Semiconducting and passive film properties of nitrogen-containing type 316LN stainless steels

The semiconducting property of passive films of nitrogen-containing type 316LN stainless steels in different electrolytic media (0.5 M NaCl, borate buffer and borate buffer + 0.5 M NaCl) was investigated by electrochemical impedance spectroscopy (EIS). The nitrogen effect on the chemical composition of the passive films was investigated using X-ray photoelectron spectroscopy, (XPS). Based on capacitance results, the semiconducting parameters obtained from the Mott-Schottky plots indicated a decrease in the donor and acceptor density (N_D and N_A) with increase in nitrogen content, and variation in the flat band potential (E_FB), depending on the electrolytic media. Thus indicating that the oxide layers of the passive film are modified by nitrogen addition. The presence of nitrogen and in the passive film was confirmed by the XPS analysis of the passive film. Cyclic polarization for pitting and repassivation corrosion studies indicated a decrease in hysteresis loop with increase in nitrogen content in 0.5 M NaCl solution. In the highest nitrogen-containing alloy (0.556 wt.% N), the hysteresis loop was small and negligible indicating that the pit initiation is minimum in this alloy. Based on the results obtained, an attempt was made to correlate the semiconducting nature of the passive films with pitting corrosion resistance.     

Electrochemical and XPS studies and the potential scan rate dependent pitting corrosion behavior of Zircaloy-2 in 5% NaCl solution

Electrochemical studies of Zircaloy-2 in 5% NaCl solution were carried out using polarization and electrochemical impedance spectroscopy (EIS). Scan rate dependent pitting behavior of Zircaloy-2 was observed when polarization experiments were carried out at different scan rates (5 mV/min, 50 mV/min and 500 mV/min). Polarization resistance, double layer capacitance, and the thickness of passive film were calculated from the EIS study. The slope of the Bode plot in the capacitive region showed non-ideal behaviour, suggesting formation of pits. A distribution of chloride concentration was seen at the surface of the electrode when the test solution was sampled from different parts of the sample surface and analyzed for chloride. Inclusion of chloride ions in the passive film was observed from the XPS study and the chloride concentration increases with the higher anodic polarization. The outermost layer of the passive film was rich in hydroxide and oxy-hydroxide. A bi-layer passive film structure on the alloy surface is proposed.     

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.     

Observation of passive films on Fe–20Cr–xNi (x = 0, 10, 20 wt.%) alloys using TEM and Cs-corrected STEM–EELS

The structure and composition of passive film formed on Fe–20Cr–xNi (x = 0, 10, 20 wt.%) alloys in deaerated pH 8.5 borate buffer solution was examined by transmission electron microscope and Cs-corrected scanning transmission electron microscope-electron energy loss spectroscopy. Thickness of the passive film on each alloy was measured to be 2.5–2.7 nm and the passive film was enriched with Cr. The passive film formed on the alloys exhibited an amorphous structure, as confirmed by the lack of diffraction contrast and by the fast Fourier transform images taken within a region of the passive film on each alloy.     

Corrosion assessment of nitric acid grade austenitic stainless steels

The corrosion resistance of three indigenous nitric acid grade (NAG) type 304L stainless steel (SS), designated as 304L1, 304L2 and 304L3 and two commercial NAG SS designated as Uranus-16 similar to 304L composition and Uranus-65 similar to type 310L SS were carried out in nitric acid media. Electrochemical measurements and surface film analysis were performed to evaluate the corrosion resistance and passive film property in 6 N and 11.5 N HNO₃ media. The results in 6 N HNO₃ show that the indigenous NAG 304L SS and Uranus-65 alloy exhibited similar and higher corrosion resistance with lower passive current density compared to Uranus-16 alloy. In higher concentration of 11.5 N HNO₃, transpassive potential of all the NAG SS shows a similar range, except for Uranus-16 alloy. Optical micrographs of all the NAG SS revealed changes in microstructure after polarization in 6 N and 11.5 N HNO₃ with corrosion attacks at the grain boundaries. Frequency response of the AC impedance of all the NAG SS showed a single semicircle arc. Higher polarization resistance (R_P) and lower capacitance value (CPE-T) revealing higher film stability for indigenous NAG type 304L SS and Uranus-65 alloy. Uranus-16 alloy exhibited the lowest R_P value in both the nitric acid concentration. Auger electron spectroscopy (AES) study in 6 N and 11.5 N HNO₃ revealed that the passive films were mainly composed of Cr₂O₃ and Fe₂O₃ for all the alloys. The corrosion resistance of different NAG SS to HNO₃ corrosion and its relation to compositional variations of the NAG alloys are discussed in this paper.     

Depassivation-repassivation behavior of type-312L stainless steel in NaCl solution investigated by the micro-indentation

Repassivation behavior of type-312L stainless steel containing 6% of molybdenum was examined in NaCl solution using in situ micro-indentation technique, together with type-304 and 316L stainless steels. High stability of the passive film formed on the type-312L stainless steel was also examined by depth profiling analysis of passive films using glow discharge optical emission spectroscopy (GDOES). In 0.9 mol dm⁻³ NaCl solution at 296 K the type-304 and 316L stainless steels are passive only up to 0.3 V (SHE), above which pitting corrosion occurs. In contrast, no pitting corrosion occurs on type-312L stainless steel. Despite the significant difference of the pitting corrosion resistance, the repassivation kinetics of the three stainless steels, examined by micro-indentation at 0.3 V (SHE), is similar. The presence of molybdenum in the stainless steel does not influence the repassivation kinetics. The charge required to repassivate the ruptured type-312L stainless steel surface increases approximately linearly with the potential, even though the passivity-maintaining current increased markedly at potentials close to the transpassive region. Repassivation occurs without accompanying significant dissolution of steel, regardless of the stability of passive state. Depth profiling analyses of the passive films on the type-312L stainless steels formed at several potentials revealed that molybdenum species enrich in the outer layer of the passive film, below which chromium-enriched layer is present. The permeation of chloride ions may be impeded by the outer layer containing molybdate, enhancing the resistance against the localized corrosion of the type-312L stainless steel.     

Electrochemical corrosion parameters of as-cast Al-Fe alloys in a NaCl solution

The aim of this study is to evaluate the general electrochemical corrosion resistance of Al-Fe alloys in the range of hypoeutectic compositions, Al-0.5 wt.% Fe and Al-1.5 wt.% Fe alloy. EIS plots, potentiodynamic polarization curves and an equivalent circuit analysis were used to evaluate the electrochemical parameters in a 0.5 M NaCl solution at 25 °C. It is shown that for an Al-0.5 wt.% Fe alloy, coarse cells tend to improve the corrosion resistance mainly due to the reduction in cellular boundaries and for an Al-1.5 wt.% Fe alloy, an opposite trend has been detected.     

Development of high Mn-N duplex stainless steel for automobile structural components

A new high Mn-Ni free (duplex stainless steel) DSS containing 18Cr-6Mn-1Mo-0.2N has been developed by examining the effects of manganese on the corrosion and mechanical properties of high Mn SSs containing 18Cr-4 ∼ 11Mn-0 ∼ 2Ni-0 ∼ 1Mo-0.2N. The alloy with 45% ferrite is found to be an optimum alloy with much higher mechanical strength and similar corrosion resistance compared with those of standard SS304. In addition, the alloy was free of precipitation of sigma phase and Cr-nitride when exposed to high temperatures due primarily to relatively low contents of Cr, N and Mo. With an increase in Mn content, the resistance to pitting and metastable pitting corrosion of high Mn DSS decreased since the number of (Mn, Cr) oxides, acting as preferential sites of pitting, increased with the Mn content.     

Pitting corrosion behaviour of austenitic stainless steels - combining effects of Mn and Mo additions

Mn and Mo were introduced in AISI 304 and 316 stainless steel composition to modify their pitting corrosion resistance in chloride-containing media. Corrosion behaviour was investigated using gravimetric tests in 6 wt.% FeCl₃, as well as potentiodynamic and potentiostatic polarization measurements in 3.5 wt.% NaCl. Additionally, the mechanism of the corrosion attack developed on the material surface was analysed by scanning electron microscopy (SEM), X-ray mapping and energy dispersive X-ray (EDX) analysis. The beneficial effect of Mo additions was assigned to Mo⁶⁺ presence within the passive film, rendering it more stable against breakdown caused by attack of aggressive Cl⁻ ions, and to the formation of Mo insoluble compounds in the aggressive pit environment facilitating the pit repassivation. Conversely, Mn additions exerted an opposite effect, mainly due to the presence of MnS inclusions which acted as pitting initiators.     

Effect of the aluminium content of AlxCrFe1.5MnNi0.5 high-entropy alloys on the corrosion behaviour in aqueous environments

High-entropy alloys (HEAs) are a newly developed family of multi-component alloys. The potentiodynamic polarization and electrochemical impedance spectroscopy of the Al_xCrFe_1.5MnNi_0.5 alloys, obtained in H₂SO₄ and NaCl solutions, clearly revealed that the corrosion resistance increases as the concentration of aluminium decreases. The Al_xCrFe_1.5MnNi_0.5 alloys exhibited a wide passive region, which extended >1000 mV in acidic environments. The Nyquist plots of the Al-containing alloys had two capacitive loops, which represented the electrical double layer and the adsorptive layer. SEM micrographs revealed that the general and pitting corrosion susceptibility of the HEAs increased as the amount of aluminium in the alloy increased.     

Effect of nitrogen addition on the stress corrosion cracking behavior of 904 L stainless steel welds in 288 °C deaerated water

This paper concerns with the effect of nitrogen addition to 904 L stainless steel (SS) welds on their stress corrosion cracking (SCC) behavior in high temperature (288 °C) and high pressure (1050 psi) water of high oxygen content (100 ppb) and high conductivity (2.5 μS/cm). For this study, 316 L SS base plate TIG welded with 904 L SS filler wire and with nitrogen contents of 0.027, 0.058 and 0.095 wt.% were used. Flat pin-loaded tensile specimens were fabricated from transverse welds, with the weld in the gauge length. Slow strain rate tests (SSRT) were carried out at a strain rate of 2.2 × 10⁻⁶ s⁻¹. The study shows that the samples, when tested in air, failed at the weld fusion zone for 0.027 and 0.058 wt.% N and at the base metal for 0.095 wt.% N. In the environment, the samples failed in the base metal except the one with least nitrogen content (0.027 wt.%). With nitrogen addition, as the failure location shifted to the base alloy, the weld seemed to acquire SCC resistance and became even more resistant than the base alloy.     

Effect of niobium on the corrosion behaviour of low alloy steel in sulfuric acid solution

This study examines the effect of niobium (Nb) addition on the electrochemical properties of low alloy steel using electrochemical techniques in a 10 wt.% sulfuric acid solution as well as surface analysis techniques. The potentiodynamic test reveals the passive behaviour of all specimens and a decrease in the passive currents with increasing Nb content. Electrochemical impedance spectroscopy (EIS) shows that the Nb-containing steels have higher passive and charge transfer resistance than the control samples. These results suggest that the interaction of elements improves the corrosion resistance of low alloy steel due to the formation of Nb, C, S, P, and Fe products on the surface.     

Corrosion behavior of glassy Ni55Co5Nb20Ti10Zr10 alloy in 1 N HCl solution studied by potentiostatic polarization and XPS

Corrosion resistance of glassy Ni₅₅Co₅Nb₂₀Ti₁₀Zr₁₀ (at.%) alloy in 1 N HCl solution was investigated with respect to the electrochemical behavior and the compositions of the passive film and the underlying alloy surface just below the passive film. The potentiostatic polarization curve indicated that the alloy was spontaneously passivated with a low passive current density of the order of 10⁻³ A m⁻². The quantitative X-ray photo-electron spectroscopy (XPS) analysis revealed that the thickness of the surface film increased linearly with an anodizing ratio of 1.5 nm V⁻¹. The high corrosion resistance of the glassy alloy was due to the formation of niobium, titanium and zirconium-enriched passive film. The growth mechanism of the passive films is also discussed.     

Electrochemical corrosion behavior of Mg-5Al-0.4Mn-xNd in NaCl solution

The electrochemical corrosion behavior of Mg-5Al-0.4Mn-xNd (x = 0, 1, 2 and 4 wt.%) alloys in 3.5% NaCl solution was investigated. The corrosion behavior of the alloys was assessed by open circuit potential measure, potentiodynamic polarization, and electrochemical impedance spectroscopy. The electrochemical results show the intermetallic precipitates with Nd behave as less noble cathodes in micro-galvanic corrosion and suppress the cathodic process. During corrosion, Al₂O₃ and Nd₂O₃, in proper ratio, is incorporated into the corrosion film, and enhances the corrosion resistance.     

Electrolyte and temperature effects on pitting corrosion of type 316LN stainless steels

Abstract

The influence of electrolyte composition and temperature on the pitting corrosion resistance of nitrogen bearing (0·015, 0·198, and 0·56%N) type 316L stainless steels has been investi gated. Anodic polarisation curves were determined in neutral chloride solution at room temperature and at elevated temperatures of 308, 318, and 333 K. Similar polarisation studies were also conducted at room temperature in 1N H₂ SO₄ and in an acidic chloride solution containing 1N H₂ SO₄ and 0·5M NaCl. The results show that the critical pitting potential E_pp in neutral chloride and acidic chloride media increases as the nitrogen content of the alloy increases, indicating that resistance to pitting increases with the addition of nitrogen. However, in 1N H₂ SO₄ solution, the transpassive potential was almost independent of the nitrogen content. As the temperature of the neutral chloride medium was increased, the E_pp values decreased, irrespective of nitrogen content. The decrease in E_pp is attributed not only to the temperature induced modification of the passive oxide film but also to chloride induced activity at the passive film/solution interface. Nevertheless, the alloy con taining 0·56% nitrogen showed better resistance to pitting at 333 K than did the alloy with 0·015% nitrogen at room temperature. SEM examination of the pitted specimens showed clear evidence of pitting for the alloy with 0·015% nitrogen, but insignificant pitting attack for the alloy with 0·56%N. However, the alloy with 0·56%N displayed some pitting attack when the experiments were conducted at 333 K.     

Corrosion behavior of Ti-Mo alloys cold rolled and heat treated

Corrosion behavior of (α + β) Ti-10Mo and β Ti-20Mo (mass%) alloys cold rolled and solution heat treated was investigated by studying the anodic polarization curves at 310 K in 5 mol% HCl solution to determine the potential use of those alloys in biomedical applications. The anodic films formed on the surfaces of the alloys were examined using X-ray photoelectron spectroscopy analysis and scanning electron microscopy. The results reveal that both of the Ti-Mo alloys cold rolled and solution treated exhibit a passive behavior in 5% HCl solution, which is attributed to the passive film formation of a mixture of MoO₃ and TiO₂. The cold rolling process does not influence the formation of passive films on the Ti-Mo alloys although it slightly increases the passive current densities. The corrosion resistance of the Ti-Mo alloys increases with Mo content and both of the Ti-Mo alloys exhibit better corrosion resistance than commercial pure Ti—the currently used metallic biomaterial.     

An electrochemical study of the effect of Li on the corrosion behavior of Ni3Al intermetallic alloy in molten (Li + K) carbonate

A study of the effect of lithium content (1, 3 and 5 wt.%) and heat treatment (400 °C during 144 h) on the corrosion behavior of Ni₃Al alloy has been carried out in a 62 mol.%Li₂CO₃-38 mol.%K₂CO₃ mixture at 650 °C using electrochemical techniques. Employed electrochemical techniques included potentiodynamic polarization curves, linear polarization resistance, LPR, electrochemical impedance spectroscopy, EIS, and electrochemical noise, measurements EN. Results have shown that the alloys exhibited an active-passive behavior regardless of the heat treatment. For alloys without heat treatment, the most corrosion resistant was the Ni₃Al base alloy, but when they were heat treated, the most corrosion resistant was the alloy containing 3%Li. EIS results showed that for short immersion tests, the corrosion process was under diffusion control, but for longer exposure times, the presence of a protective scale was evident. All the alloys were highly susceptible to a localized type of corrosion according to EN measurements and supported by SEM micrographs.     

The Estimation of Localized Corrosion Behavior of Ni-Based Dental Alloys Using Electrochemical Techniques

The aim of this study is to investigate the electrochemical behavior of the five non-precious Ni-based dental casting alloys in acidified artificial saliva. For comparison, nickel was also investigated. In order to study the localized corrosion resistance, the cyclic potentiodynamic polarization (CCP) and electrochemical impedance spectroscopy were performed. Scanning electron microscopy (SEM) observations were made after the CCP tests. The Ni-Cr alloys with chromium (14-18%) contents were susceptible to localized corrosion. The Ni-Cr-Mo alloy with contents of chromium (??13%) and molybdenum (9%) presents a dangerous breakdown, but have a zero corrosion potential so that the difference between them is around 650?mV. The Ni-Cr-Mo alloys with higher chromium (22-25%) and molybdenum (9-11%) contents had a much larger passive range in the polarization curve and were immune to pitting corrosion. Pitting resistance equivalent (PRE) of about ??54 could provide the Ni-based alloy with a good pitting corrosion resistance.     

Semiconductor properties and protective role of passive films of iron base alloys

Semiconductor properties of passive films formed on the Fe-18Cr alloy in a borate buffer solution (pH = 8.4) and 0.1 M H₂SO₄ solution were examined using a photoelectrochemical spectroscopy and an electrochemical impedance spectroscopy. Photo current reveals two photo action spectra that derived from outer hydroxide and inner oxide layers. A typical n-type semiconductor behaviour is observed by both photo current and impedance for the passive films formed in the borate buffer solution. On the other hand, a negative photo current generated, the absolute value of which decreased as applied potential increased in the sulfuric acid solution. This indicates that the passive film behaves as a p-type semiconductor. However, Mott-Schottky plot revealed the typical n-type semiconductor property. It is concluded that the passive film on the Fe-18Cr alloy formed in the borate buffer solution is composed of both n-type outer hydroxide and inner oxide layers. On the other hand, the passive film of the Fe-18Cr alloy in the sulphuric acid consists of p-type oxide and n-type hydroxide layers. The behaviour of passive film growth and corrosion was discussed in terms of the electronic structure in the passive film.     

Temperature effect in the corrosion resistance of Ni-Fe-Cr alloy in chloride medium

The corrosion susceptibility of alloy 33 in 0.5 mol/L sodium sulphate solutions containing or not 0.1 mol/L sodium chloride was tested at three different temperatures: 22 °C, 40 °C and 60 °C. Electrochemical studies were performed using corrosion potential measurements (E_corr) as well as potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. Corrosion potential measurements showed that alloy 33 was passivated by a previously air formed film which was not destroyed during immersion in both solutions. No corrosion was observed during these tests although the temperature affected the film. Potentiodynamic polarization experiments showed that at high anodic potentials the previous film was broken up, and localized corrosion occurred in both solutions and at the three temperatures tested. Electrochemical impedance spectroscopy tests confirmed the presence of a stable passive film on the alloy surface at open circuit potential. Mott-Schottky analysis indicated that the passive film is an n-type semiconductor due to the presence of point defects of donor species, such as oxygen vacancies and interstitial metallic cations. As the potential increases the Cr(III) present in the barrier layer oxidizes producing Cr(VI) soluble species. The dissolution creates metallic cation vacancies that are acceptor species and the film changes from n-type to p-type semiconductor. The passive film rupture and the following localized attack are related to the drastic oxidative dissolution of the film at high anodic potentials, independent of its p-nature, chloride presence or increased temperature.