Relation Between Phase Transition of Martensite Induced by Deformation and Pitting Susceptibility of 1Cr18Ni9Ti Stainless Steel in Acidic NaCl Solution
Wing-yan Ng
Hong Kong Polytechnic University
Hong Kong
email: bcwyng@hkpucc.polyu.edu.hk
Chunchun Xu Ruifen Xu and Weizhen Ouyang
Beijing University of Chemical Technology
Beijing, China
ABSTRACT
Electrochemical Impedance Spectroscopy (EIS), X-ray Photoelectron
Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) were
employed to study the effect of '-martensite induced
by deformation on pitting susceptibility of 1Cr18Ni9Ti Stainless
Steel in 0.5 M HCl + 0.05 M NaCl solution. EIS characterizes the
contribution of a'-martensite contents
and the development of pitting corrosion. When the content of
a'-martensite is less than 6% and more
than 22%, pitting corrosion of this material increases with the
increase of a'-martensite contents.
However, when the content is between 6 - 22%, pitting corrosion
decreases with the increase of a'-martensite.
XPS and AES studies on the film formed on the specimen confirmed
that when a'-martensite content is
less than 6%, the film formed is thinner, leading to reduced coverage
and inferior protective characteristics. When the a'-martensite
content is between 6% and 22%, the film is thicker with higher
chromium content. The film is more compact and possesses better
corrosion resistance. When the a'-martensite
content is higher than 22%, the chromium content of the film decreases,
thus lowering the protective properties of the film.
INTRODUCTION
The consequence of deformation-induced martensite on pitting corrosion of austenite stainless steel (SS) in acidic chloride solution has been the subject of studies recently. Sunada et al 1 correlated pitting corrosion and volume fraction of martensite-induced deformation of Type 304 SS in H2SO4-NaCl solution by electrochemical measurements coupled with optical microscopic observation. They showed that the number of pits formed bore direct relations with martensite contents. The immediate environment, such as NaCl concentration, temperature and applied electrical potential played an important role in inducing corrosion. Xu et al 2 studied the phase transition of martensite-induced deformation of Type 321 SS in NaCl solution by SEM, XRD and TEM. The part played by martensite on pitting susceptibility was also studied by electrochemical hysteresis technique and by linear polarization measurement. Results 2 showed that when the ferromagnetic phase (martensite phase) was below 4.6% and above 25.5%, pitting susceptibility increased. However, when the ferromagnetic phase was in the range of 4.6 - 25.5% pitting susceptibility decreased.
The contribution of deformation-induced martensite on pitting corrosion for austenitic SS in active corrosion state has not been fully understood. The present study reports the change of corrosion resistance of 1Cr18Ni9Ti (SUS 321) SS with a'-martensite induced by low temperature deformation in acidic NaCl solution. An attempt is made to correlate the change in corrosion behavior with the formation of martensite.
EXPERIMENTAL PROCEDURE
The compositions of the SUS 321 SS samples were determined as (wt %): Cr, 17.27; Ni, 10.34; C, 0.08; Si, 0.87; Mn, 0.77; S, 0.013; Ti, 0.60; and balance was Fe. The material was solution annealed at 1050oC for 30 min., followed with water quenching.
Low temperature (-70oC) elongation method was used to induce the test specimens with different martensite contents. The exposed test area of the specimens was 10 mm x 10 mm, which was polished with grade 800 emery paper prior to measurements.
EIS was employed for studying the effect of martensite-induced deformation on pitting susceptibility of the material in 0.5 M HCl + 0.05 M NaCl solution at 50oC. Measurements were made with PAR 398 Corrosion Measurement System at the corrosion potential of the specimen from 0.05 kHz to 100 kHz for specimens of martensite contents from 0.08% -32%.
Specimens of 3%, 22% and 30% martensite contents were prepared. They were immersed in 0.5 M HCl + 0.05 M NaCl at 50oC for 15 hours. The specimens were then analyzed with XPS and AES.
RESULTS AND DISCUSSIONS
EIS Measurements
EIS measurements were performed on the specimens with a range
of induced martensite contents of volume fraction of 0.08%, 0.1%,
6.3%, 11%, 18%, 20%, 22%. 25%, 27.5%, 29% and 32%. These measurements
were conducted in 0.5 M HCl + 0.05 M NaCl. Results are shown in
Fig.1.
The untreated sample, curve a in Fig.1A, with minimal amount of a'-martensite gives the characteristic semi-circle behavior. The increase of a'-martensite distorts and diminishes both the shape and the dimensions of the EIS spectra, as shown from curves a to curve c in Fig.1A. The trend is reversed when the a'-martensite contents in the specimens increase, as shown in curves c to g in Fig.1B. As the a'-martensite contents increase from 6.3% to 22%, EIS spectra expand. Again, the trend is reversed when a'-martensite contents in the specimens increase from 25% to 32%, as illustrated in curves k to h in Fig.1C. Its size decreases with a'-martensite content.
The distortions of high and low frequency regions signify the contributions of capacitance and inductance of the equivalent circuit 3 . In the low frequency regions, shrinkages of the real component of the impedance spectra as shown in curves b, c, k & j in Fig.1 characterize the occurrence of corroded products adsorbed on the metal surface that leads to pitting corrosion. Stains were clearly observed on the specimens after each measurement. These stains were visually similar to the stain and corroded products formation in preliminary prolonged immersion tests on similar deformation-induced specimens on which corrosion products were observed.
The shrinkages of the real component in the impedance spectra with the increase of a'-martensite contents at the low frequency end of the EIS spectra, as shown in curves b, c and k and the dispersion effect in the low frequency regions as shown in curves a, f & g , suggest the rapid changes and instabilities of the charges transfer resistance and associated electrochemical parameters of its equivalent circuit during corrosion. The occurrence of defects in the lath-type a'-martensite as well as its anodic behavior generates nonuniformity of the surface film. During corrosion, adsorption of chloride preferentially occurs on a'-martensite sites. Dissolution of metal chlorides leads to the deterioration of the film and the decrease in charge transfer resistance. A sequence of events may take place viz. anodic dissolution - film formation - dissolution - film restoration..... , leading to pitting corrosion. These erratic changes associated with variations in charge transfer resistances contributes to shrinkages and dispersions of the real component of the impedance spectra.
Fig. 2 shows the relation between polarization resistance, Rp , charge transfer resistance, Rt , and the interfacial capacity, Cd obtained from these EIS measurements with varying a'-martensite contents. When the a'-martensite content is in the region of 0 - 6% and above 22% Rt and Rp decrease with the increase of a'-martensite, but Cd increases. This is an indication of active dissolution of metal i.e. the protective power of the surface film has decreased. Active-passive transitions occur at 6% and at 22% a'-martensite contents. The surface film formed on the metal thickens, i.e. its protective power increases. Pitting corrosion susceptibility of the material decreases with the increase of a'-martensite contents.
EIS characterizes the contributions from a'-martensite contents and the development of pitting corrosion. Austinite-martensite transformation of austenitic SS also possesses plastic deformation, giving defects such as dislocation, vacancy etc. in the martensite phase. The difference in the austenite phase and the martensite phase in metallographic structure causes electrochemical nonuniformity of the material 4 . These phases form a galvanic cell. The martensite phase constitutes the anode and is prone to selective corrosion. It nucleates to a small area and propagates to pit corrosion.. Because of the amount of chloride adsorption on the metal surface coupled with the protective characteristics of the film formed on austenite, pitting corrosion susceptibility of this material is directly related with a'-martensite contents.
Fig. 1 Impedance spectra for 321 stainless steel with different ferromangetic phase contents in 0.5 M NaCl + 0.05 M HCl solution, t = 55o C.
Surface Film Analysis
XPS and AES analysis give further information on the state of corrosion of the surface film. The corroded products are adsorbed on the metal surface. These analytical techniques confirmed the presence of Fe2+, Fe2O3 , FeOOH- ,Cr2O3 , Ni2O3 , Ni2+ and Cl- species. On the basis of XPS and AES analysis on metal dissolution processes, Zhang 5 proposed that during the dissolution of Grade 304 SS in a solution of HCl + NaCl, the adsorbed hydroxyl (MOH)ads and chloride (MCl)ads species as well as the hydroxyl-chloride composite (MOHCl)com of Fe, Cr and Ni were involved (M refers to either one of these metals). When the contents of a'-martensite is less than 6% , the film formed contains only the chloride of Fe, Cr and Ni. The thickness of the film is less than 80 Å. This thin film provides inferior protective power. When the a'-martensite content is in the range of 6 - 22%, the film is much thicker with higher chromium content. The film is more compact and possesses much better corrosion resistance. However, when the a'-martensite content is higher than 22%, the chromium content of the film decreases and there is an enrichment of chloride content. These changes lower the protective properties of the film. Results from surface analysis of the film are consistent with those obtained from EIS measurements.
CONCLUSION
ACKNOWLEDGEMENT
The authors would like to thank the Chinese Natural Sciences Foundation,
State Key Laboratory of Metal Corrosion and Protection and the
Hong Kong Polytechnic University for financial support
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