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.     

Changes in the passive layer of corrugated austenitic stainless steel of low nickel content due to exposure to simulated pore solutions

In this work, changes undergone at the passive layer of a new type of corrugated austenitic stainless steel (low Ni, high Mn 204Cu type) when exposed to solutions simulating that contained in the pores of concrete have been studied. Changes in the nature of the passive layer have been characterized by X-ray photoelectronic spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). Particular focus has been put on the influence of the presence of chlorides and/or carbonation in the solution. Changes in the passive layer due to the passivation treatment that is often applied to corrugated stainless steels during manufacturing processes have also been considered. The results obtained on the 204Cu type steel have been compared with those obtained on more traditional, high Ni, austenitic AISI 304 grade and duplex SAF 2205 grade. During the immersion in simulated pore solutions, 204Cu type suffers more intense redox processes than other studied stainless steels. Moreover, it shows less Cr-rich protective passive layers in these media.     

The semiconducting properties of passive films formed on AISI 316 L and AISI 321 stainless steels: A test of the point defect model (PDM)

The semiconductor properties of passive films formed on AISI 316L in 1 M H₂SO₄ in three temperatures and AISI 321 in 0.5 M H₂SO₄ were studied by employing Mott–Schottky analysis in conjunction with the point defect model (PDM). Based on the Mott–Schottky analysis in conjunction with PDM, it was shown that the calculated donor density decreases exponentially with increasing passive film formation potential. Also, the results indicated that donor densities increased with temperature. By assuming that the donors are oxygen ion vacancies and/or cation interstitials, the diffusion coefficients of the donors for two stainless steels are calculated.     

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 nitrogen on the passivity of Fe-20Cr alloy

Influences of nitrogen on the passivity of Fe-20Cr-(0, 1.1)N alloys were examined by in situ electrochemical techniques. Nitrogen was incorporated in the form of (Fe, Cr)-nitrides in the passive film, and Cr was enriched in the film of the alloy with nitrogen. Photocurrent analysis demonstrated that the structure of passive film formed on Fe-20Cr-1.1N alloy is Cr-substituted γ-Fe₂O₃ with (Fe, Cr)-nitrides. Mott-Schottky analysis revealed that the film formed on Fe-20Cr-1.1N contained higher Cr⁶⁺ and lower Cr³⁺ vacancy concentrations compared with that on Fe-20Cr alloy. All of these results were associated with the enhanced protectiveness of the film on Fe-20Cr-1.1N.     

Simulation analysis of electrochemical nature of real passive films with artificial passivation films

Simulation experiments using oxide thin films with analogues composition and thickness of passive films on Fe-Cr alloys have been performed to reveal the function of alloying elements in passive films. The objective of the present paper is to review recent results of the simulation experiments using Fe₂O₃-Cr₂O₃ films. The thinning rate of the films was measured as a function of potential in 1 M HCl by in-situ ellipsometry. The potential-dependent change of the rate well simulated the potential-dependent dissolution behavior of Fe-Cr alloys. The composition of the films having resistance against given environment was obtained by the simulation experiments.     

Influence of long-term ageing in solution containing chloride ions on the passivity and the corrosion resistance of duplex stainless steels

The influence of long-term ageing in NaCl on the passivity and the electrochemical behavior of UNS S32304 is studied. The passive film thickness, the Cr/Fe ratio and the chloride content were significantly increased after ageing. The chloride distribution depends on residual stresses, sample microstructure and surface preparation. Local electrochemical measurements revealed that pitting potentials are between 250–550 mV vs. SCE after electropolishing. The higher the chloride content, the lower the local pitting potential. It was also shown that the presence of chloride was balanced by the enrichment in chromium after ageing. Then no pitting potential could be measured.     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

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.     

Factors that influence the release of metals from stainless steels exposed to physiological media

Release rates of chromium, nickel and iron from grade 304 stainless steel with three different surface finishes, BA, 2B and 2D, have been determined after exposure to artificial lysosomal fluid. Metal release rates are discussed in relation to corrosion resistance, compositional changes of the outermost surface film of the stainless steel and to measurements of the effective surface area and roughness. The total metal release decreased in the following sequence: 2D > 2B ≈ BA, and was primarily related to the electrochemically active surface area. No direct correlation was observed between corrosion resistance and metal release rates.     

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.     

EIS assessment of critical pitting temperature of 2205 duplex stainless steel in acidified ferric chloride solution

In this study, critical pitting temperature (CPT) of 2205 duplex stainless steel (DSS2205) was assessed using electrochemical impedance spectroscopy (EIS) in ferric chloride solution. In order to verify the results other methods such as ASTM G 48, potentiodynamic and potentiostatic polarisation and zero resistance ammeter (ZRA) were also employed. The results show a strong close relation between the results of this method by those of previous methods. CPT of the alloy is 40 °C based on standard method and 44 °C, 49 °C according to the ZRA and potentiostatic methods. Both potentiodynamic and EIS methods give an almost identical CPT value.     

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.     

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.     

Pitting and uniform corrosion source recognition using acoustic emission parameters

Acoustic emission (AE) can be used to detect corrosion in various applications. In cases of severe background noise, good noise elimination techniques and source identification approaches are required to increase signal-to-noise ratio. In this paper, a new AE parameter is presented as an alternative to AE activity for classification of corrosion sources. The duration and frequency of AE signals are found to be helpful in identifying the source of pitting and uniform corrosion processes. Results are explained in terms of theories of corrosion mechanisms and AE, as well as using experiment results. Finally, new AE source-based approaches to monitor both corrosion processes are proposed.     

The corrosion behaviour of Zr-based bulk metallic glasses in 0.5 M NaCl solution

The corrosion behaviour of eutectic Zr₅₀Cu₄₀Al₁₀ and hypoeutectic Zr₇₀Cu₆Al₈Ni₁₆ bulk metallic glasses (BMGs) was studied by electrochemical measurements, scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Zr₅₀Cu₄₀Al₁₀ BMG was highly susceptible to pitting corrosion in naturally aerated 0.5 M NaCl solution at 30 °C. In contrast, Zr₇₀Cu₆Al₈Ni₁₆ BMG passivated spontaneously under the same condition. EDX results for Zr₅₀Cu₄₀Al₁₀ indicated that enrichment of Cu, Cl and O occurred in the pitted region, while for Zr₇₀Cu₆Al₈Ni₁₆ BMG, no significant difference was found in the surface composition from the specimens before and after immersion in the solution. XPS analysis including angle-resolved measurements for Zr₇₀Cu₆Al₈Ni₁₆ BMG revealed that zirconium cation (Zr⁴⁺) was highly concentrated in both air-formed and passive films. Furthermore, the concentration of Zr⁴⁺ ions after immersion for 24 h or more showed tendency to increase with decreasing take-off angle, indicating that the exterior part of the passive film consisted exclusively of zirconium oxyhydroxide. The high corrosion resistance of Zr₇₀Cu₆Al₈Ni₁₆ BMG was attributed to the formation of homogeneous and stable passive film enriched with zirconium.     

Weld oxide formation on lean duplex stainless steel

Weld oxides have a strong influence on corrosion resistance, but have hitherto only been studied to a limited extent for duplex stainless steels. X-ray photoelectron spectroscopy (XPS) has here been used to study heat tint formed on gas tungsten arc (GTA) welds on the commercial duplex grades LDX 2101 (EN 1.4162/UNS S32101) and 2304 (EN 1.4362/UNS S32304) welded with and without nitrogen additions to the shielding gas. The process of heat tint formation is discussed in terms of transport phenomena to explain the effect of atmosphere, temperature and composition. The oxides formed were found to be enriched in manganese and corrosion testing shows that nitrogen has a strong influence on the weld oxide. A mechanism is proposed including evaporation from the weld pool and subsequent redeposition.     

Some aspects in designing passive alloys with an enhanced corrosion resistance

The technical and biomedical use of alloys containing reactive metals in aggressive environments is possible due to the spontaneous formation of thin oxide layers which slow down corrosion reactions by several orders of magnitude. Oxide films of important technical metals like Cr and Cu are cation semiconductors, and therefore they are electrochemically unstable and susceptible to anodic decomposition in the contact with electrolyte solutions. Alloying with elements that easily undergo oxidation and formation of highly charged cations, which interact electrostatically with the mobile cation vacancies, results in an enhanced electronic conductivity of their oxide films.This is an effective way how to design an alloy with significantly greater corrosion resistance in aggressive environments in comparison with its constitutive elements. The present paper describes several selected systems investigated under conditions of their real application.