated film were due to the effect of NaH2PO2 in the plating bath. The measured BE of P in the film is 133.85 eV which is much higher than that of pure phosphorus (129.9 eV) [20]. So the element P is present in the film in an oxi-
Fig. 3. XPS analysis of the Pd film surface: (a) survey, (b) Pd3p3/2 and O1s of the electroplated palladium film, (c) Pd3p3/2 and O1s of the electroless plated palladium film.
dized state. The measured BE for N is 399.92 eV, very close to the value of N in Pd(NH3)4Cl2 (400.1 eV) [21] which was present in the electroless plating solution. 3.2. Corrosion resistance of the palladium plated samples 3.2.1. Weight loss tests Table 4 shows weight loss testing results of the palladium plated stainless steel samples in boiling 20% H2SO4 and boiling 20% H2SO4 + 0.01 M NaCl solutions. The result for original 316L stainless steel samples is also shown as comparison. For 316L stainless steel samples, as soon as the samples were immersed in the solution, severe corrosion happened and a large amount of bubbles was observed on the surface. The sample size diminished obviously after only 2 h of immersion. While little weight losses were mea-
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Table 3 Measured binding energy values and atom% of the elements in films by XPS Name
C1s
O1s
Electroless plated sample
Electroplated sample
P2p3/2
Pd3d
N1s
C1s
O1s
Pd3d
Peak BE (eV) At.%
284.99 45.88
531.651 8.11
133.85 2.52
335.59, 340.88 34.72
399.92 8.78
285. 71.62
531.651 4.05
335.63, 340.88 24.33
Table 4 Weight loss testing results for the samples Sample
Time (h)
Corrosion rate in boiling 20% H2SO4 (g m2 h1)
Corrosion rate in boiling 20% H2SO4 + 0.01 MCl (g m2 h1)
Corrosion phenomena
316L stainless steel with electroless plated Pd film (1 lm)
240
0.014
0.020
Intact film
316L stainless steel with electroplated Pd film (1 lm)
240
0.008
–
Intact film
316L stainless steel with electroplated Pd film (2 lm)
240
0.007
0.011
Intact film
Original 316L stainless steel
2
288.4
549.6
Severe corrosion
a
1.2 1.0 0.8
E (VSCE)
sured for both palladium plated samples after 240 h of immersion. The electroplated sample showed lower corrosion rate than the electroless plated sample in the testing solutions both with and without Cl. Longer term immersion tests were carried out for the palladium plated samples in boiling 20% H2SO4 solutions with or without 0.01 M NaCl up to 25 days. Both the electroless plated and electroplated samples showed little corrosion after the 25 days of immersion, indicating quite good performance in the strong corrosive mediums.
1. 316L blank 2. electroless plating 3. elrctroplating
0.6
2
3
0.4 0.2 0.0
3.3. Polarization measurements
1
-0.2 -0.4 10-8
10-7
10-6
10-5
10-4
10-3
10-2
i ( A/cm2)
b
1.2 1.0 0.8
E (VSCE)
Fig. 4a shows polarization curves of 316L stainless steel samples with and without Pd films in boiling 20% H2SO4 solutions (102 °C). For 316L stainless steel, measured corrosion potential was about 0.2 V, at which the corresponding corrosion current density was quite high and active dissolution happened for the steel without films. However, for Pd plated samples, corrosion potentials were greatly raised to passive region, and stable passivation was obtained for both Pd plated samples. Corrosion potential of the electroplated sample is about 0.4 V, more positive than that of the electroless plated sample (about 0.2 V). The anodic current densities for Pd plated samples were three orders of magnitude lower than original stainless steel. For curve 2 and curve 3 in Fig. 4a, a current step was observed at higher potential range, which may be explained by oxidation of palladium (Pd/Pd2+ equilibrium potential + 0.743 VSCE). Fig. 4b shows the influence of chloride ions on polarization behaviors of 316L stainless steel samples with and without Pd films in 20% H2SO4 solutions. Chloride ions effectively damaged passivation of stainless steels. With addition of 0.01 M NaCl to H2SO4 solution, for all the samples the anodic current densities tended to increase. For 316L stainless steel without Pd films, the anodic current density in the active region showed a remarkable increase due to chlorides. However, for palladium plated samples the corrosion potentials maintained almost the same as they were in the solution without Cl. This result means that in corrosive solutions with trace Cl, the plated palladium films still raised corrosion resistance of 316L stainless steel. This is reasonable because the enhanced passivation by palladium film would make the stainless steel more resistant to trace chloride ions. However, higher chloride concentration still can damage corrosion resistance of the palladium plated samples. It is seen that in Fig. 4b for both electroless plated and electroplated samples there are current increases on the polarization curves which may be due
1. 316L blank 2. electroless plating 3. elrctroplating
0.6
3
2
0.4 0.2
1
0.0 -0.2 -0.4 -0.6 10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
2
i (A/cm ) Fig. 4. Polarization curves of 316L stainless steel samples with and without Pd films in: (a) boiling 20% H2SO4 solution, (b) boiling 20% H2SO4 + 0.01 M NaCl solution.
to the reaction between palladium and chlorides: Pd þ 4Cl ! 2 PdCl4 þ 2e (The reaction equilibrium potential is +0.38 VSCE). Table 5 shows the variations of corrosion potentials of the electroless plated and electroplated samples with plating time in boiling 20% H2SO4 solution. With the plating time prolonged, corrosion potential of the electroless plated samples remained stable in certain range around 0.2 V, while that of electroplated samples increased from 0.17 V until reached 0.4 V. The higher porosity of
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J. Tang, Y. Zuo / Corrosion Science 50 (2008) 2873–2878 Table 5 Corrosion potentials of electroless plated and electroplated samples vs. plating time in boiling 20% H2SO4 solution Electroless plating Plating time (min) Thickness (lm) Corrosion potential (V)
30 <0.5 0.227
Electroplating 60 0.6–0.8 0.218
the electroplated film may be the reason for its more negative potential when the film was very thin. With the film thickened, the effect of porosity was diminished and the coupled potential between palladium film and the matrix would increase to the stable value. 3.4. Electrochemical impedance measurements Fig. 5 shows the measured electrochemical impedance spectra of 316L stainless steel samples with and without Pd films in 20% H2SO4 solution at open circuit potentials. It is seen that in 20% H2SO4 solution at 102 °C 316L stainless steel showed very small impedance, indicating that the steel was in active dissolution state. The palladium plated samples showed much higher impedance values. The impedance values for both electroplated and electroless plated samples increased by 3–4 orders of magnitude compared with the sample without Pd films. It is an indication that stable passivation was established on the surface of the samples. The results are quite consistent with those obtained from polarization tests. Palladium plating on stainless steel effectively improved passive ability of the steel in strong reductive corrosive environments. The porosity tests showed that the electroless plated film has lower porosity, but corrosion rate of the electroless plated samples is a little higher than that of the electroplated film. This result indicates that the increase of corrosion resistance by Pd plating on stainless steel does not rely on the barrier effect of the films. The standard oxidation/reduction potential of Pd/Pd2+ is +0.987 VSHE or +0.743 VSCE, much higher than that of stainless steels. As a cathodic film, the function of palladium film to improve corrosion resistance of stainless steel is mainly due to its effect of promoting passivation. In strong reductive corrosion mediums like boiling sulfuric acid solutions, the passive film on 316L stainless steel is not stable and active dissolution happens at open circuit potential. However, the polarization curves in Fig. 4 show that passivation is possible at higher potential range. With palladium films, the open
1800 1600 1400
0.5 0.5 0.174
1 1 0.340
2 2 0.40
circuit potentials of stainless steel in boiling sulfuric acid solutions were raised to higher potential range where passivation was again established on the surface. The measured corrosion potential may more or less reflect the stability of passivation under the experimental conditions. The samples with electroless plated Pd film showed more negative corrosion potential and slightly higher corrosion rate than the samples with electroplated Pd films, which should be attributed to the presence of phosphorus and nitrogen in the film. The almost pure palladium film without impurities by electroplating may lead the stainless steel surface to higher open circuit potential and relatively higher passive ability. Another advantage of electroplating is the plating time needed for the same film thickness is much shorter than electroless plating. Both of the palladium plating techniques may be used to improve corrosion resistance of stainless steels in strong reductive corrosion mediums. 4. Conclusions
316L blank
Palladium films with good adhesive strength were deposited on 316L stainless steel by electroless plating and electroplating. The films were uniform with the thickness about 0.5–2 lm. The electroless plated palladium film was mainly consisted of palladium, phosphorus and nitrogen, and the electroplated palladium film was almost pure palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples by both electroplating and electroless plating methods showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling 20% dilute sulfuric acid solutions corrosion rates of the palladium plated 316L stainless steel samples were four orders of magnitude lower than that of the original 316L stainless steel samples. When the solution contained 0.01 M NaCl, the palladium plated samples also showed better corrosion resistance. In comparison, the electroplated samples showed better corrosion resistance than electroless plated samples, which may be attributed to less impurities and thereby higher corrosion potential for the former.
electroplating
References
electroless plating
1200
Z'' (Ω •cm2)
120 1.2–1.5 0.203
1.0
1000 800 600
0.5
400 200
0.0
0 0.0
-200 0
500
1000
0.5
1500
2000
1.0
2500
3000
2
Z' (Ω •cm ) Fig. 5. Electrochemical impedance spectrum of 316L stainless steel samples with and without Pd films in boiling 20% H2SO4 solution.
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[15] J. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Co., 1992. [16] M. Brun, A. Berthet, J.C. Bertolini, J. Electron. Spectrosc. Relat. Phenom. 104 (1999) 55. [17] G. Schoen, J. Electron Spectrosc. Relat. Phenom. 1 (1972) 377. [18] H.M. Wang, J.F. Deng, S.Z. Dong, Y.M. Qiu Chin, J. Chem. Phys. 4 (1991) 59.
[19] H. Gabasch, W. Unterberger, K. Hayek, B. KlÖtzer, E. Kleimenov, D. Teschner, S. Zafeiratos, M. Hävecker, A. Knop-Gericke, R. SchlÖgl, J.Y. Han, F.H. Ribeiro, B. Aszalos-Kiss, T. Curtin, D. Zemlyanov, Surf. Sci. 600 (2006) 2980. [20] C.E. Myers, H.F. Franzen, J.W. Anderegg, Inorg. Chem. 24 (1985) 1822. [21] V.I. Nefedov, I.A. Zakharova, I.I. Moiseev, M.A. Porai-Koshits, M.N. Vargoftik, A.P. Belov, Zh. Neorg. Khimii. 18 (1973) 3264.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Study on corrosion resistance of palladium films on 316L stainless steel by electroplating and electroless plating Junlei Tang, Yu Zuo * 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 27 March 2008 Accepted 18 July 2008 Available online 29 July 2008 Keywords: A. Stainless steel C. Acid corrosion C. Passivity
a b s t r a c t Palladium films with good adhesive strength were deposited on 316L stainless steel by electroless plating and electroplating. Scanning electronic microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, weight loss tests and electrochemical methods were used to study the properties of the films. The electroless plated palladium film mainly consisted of palladium, phosphorus and nitrogen, and the electroplated palladium film was almost pure palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples prepared by both methods showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling 20% dilute sulfuric acid solution, the corrosion rates of the palladium plated 316L stainless steel samples were four orders of magnitude lower than that of the original 316L stainless steel samples. In the solution with 0.01 M NaCl, the palladium plated samples also showed better corrosion resistance. In comparison, the electroplated samples showed slightly better corrosion resistance than electroless plated samples, which may be attributed to less impurities and thereby higher corrosion potential for the former. Ó 2008 Published by Elsevier Ltd.
1. Introduction It is well known that stainless steels owe their corrosion resistance largely to the formation of passive films on the alloy surface. Stainless steels show good corrosion resistance in oxidizing corrosive mediums where the passive films formed on the surface are stable. However, in many reductive corrosion mediums such as boiling dilute sulfuric acid solutions or boiling acetic plus formic acids, passivity can not be steadily established on the surface and active corrosion happens for stainless steels. For the alloys with passive ability such as titanium alloys or stainless steels, if corrosion potential of the alloy is raised from active potential into passive region by applied anodic current or by alloying with elements with higher oxidation/reduction potentials, corrosion resistance would be improved. Different techniques were reported for palladium deposition on titanium, including vacuum evaporation, ion beam mixing, etc. [1]. For example, palladium deposition on titanium surface by ion beam mixing may effectively improve corrosion resistance of titanium in reductive acidic solutions [2–5]. There were also some reports on deposition of palladium on stainless steels. However, almost all the studies were aimed to prepare catalytic membrane reactors [6–8], since palladium film is permeable for hydrogen and catalytically active to many hydrogen-involved reactions. Hughes and co-workers [9] studied the method to deposit a Pd composite membrane on porous stainless steel tube * Corresponding author. Tel.: +86 10 64423795; fax: +86 10 64434908. E-mail address: [email protected] (Y. Zuo). 0010-938X/$ - see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.corsci.2008.07.014
by electroless plating. The membrane possessed high hydrogen selectivity and thermal stability at high temperature. Matsumura and co-workers [10–12] also prepared Pd membrane on porous stainless steel tube by electroless plating. Shi et al. [13] studied the initial formation of palladium membrane on a porous stainless steel substrate by electroless method and the morphology of pores in the membrane was examined. The effects of surface activity, defects and mass transfer on hydrogen permeation in palladium-porous stainless steel membrane were studied by Guazzene et al. [14]. In this paper, palladium deposition on 316L stainless steel was carried out by both electroless plating and electroplating, and corrosion behaviors of the Pd films/stainless steel in boiling 20% H2SO4 solution were studied. The palladium films on the surface of 316L stainless steel improved passive ability and corrosion resistance of the stainless steel, which were due to a positive shift of corrosion potential. The difference in composition and corrosion resistance for the palladium films produced by electroless plating and electroplating were investigated. 2. Experimental methods The testing material was rolled 316L stainless steel which was solution annealed and quenched, with a thickness of 3 mm. The steel composition was 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%, Fe% Bal. The samples were cut to the size of 20 10 mm, finished with abrasive papers up to 1000#, then were decreased first in alcohol then in a basic solution (Na2CO3: 30 g/L, Na3PO4: 30 g/L, NaOH:
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50 g/L) at 70 °C for 10 min, followed by acidic etching in 20% H2SO4 solution at 80 °C for about 30 s (electroless plating) or at 65 °C for about 5 min (electroplating), and rinsed in deionized water. The electroless plating condition was as follows: PdCl2: 2 g/L, NH3 H2O (28%): 150 ml/L, HCl: 4 ml/L, NH4Cl: 27 g/L, NaH2PO2: 12 g/L, temperature: 50 °C, pH 9.5–10, time: 30–60 min. The thickness of the electroless plated palladium film was about 0.5–1 lm. The electroplating condition was as follows: PdCl2: 15 g/L, NH3 H2O (28%): 15 ml/L, NH4Cl: 50 g/L, (NH4)2PO4: 50 g/L, current density: 1 A/dm2, temperature: 40 °C, pH 7–8, time: 3 min. The thickness of the electroplated palladium film was about 2–3 lm. Energy dispersive X-ray spectroscopy (EDX) and X-rays photoelectron spectroscopy (XPS) were used to analyze the composition of the films. The film surface was observed by using scanning electronic microscope (SEM). The porosity of the film was measured by following method: A filter paper (area 10 mm 10 mm) was wetted by the solution of K3Fe(CN)6 (10 g/L) and NH4Cl (30 g/L), adsorbed on the film surface and kept for 15 min, then was dried on a clean glass. The blue spots on the dried filter paper were counted. For each sample five tests in different areas were conducted then the average value was taken. Micro-hardness of the film was measured by a HXS-1000A digital micro-hardness tester. The film thickness was measured with SEM according to the crosssection images of the samples. The adhesive strength of the film to stainless steel matrix 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. Corrosion behaviors of the palladium plated samples in 20% sulfuric acid solutions were studied by polarization tests, electrochemical impedance tests and weight loss tests at boiling temperature. The samples were sealed with phenolic resin, leaving an area of 0.5 cm2 exposed to the solution. Polarization measurements were performed with an EG & G model 273A potentiostat. The sample was first polarized from the open circuit potential to a cathodic potential which was 100 mV negative to the open circuit potential, then was polarized in the positive direction with a potential scanning rate of 1 mV/s. A saturated calomel electrode (SCE) was used as the reference electrode, and the counter electrode was platinum. Because the testing temperature was relatively high, the potential was measured by an external calomel electrode coupled to the cell through a Luggin capillary. Electrochemical impedance spectroscopy (EIS) measurements were performed with a Model 5210 lock in amplifier connected to the cell via a Model 273A potentiostat, at the open circuit potential with a 10 mV perturbation. The frequency was ranged from 100 kHz to 10 mHz. All the reagents used were analytic grade, and the solutions were open to the air.
3. Results and discussion 3.1. Features of the palladium films deposited by electroless plating and electroplating Smooth surface was observed for the palladium films on stainless steel. The electroless plated film showed black color and the electroplated film showed gray color. For the plating time of 1 h, the obtained electroless plated palladium film was about 1 lm in thickness. For plating time of 2 min, the obtained electroplating palladium film was about 2 lm. Fig. 1 shows surface morphology of the two films under SEM. Because the plated palladium films were very thin, the traces left by surface finishing were still discernible. Both the films were continuous, compact and covered the surface well. The measured results for porosity of the two films (Table 1) indicate that the porosity of the film by electroless plating was lower.
Fig. 1. The surface views of the Pd films under SEM: (a) electroless plating, (b) electroplating.
Table 1 Porosity testing results for the two films Electroless plated film
Electroplated film
0.75 sopt/cm2
2 sopt/cm2
Table 2 shows the measured micro-hardness values of the palladium plated samples. Because the films were very thin, the measured values could not reflect the hardness of the palladium films exactly. However, the result indicated that the electroplated film had higher hardness than electroless plated film. The adhesive strength was examined by cross–cut tape test according to ASTM-D3359-90 (Standard Test Method for Film Adhesion by Cross–Cut Tape Test), and the adhesion of both the films to stainless steel reached 4B level, which means the adhesion was quite good. The compositions of the plated palladium films were measured with EDX. For electroless plated film, Pd, P, Fe, Cr and Ni were clearly observed on the spectrum plot as shown in Fig. 2. However, the existence of Fe, Cr and Ni were due to the stainless steel matrix since the film thickness is less than 1 lm. The electroplated film was mainly composed of palladium. XPS analysis was carried out to further understand the states of the elements in the films. Fig. 3a shows the wide scan XPS spectra on surface of the palladium plated films. The binding energy (BE) values and the atomic percentages for the elements are shown in Table 3. It is seen that there is some difference between compositions of electroless plated film and electroplated film. The electroplated film surface was mainly composed of palladium, oxygen, and carbon which
J. Tang, Y. Zuo / Corrosion Science 50 (2008) 2873–2878
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Table 2 Micro-hardness testing result 316L SS matrix
Electroless plating
Electroplating
245 HV
219 HV
270 HV
Fig. 2. The composition analysis of Pd plated stainless steel by EDX: (a) electroless plating, (b) electroplating.
was added for calibration of the spectrum. It was reported [15] that for metallic palladium, binding energy values of the spin-orbit couple peaks of Pd3d are 335.30 eV and 340.65 eV, respectively, which are very close to the measured values in Table 3. When Pd is oxidized, binding energy of Pd3d5/2 may shift from 335.4 eV to 336.8 eV [16]. Hence the results in Table 3 suggest that palladium in the surface film was present in metallic state. In addition, binding energy of Pd3p3/2 (532.4 eV) [17] is close to that of O1s which was reported near 531.8 eV when O2 adsorbed on Pd–Ag film [18]. Thus the peak around 532 eV in Fig. 3a should be the overlapped result of Pd3p3/2 peak and O1s peak. Fig. 3b shows the deconvolution of the peaks for the electroplated palladium film, where oxygen should come from the adsorbed oxygen on the surface because the BE values for O1s in PdxOy was reported as 529.65– 530.38 eV [19], obviously different from the measured values. The electroless plated film was composed of Pd, O, P, N and C. It is seen in Table 3 and Fig. 3c that the binding energy values of both Pd and O are almost the same as the values for electroplated film, indicating that palladium was present in the same state in the films prepared by electroplating or electroless plating. Phosphorus and nitrogen in electroless plated film were due to the effect of NaH2PO2 in the plating bath. The measured BE of P in the film is 133.85 eV which is much higher than that of pure phosphorus (129.9 eV) [20]. So the element P is present in the film in an oxi-
Fig. 3. XPS analysis of the Pd film surface: (a) survey, (b) Pd3p3/2 and O1s of the electroplated palladium film, (c) Pd3p3/2 and O1s of the electroless plated palladium film.
dized state. The measured BE for N is 399.92 eV, very close to the value of N in Pd(NH3)4Cl2 (400.1 eV) [21] which was present in the electroless plating solution. 3.2. Corrosion resistance of the palladium plated samples 3.2.1. Weight loss tests Table 4 shows weight loss testing results of the palladium plated stainless steel samples in boiling 20% H2SO4 and boiling 20% H2SO4 + 0.01 M NaCl solutions. The result for original 316L stainless steel samples is also shown as comparison. For 316L stainless steel samples, as soon as the samples were immersed in the solution, severe corrosion happened and a large amount of bubbles was observed on the surface. The sample size diminished obviously after only 2 h of immersion. While little weight losses were mea-
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Table 3 Measured binding energy values and atom% of the elements in films by XPS Name
C1s
O1s
Electroless plated sample
Electroplated sample
P2p3/2
Pd3d
N1s
C1s
O1s
Pd3d
Peak BE (eV) At.%
284.99 45.88
531.651 8.11
133.85 2.52
335.59, 340.88 34.72
399.92 8.78
285. 71.62
531.651 4.05
335.63, 340.88 24.33
Table 4 Weight loss testing results for the samples Sample
Time (h)
Corrosion rate in boiling 20% H2SO4 (g m2 h1)
Corrosion rate in boiling 20% H2SO4 + 0.01 MCl (g m2 h1)
Corrosion phenomena
316L stainless steel with electroless plated Pd film (1 lm)
240
0.014
0.020
Intact film
316L stainless steel with electroplated Pd film (1 lm)
240
0.008
–
Intact film
316L stainless steel with electroplated Pd film (2 lm)
240
0.007
0.011
Intact film
Original 316L stainless steel
2
288.4
549.6
Severe corrosion
a
1.2 1.0 0.8
E (VSCE)
sured for both palladium plated samples after 240 h of immersion. The electroplated sample showed lower corrosion rate than the electroless plated sample in the testing solutions both with and without Cl. Longer term immersion tests were carried out for the palladium plated samples in boiling 20% H2SO4 solutions with or without 0.01 M NaCl up to 25 days. Both the electroless plated and electroplated samples showed little corrosion after the 25 days of immersion, indicating quite good performance in the strong corrosive mediums.
1. 316L blank 2. electroless plating 3. elrctroplating
0.6
2
3
0.4 0.2 0.0
3.3. Polarization measurements
1
-0.2 -0.4 10-8
10-7
10-6
10-5
10-4
10-3
10-2
i ( A/cm2)
b
1.2 1.0 0.8
E (VSCE)
Fig. 4a shows polarization curves of 316L stainless steel samples with and without Pd films in boiling 20% H2SO4 solutions (102 °C). For 316L stainless steel, measured corrosion potential was about 0.2 V, at which the corresponding corrosion current density was quite high and active dissolution happened for the steel without films. However, for Pd plated samples, corrosion potentials were greatly raised to passive region, and stable passivation was obtained for both Pd plated samples. Corrosion potential of the electroplated sample is about 0.4 V, more positive than that of the electroless plated sample (about 0.2 V). The anodic current densities for Pd plated samples were three orders of magnitude lower than original stainless steel. For curve 2 and curve 3 in Fig. 4a, a current step was observed at higher potential range, which may be explained by oxidation of palladium (Pd/Pd2+ equilibrium potential + 0.743 VSCE). Fig. 4b shows the influence of chloride ions on polarization behaviors of 316L stainless steel samples with and without Pd films in 20% H2SO4 solutions. Chloride ions effectively damaged passivation of stainless steels. With addition of 0.01 M NaCl to H2SO4 solution, for all the samples the anodic current densities tended to increase. For 316L stainless steel without Pd films, the anodic current density in the active region showed a remarkable increase due to chlorides. However, for palladium plated samples the corrosion potentials maintained almost the same as they were in the solution without Cl. This result means that in corrosive solutions with trace Cl, the plated palladium films still raised corrosion resistance of 316L stainless steel. This is reasonable because the enhanced passivation by palladium film would make the stainless steel more resistant to trace chloride ions. However, higher chloride concentration still can damage corrosion resistance of the palladium plated samples. It is seen that in Fig. 4b for both electroless plated and electroplated samples there are current increases on the polarization curves which may be due
1. 316L blank 2. electroless plating 3. elrctroplating
0.6
3
2
0.4 0.2
1
0.0 -0.2 -0.4 -0.6 10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
2
i (A/cm ) Fig. 4. Polarization curves of 316L stainless steel samples with and without Pd films in: (a) boiling 20% H2SO4 solution, (b) boiling 20% H2SO4 + 0.01 M NaCl solution.
to the reaction between palladium and chlorides: Pd þ 4Cl ! 2 PdCl4 þ 2e (The reaction equilibrium potential is +0.38 VSCE). Table 5 shows the variations of corrosion potentials of the electroless plated and electroplated samples with plating time in boiling 20% H2SO4 solution. With the plating time prolonged, corrosion potential of the electroless plated samples remained stable in certain range around 0.2 V, while that of electroplated samples increased from 0.17 V until reached 0.4 V. The higher porosity of
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J. Tang, Y. Zuo / Corrosion Science 50 (2008) 2873–2878 Table 5 Corrosion potentials of electroless plated and electroplated samples vs. plating time in boiling 20% H2SO4 solution Electroless plating Plating time (min) Thickness (lm) Corrosion potential (V)
30 <0.5 0.227
Electroplating 60 0.6–0.8 0.218
the electroplated film may be the reason for its more negative potential when the film was very thin. With the film thickened, the effect of porosity was diminished and the coupled potential between palladium film and the matrix would increase to the stable value. 3.4. Electrochemical impedance measurements Fig. 5 shows the measured electrochemical impedance spectra of 316L stainless steel samples with and without Pd films in 20% H2SO4 solution at open circuit potentials. It is seen that in 20% H2SO4 solution at 102 °C 316L stainless steel showed very small impedance, indicating that the steel was in active dissolution state. The palladium plated samples showed much higher impedance values. The impedance values for both electroplated and electroless plated samples increased by 3–4 orders of magnitude compared with the sample without Pd films. It is an indication that stable passivation was established on the surface of the samples. The results are quite consistent with those obtained from polarization tests. Palladium plating on stainless steel effectively improved passive ability of the steel in strong reductive corrosive environments. The porosity tests showed that the electroless plated film has lower porosity, but corrosion rate of the electroless plated samples is a little higher than that of the electroplated film. This result indicates that the increase of corrosion resistance by Pd plating on stainless steel does not rely on the barrier effect of the films. The standard oxidation/reduction potential of Pd/Pd2+ is +0.987 VSHE or +0.743 VSCE, much higher than that of stainless steels. As a cathodic film, the function of palladium film to improve corrosion resistance of stainless steel is mainly due to its effect of promoting passivation. In strong reductive corrosion mediums like boiling sulfuric acid solutions, the passive film on 316L stainless steel is not stable and active dissolution happens at open circuit potential. However, the polarization curves in Fig. 4 show that passivation is possible at higher potential range. With palladium films, the open
1800 1600 1400
0.5 0.5 0.174
1 1 0.340
2 2 0.40
circuit potentials of stainless steel in boiling sulfuric acid solutions were raised to higher potential range where passivation was again established on the surface. The measured corrosion potential may more or less reflect the stability of passivation under the experimental conditions. The samples with electroless plated Pd film showed more negative corrosion potential and slightly higher corrosion rate than the samples with electroplated Pd films, which should be attributed to the presence of phosphorus and nitrogen in the film. The almost pure palladium film without impurities by electroplating may lead the stainless steel surface to higher open circuit potential and relatively higher passive ability. Another advantage of electroplating is the plating time needed for the same film thickness is much shorter than electroless plating. Both of the palladium plating techniques may be used to improve corrosion resistance of stainless steels in strong reductive corrosion mediums. 4. Conclusions
316L blank
Palladium films with good adhesive strength were deposited on 316L stainless steel by electroless plating and electroplating. The films were uniform with the thickness about 0.5–2 lm. The electroless plated palladium film was mainly consisted of palladium, phosphorus and nitrogen, and the electroplated palladium film was almost pure palladium. XPS analysis indicated that palladium was present in the films as metal state. The palladium plated stainless steel samples by both electroplating and electroless plating methods showed excellent corrosion resistance in strong reductive corrosion mediums. In boiling 20% dilute sulfuric acid solutions corrosion rates of the palladium plated 316L stainless steel samples were four orders of magnitude lower than that of the original 316L stainless steel samples. When the solution contained 0.01 M NaCl, the palladium plated samples also showed better corrosion resistance. In comparison, the electroplated samples showed better corrosion resistance than electroless plated samples, which may be attributed to less impurities and thereby higher corrosion potential for the former.
electroplating
References
electroless plating
1200
Z'' (Ω •cm2)
120 1.2–1.5 0.203
1.0
1000 800 600
0.5
400 200
0.0
0 0.0
-200 0
500
1000
0.5
1500
2000
1.0
2500
3000
2
Z' (Ω •cm ) Fig. 5. Electrochemical impedance spectrum of 316L stainless steel samples with and without Pd films in boiling 20% H2SO4 solution.
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