Journal of The Electrochemical Society, 146 (6) 2107-2112 (1999) 2107 S0013-4651(98)01-077-5 CCC: $7.00 © The Electrochemical Society, Inc. Hydrogen damage is a severe problem that refers to the degrada- tion of mechanical properties of a metal caused by the presence of, or interaction with, hydrogen. The reduction of hydrogen ions is a source of atomic hydrogen. Hence, corrosion reactions, the applica- tion of cathodic protection and electroplating, etc., are major sources for introducing hydrogen into metals. During these hydrogen-gener- ating processes, some of the reduced hydrogen may be absorbed on the metal surface and diffuse into the metals. 1-5 The presence of hydrogen in metals has a significant influence on the mechanical properties and electrochemical behavior of metals. For instance, increased hydrogen content results in the low stress fracture and plastic reduction of steels. 6,7 Dissolution of hydrogen in metals was proven to affect the general corrosion of metals 1,8 and to induce stress corrosion cracking 9,10 and pitting corrosion. 11,12 It is known that the nature of passive films on metals and alloys is an ultimate factor which controls their corrosion behavior. 13,14 Therefore, knowledge about electronic structure of the passive film is required for better understanding of the corrosion process. For this reason, much work has been devoted to the study of the electronic properties and chemical composites of passive films on metals and alloys. 13,15-19 Most passive films formed on stainless steels behave like semiconductors. Based on the fact that the potential-dependent transpassive dissolution varies with the electronic properties of the passive film, Sato 20 proposed a breakdown mechanism of the pas- sive film on metals: the electrochemical stability of a passive film strongly depends on the electron energy band structure in the film. Bohni, et al. 21 studied the semiconductive properties of the passive films on 304 SS using Mott-Schottky analysis and photoelectro- chemical methods. They found the coincidence of the onset potential for pit nucleation with the flatband potential of passive film. Bianchi et al. 14 concluded that the high susceptibility of stainless steel to pit- ting nucleation was connected to n-type conductivity of the oxide film. Nevertheless, the reason for the existence of this correlation is still not clear, and little research has been done to investigate the effect of hydrogen dissolved in metals on electronic structures of passive films on metals. 22 In this work, the new experimental results on the reversion of the electronic properties of passive film induced by hydrogen dissolved in AISI 310 SS were presented. The experimental results were ana- lyzed by taking into account the susceptibility of specimens to pit- ting corrosion with or without hydrogen and were interpreted by the band model of different conductivity types of passive films. Experimental The test material was commercial 310 SS foil, 0.01 cm thick. The main composition (wt %) was 23.5 Cr, 18.4 Ni, 1.76 Mn, 0.70 Si, 0.15 Mo, 0.16 Cu, 0.09 Ti, 0.02 Nb, 0.06 C, 0.02 P, 0.02 S, and Fe in balance. The solution for hydrogen charging was 0.5 M H 2 SO 4 + 250 ppm As 2 O 3 . The solution for the impedance and polarization measurements was borate buffer solution (0.02 M H 3 BO 3 + 0.005 M Na 2 B 4 O 7 10 H 2 O, pH 8.45). Noise resistance was meas- ured in the 0.02 M H 3 BO 3 + 0.005 M Na 2 BO 4 O 7 10 H 2 O + 0.6% FeCl 3 solution (pH 8.45). The polarization curves for determining the anodic and cathodic transfer coefficients were performed in the 0.02 M H 3 BO 3 + 0.005 M Na 2 B 4 O 7 10 H 2 O solution containing 1 mM Fe(CN) 6 3- + 1 mM Fe(CN) 6 4- . All solutions were prepared with deionized water and analytical grade reagents. The samples with various hydrogen contents were prepared by galvanostatic polarization at various cathodic current densities for different charging times. Before prepassivation or polarization meas- urements, all the charged and uncharged samples were ground with 600 grit SiC grinding papers and then rinsed successively with deionized water and ethanol. Prior to measuring polarization diagrams, both uncharged and charged specimens were polarized cathodically in testing solution at the potential -300 mV vs. the corrosion potential for 5 min to reduce the air-formed passive films. Prior to measuring the imped- ance, transfer coefficients, and electrochemical noise resistance, all of the working electrodes were prepassivated at 0.7 V for 2 h. The high potential of 0.7 V was chosen as the passivation potential because (i) prepassivation should be carried out at the highest poten- tial in the potential range for Mott-Schottky plots to reduce the influ- ence of the film-forming current and faradaic process to the imped- ance signals, and (ii) the Mott-Schottky plots have to be detected over a wide potential range in order to determine accurately the straight line and the slope of the line. The amount of hydrogen released from the charged specimens after passivation was measured under the condition of mechanical vacuum by using a U-tube filled with mercury. For measurements of impedance and polarization curves, a three- electrode cell was used which included a saturated calomel electrode Effects of Hydrogen on Semiconductivity of Passive Films and Corrosion Behavior of 310 Stainless Steel M. Z. Yang, a J. L. Luo, a, * Q. Yang, a L. J. Qiao, a Z. Q. Qin, b and P. R. Norton b a Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6 b Department of Chemistry and Interface Science, Western University of Western Ontario, London, Ontario, Canada N6A 5B7 The effects of hydrogen on the semiconductive properties and compositions of passive films on AISI 310 stainless steel (310 SS) and their corrosion behavior were investigated by Mott-Schottky plot, polarization, noise resistance analyses, and secondary ion mass spectrometry (SIMS). The results indicate that the susceptibility of 310 SS to pitting is strongly influenced by hydrogen pre- sent in the specimens. The conductivity type of the passive film formed at 0.7 V vs. saturated calomel electrode on the specimens charged with hydrogen at the quantity higher than 3.6 C cm -2 is p-type while that formed on the uncharged specimen is n-type, i.e., the presence of hydrogen in 310 SS causes an inversion of conductivity type of a surface film from p-type to n-type. When the concentration of ionized hydrogen in the passive film on the 310 SS, i.e., the donor density reaches the saturated value, the aver- age content of the diffusible hydrogen released from a specimen measured by a U-tube filled with mercury under the condition of mechanical vacuum is 3.8  10 20 cm -3 . The donor concentration evaluated from the Mott-Schottky slope is about in the order of 10 19 cm -3 . The susceptibility of 310 stainless steel (SS) to pitting can be correlated to the electronic properties and chemical com- positions of the passive film. The high susceptibility of the charged specimens to pitting corresponds to the n-type conductivity of the passive film. While low susceptibility is connected to p-type conductivity and high content of chromium and nickel oxides. Such a correlation can be explained with the band model of a passive film. © 1999 The Electrochemical Society. S0013-4651(98)01-077-5. All rights reserved. Manuscript submitted January 27, 1998; revised manuscript received March 1, 1999. * Electrochemical Society Active Member. z E-mail: jingli. luo@ualberta.ca Downloaded 01 Apr 2009 to 193.146.171.29. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp