XPS analysis of the passive film formed on austenitic stainless steel coated with conductive polymer

Improvement of the passivation behavior of Type 304 austenitic stainless steel (SS) by coating with conductive polymers (CPs), like polyaniline (PANI) and poly(o-phenylenediamine) (PoPD), followed by exposure in an acid solution has been demonstrated. The passive films formed on SSs (after peeling off the polymer layer) are compared with those formed during anodic polarization under the same exposure condition. The passive films beneath the CPs are thicker and less hydrated than those formed on uncoated stainless steel. The polymer layer enhances the enrichment of chromium and nickel in the entire passive oxide, forming a more protective film than that formed during anodic polarization. The elemental distribution within the passive film is different in the two modes of passivation. The type of the polymer influences on the composition of the passive film. The best passivation is obtained by PoPD, with the passive film resulting in significant resistance of the SS to pitting corrosion in the 3% NaCl solution. The oxide film of this steel is characterized, in its inner and outer layers, by the highest ratio of Cr(OH)₃/Cr₂O₃ and the lowest content of iron species.     

Laser Raman microscopic studies of passive films formed on type 316LN stainless steels during pitting in chloride solution

The surface films formed on type 316LN stainless steels (SS) with different nitrogen contents, during potentiodynamic polarization in acidified 1 M NaCl solution, were characterized by Laser Raman Spectroscopy (LRS). LRS confirmed the presence of oxides and oxychlorides of iron and chromium, hydrated chlorides and nitrates in the film. Raman mapping showed increasing nitrate content in the film with increasing nitrogen content. The film on the uncorroded material showed the presence of chromium and molybdenum oxides. The improvement in pitting corrosion resistance of type 316LN SS with increasing nitrogen content was attributed to increased amount of nitrates in the passive film.     

Real time pit initiation studies on stainless steels: The effect of sulphide inclusions

It has long been accepted that manganese sulphide favours pitting on stainless steels. However, there are different standpoints on the most important mechanism for pit initiation; due to dissolution of sulphide inclusions, chromium depletion around the inclusion or mechanical rupture of the passive film by metal chlorides. Analysing the pitting potential and metastable pitting rates on different grades of stainless steels has rationalised the effect of sulphide content on pitting corrosion resistance. In situ atomic force microscopy (AFM) has been used in conjunction with conventional electrochemical techniques for imaging real time pit initiation events.     

XPS analysis of manganese in stainless steel passive films on 1.4432 and the lean duplex 1.4162

Passive films were compared on two stainless steels: the recent lean duplex EN 1.4162 and EN 1.4432 (316L). For alloys with significant amount of manganese and nickel, the Mn 2p_3/2 peak will overlap with the Ni-LMM. To resolve this overlap, Ni 2p_3/2 to Ni-LMM intensity ratios were recorded on 1.4432, compensated for overlayer thickness, and then used to fix the Ni-LMM intensities in the Mn 2p spectra on the duplex material. Manganese was found in oxidation states II and V/VI; its film content was not dependent on the bulk composition.     

Effects of temperature on the oxide film properties of 304 stainless steel in high temperature lithium borate buffer solution

The paper mainly investigated the effects of temperature on the oxide film properties of 304SS in lithium borate buffer solution by electrochemical measurements and XPS analysis. As temperature increased, the protective property of the film degraded and structure varied from a single layer to a double-layer. Whatever the temperature, the oxide film exhibited an n-type and p-type semiconductor in the potential range above and below the flat band potential, respectively. The electronic properties were assigned to a Fe–Cr spinel inner layer and a defective Fe–Cr oxide outer layer. The related growth mechanisms of the oxide film were also discussed.     

Effect of nickel alloying on crevice corrosion resistance of stainless steels

The crevice corrosion behaviour of stainless steels containing 25 mass% Cr, 3 mass% Mo and various amounts of Ni was investigated in natural seawater. The results showed that ferritic steels containing nickel were more resistant to corrosion than both ferritic steels without nickel and austenitic steels. The superiority of the Ni bearing ferritic steel over the other steels was in close agreement with the depassivation pH of those steels in acidic chloride solutions. The results showed that the addition of Ni to ferritic steel was effective in decreasing the depassivation pH and the dissolution rate in acidic chloride solutions at crevices.     

Electrochemical behaviour of high nitrogen stainless steel in acidic solutions

The electrochemical behaviour of high nitrogen stainless steel in acidic solutions was studied by potentiodynamic polarization, EIS, Mott-Schottky and XPS. The passive film formed in neutral NaCl solution was very stable, but the stability of the film decreased with the addition of H₂SO₄ into the solution. The passive film formed in acidic Na₂SO₄ has a superior protective ability than that in acidic NaCl solution. The stability of the film formed in tested solution decreased with increase of applied potentials. The film formed on steel surface was of n-type semiconductor. Chloride penetration mechanism was proposed for the observed passive film breakdown.     

Importance of initial surface film in the degradation of stainless steels by atmospheric exposure

The surface compositions of stainless steels types 304, 316, 430, and 444 combined with four types of surface finishes, 2B finish, hairline polishing, mirror polishing, and bright annealing, were measured by ICP-AES, EPMA, and XPS before exposure. The surface finish that most enriched the chromic species in the surface film was mirror polishing, followed by bright annealing, 2B finish, and hairline polishing. The order of corrosion-resistance was type 444, type 316, type 304, and type 430. The surface compositions were correlated with the rating number and pitting depth after exposure to atmospheric environments. The rating number had a high positive correlation with the concentration of Cr in the surface film, and had a slight correlation with the near-surface composition measured by EPMA at 12 kV, but did not show any correlation with bulk composition within the composition range covered in the present work. This same trend was observed for pitting depth. It was concluded that the cationic concentration of Cr in the surface film before atmospheric exposure is the prime factor in controlling the resistance of stainless steels to atmospheric corrosion.     

Corrosion behaviour of low temperature plasma carburised 316L stainless steel in chloride containing solutions

The electrochemical corrosion behaviour of the carburised (expanded austenite) layer on 316L austenitic stainless steel produced by low temperature plasma carburising has been studied in 0.5 M NaCl and 0.5 M HCl + 0.5 M NaCl solutions. The present work focuses on the variation of the corrosion behaviour of the carburised layer with depth from the surface and the effect of carbon concentration on electrochemical behaviour. The results show that the carburised layer has excellent resistance to localised corrosion. There exists a critical carbon concentration, above which the expanded austenite possesses excellent resistance to both metastable pit formation and pit growth.     

Electrochemical behavior of sintered oxide dispersion strengthened stainless steels

The electrochemical behavior of powder metallurgy (P/M) oxide dispersion strengthened stainless steels (SS) (316L and 434L) have been compared with standard 430 and 316 wrought samples in 0.05 mol/l sulfuric acid. The effects of sintering temperature and yttria addition on the electrochemical behavior have been studied. The behaviour of the dispersion strengthened SS was comparable to that of the straight P/M samples. The straight P/M samples sintered at 1400 °C exhibited better corrosion resistance compared to the samples sintered at 1250 °C and this has been correlated to sintered densities. The P/M austenitic SS were superior to the P/M ferritic SS. Pitting resistance, studied by cyclic polarization experiments in 3.56 wt.% NaCl, of the P/M samples were comparable to the wrought samples. The addition of Y₂O₃ did not affect the pitting resistance.     

Environmental factors affecting pitting corrosion of type 304 stainless steel investigated by electrochemical noise measurements under potentiostatic control

Electrochemical noise measurements on anodically polarised type 304 stainless steel surfaces in contact with buffer solutions of neutral pH were performed to study the effect of chloride ions in the nucleation of pitting corrosion. Passive layer stability and susceptibility to pitting corrosion after pickling and passivation at different environmental conditions were also investigated by means of electrochemical current noise measurements under cathodic and anodic polarisation. According to the obtained experimental results pits nucleate independently on the presence of chloride ions. It has been also shown that protectiveness of stainless steel surfaces after pickling strongly depends on the relative humidity of the environment in which the surface is subsequently passivated.     

Hydrogen effects on the passive film formation and pitting susceptibility of nitrogen containing type 316L stainless steels

The effects of hydrogen on the passivity and pitting susceptibility of type 316L stainless steels have been investigated with alloys containing different nitrogen contents (0.015, 0.198 and 0.556 wt.% N). The study revealed that electrochemically pre-charged hydrogen significantly reduced the pitting resistance of alloys conatining 0.015 and 0.198 wt.% nitrogen contents. In alloy with highest nitrogen content (0.556 wt.% N), an increase in the passive film current density with hydrogen was observed without affecting breakdown potential. Auger electron spectroscopy (AES) analysis of the passive film indicated the presence of nitrogen in the passive film. On other hand, for hydrogen charged samples, nitrogen was found to be significantly less in the passive film. In Electrochemical impedance spectroscopy (EIS) measurement, the decrease in semi-circle radius of Nyquist plot, and the polarization resistance, R_P associated with the resistance of the passive film was observed with hydrogen, indicating that hydrogen decreased the stability of the passive film. The present investigation indicated that precharged hydrogen deteriorated the passive film stability and pitting corrosion resistance in these alloys, and the increase in nitrogen content of the alloy offsets the deleterious effect of precharged hydrogen.     

XPS studies on passive film formed on stainless steel in a high-temperature and high-pressure methanol solution containing chloride ions

The composition and structure of passive film formed on 316L SS immersed in an anhydrous methanol solution (water content < 0.05 wt%) containing 0.42 wt% LiCl at 323-473 K were investigated by X-ray photoelectron spectroscopy (XPS), and compared with those of film formed in an aqueous solution. The passive film formed in the methanol solution was mainly composed of Fe and Cr oxides, and it possessed a double-layered structure consisting of an Fe oxide-rich outer layer and a Cr oxide-rich inner layer. Dissolution of the Fe-rich layer and densification of the Cr-rich inner layer were observed, especially at high temperatures. However, these were suppressed in an aerated methanol solution at 423 K or below, probably due to the barrier effect of adsorbed oxygen. No Ni compound contributed to composing the passive film, even at higher temperatures. The ratio of OH⁻ to O²⁻ was small and decreased with an increase in temperature (the presence of oxygen suppressed the decrease, especially at 423 K or below). The chloride ions were concentrated in the Fe-rich outer layer, and they penetrated more deeply than that in the aqueous solution into the passive film formed in the methanol solution.     

Effects of Ce, La and Ba addition on the electrochemical behavior of super duplex stainless steels

The effects of rare earth metal (REM: Ce, La) and Ba addition on aqueous corrosion properties of super duplex stainless steels (SDSS) were investigated by electrochemical tests and surface analyses. The results of potentiodynamic test indicated that the passive range increased by the addition of Ce, La, and Ba, indicating increased relative resistance to localized corrosion. The EIS measurements showed that the Ce-La-Ba-bearing alloys exhibited higher R_ct and R_p values than the Ce-La-Ba-free alloy at the passive and breakdown states. Furthermore, the additions of REMs and Ba together promoted the formation of dense chromium-enriched passive film.     

Corrosion and passivation of low-temperature nitrided AISI 304L and 316L stainless steels in acidified sodium sulphate solution

304L and 316L steels were nitrided at 425 °C for 30 h and examined at various depths in 0.1 M Na₂SO₄ acidified to pH 3.0. In the near-surface region with about 7-14 wt% N, at potentials of active state anodic currents were much higher than those for untreated steels, whereas in deeper regions with <7 wt% N the currents were only slightly increased in comparison with untreated steels or they were even lower in passive and transpassive states. Surface films were composed of oxygen-containing species on top and of Cr-N species in deeper layers. It is suggested that strong corrosion of near-surface regions is associated with nitride precipitates. Beneficial effect of low nitrogen concentrations can be due to initially accelerated corrosion which leads to larger amounts of passivating species and to the accumulation of corrosion resistant chromium nitrides.     

Impact of temperature excursion on stress corrosion cracking of stainless steels in chloride solution

The impact of a temperature excursion on the subsequent stress corrosion crack growth at the normal operating temperature has been investigated for 321 stainless steel (UNS32100) and 316L stainless steel (UNS31603) using precracked compact tension specimens. Although the data are preliminary the indication is that once crack growth has initiated in 321 SS at the elevated temperature, 130 °C in this study, the crack growth may be sustained at the lower temperature (40 °C), at least over the exposure time of about 700 h. However, the growth rate of 316L SS at the lower temperature was significantly lower than for 321 SS and tended to zero after 2000 h. For the 316 SS a temperature transient should not impact on structural integrity, provided it is short in duration.     

Breakdown of passive film on nickel in borate solutions containing halide anions

The effect of Cl⁻, Br⁻ and I⁻ anions as aggressive agents on the anodic behaviour of nickel electrode in deaerated Na₂B₄O₇ solutions have been investigated by galvanostatic polarization technique. Lower concentrations of the halide anions have no effect on the mechanism of nickel passivation. An increase in the halide anions concentration causes oscillation of the potential in the oxygen evolution region. This could be attributed to the destruction of the passivity by halide anions and repassivation of the film by anodic current and/or OH⁻ anions. Higher aggressive anion concentrations cause breakdown of the passive film and initiated pitting corrosion. As the temperature increases, the breakdown potential is shifted towards the more negative direction. On the other hand, as the pH of the solution increases, the breakdown potential is shifted toward more positive direction, indicating increased protection of the passive film. The activation energy, , of the oxide film formation in the presence of Cl⁻ anions was calculated and was found to be 21 kJ/mol.     

Critical pitting temperature for Type 254 SMO stainless steel in chloride solutions

The variation with time of the open circuit potential of high molybdenum containing stainless steel (Type 254 SMO) was measured in 4% sodium chloride solution in the temperatures range 30-100 °C. The plot of steady state potentials as function of temperature showed an inflection at 50 °C, attributed to the decrease of oxygen solubility in test solution above 50 °C. Potentiodynamic cycling anodic polarization technique was used to determine the critical pitting potential (E_pit) and the critical protection potential (E_prot) of the steel in 4-30% NaCl solutions at temperatures between 30 and 100 °C. By plotting the two values versus solution temperature, the corresponding critical pitting (CPT) and the critical protection (CPrT) temperatures were determined. Both parameters decreased with increasing chloride content. Above the CPT, E_pit and E_prot decreased linearly with log[Cl⁻]. The addition of bromide ions to the solution shifted both E_pit and E_prot towards positive values. In 4% NaCl, E_pit increased linearly with pH in the range 1-10. The combined effect of chloride ion concentration and pH on the morphology of the pits was examined by scanning electron microscopy (SEM) following potentiodynamic cycling anodic polarization.     

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

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.     

Effect of solution concentration on semiconducting properties of passive films formed on austenitic stainless steels

The semiconductor properties of passive films formed on AISI 316L in three acidic solutions were studied by employing Mott-Schottky analysis in conjunction with the point defect model (PDM). Based on PDM, the key parameters for passive film growth are the diffusivity and density of the defects within the film. The results indicated that donor densities are in the range 1-4 × 10²¹ cm⁻³ and increased with solution concentration. By assuming that the donors are oxygen ion vacancies and/or cation interstitials, the diffusion coefficient of the donors in three acidic solutions are calculated to be approximately 1-5 × 10⁻¹⁶ cm²/s.