Pitting transients analysis of stainless steels at the open circuit potential

Metastable pitting of stainless steels in chloride containing oxidising electrolytes is investigated at rest potential using a new experimental technique allowing to record simultaneously the potential and corrosion current variations. Different industrial surface conditions (BA and 2B) are tested for both ferritic (FeCr type) and austenitic (FeCrNi type) stainless steels. It was shown that the number of pitting events decreases with the exposure time and that BA condition provides better resistance to pitting than 2B. As far as pitting mechanisms are concerned, the potential recovery after pitting does not reflect the pit repassivation but rather refers to the discharge of the surface capacity. Analysing the pitting transients provides quantitative information on the cathodic reaction through the passive film (transfer resistance and surface capacitance). Differences in pitting transient shapes are discussed as well.     

Uniformity of passive films formed on ferrite and martensite by different inorganic inhibitors

A comparison of uniformity of passive films formed on ferrite and martensite by three kinds of inorganic inhibitors, chromate, bicarbonate and nitrite, has been made by anodic polarization curve and AC impedance measurements. It was found, by anodic polarization curve analyses, that there was different pitting susceptibility of passive films formed on ferrite and martensite by chromate or bicarbonate. In the solution containing chloride ions, the broken potential of the passive film formed on ferrite by chromate or bicarbonate was more negative than that on martensite. However, the passive films formed on both ferrite and martensite by nitrite had similar pitting susceptibility. Their broken potentials were almost the same. The difference in pitting susceptibility of the passive films was explained by Mott-Schottky analyses. It was found that the passive film formed on ferrite by chromate or bicarbonate had a higher donor concentration than the passive film on martensite. The passive films with higher donor concentrations were more sensitive to chloride ions. However, the passive films formed on both ferrite and martensite by nitrite had the same donor concentration. A uniform passive film can be formed on a martensite mild steel with ferrite bands by nitrite.     

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.     

Effect of the reinforced boron carbide particulate content of AA6061 alloy on formation of the passive film in seawater

The effect of boron carbide (B₄C) reinforcement on the corrosion of AA6061 alloy was studied by investigating passive films formed in seawater. The higher passive current and its potential-dependence for these composites indicated formation of porous passive film. Electrochemical impedance spectroscopy (EIS) graph suggests that the alloy surface is partly or totally active. The formed passive film is n-type semiconductor junction in nature. The difference between corrosion potential (E_corr) and potential at zero charge (PZC) suggests that the chloride ions responsible for film breakdown exist within the passive film. A suitable mechanism is proposed for the passive film breakdown.     

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.     

The role of corrosion-resistant alloying elements in passivity

Passivity of alloys containing corrosion-resistant elements were reviewed. Chromium and valve metals except aluminum form stable oxyhydroxide films even in aggressive hydrochloric acids. Molybdenum forms a passive MoO₂ film in the active region of stainless steels and hence decreases the active dissolution current. In the passive region of transition metals and valve metals, molybdenum is generally in the transpassive state and dissolved. However, if the outer oxyhydroxide film is stable the inner MoO₂ film is protected by the outer oxyhydroxide film and the MoO₂ film acts as the effective barrier against diffusion of matters through the film. Thus the passive current density of 30Cr-2Mo ferritic stainless steel is more than two orders of magnitude lower than that of 30Cr steel without molybdenum in 1 M HCl.     

Influence of alloying on the passive behaviour of steels in bicarbonate medium

The electrochemical behaviour of passive films on carbon steel, chromium steel and high speed steel (HSS) was studied in 0.5 M NaHCO₃. The influence of the presence of 10 mM potassium chloride on passive film stability, and of the heat treatment of chromium steel and HSS on the respective film properties, was also investigated. Polarization curves showed that the alloy composition exerts a significant influence on the onset of passive film formation, chromium and HSSs passivating more easily as a result of the higher alloying content. Values of passive current, obtained both potentiodynamically and at fixed potential, were higher for HSS than for carbon and chromium steels, which was attributed to the contributions of Mo and W to the passive film composition. The presence of chloride ion delays the passivation of chromium and HSSs, but does not interfere with passive film stability, as shown by the chronoamperometric profile and passive current values. In the case of carbon steel, with no alloying elements, chloride ion diminishes film stability but does not influence passive film formation. Heat treatment, which alters the microstructure, does not alter the properties of the passive films except for a small effect on those formed potentiodynamically on HSS.     

The influence of temperature on the passivation behaviour of a super duplex stainless steel in a boric-borate buffer solution

The passivation behaviour of a super duplex stainless steel has been investigated in a boric-borate buffer solution with and without chloride additions, at different temperatures, by means of potentiodynamic and potentiostatic techniques. X-ray photoelectron spectroscopy was used to detect surface compositional changes of anodic passive films formed as a function of electrode potential and temperature. The resulting passive films were all chromium enriched with respect to the bulk composition, and their thickness and ratio were found to vary significantly. In particular, the films formed in the passive domain were more enriched with chromium compared with those grown in the secondary passive region. At a constant potential the temperature increase produced a reduction in the chromium content of the film. These findings can be explained by the different ionic mobilities and dissolution rates at the film/solution and film/alloy interfaces. Pitting corrosion was observed only in the transpassive zone. A temperature increase from 23 to 60 °C did not produce a decrease in the pitting potential. This behaviour can be explained by the parallel action of two different phenomena, the change in the composition and structure of the passive film and the velocity increase in the process, which yield localized rupture of the passivity by the nucleation and growth of pits in connection with chromium depletion at high potentials.     

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 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.     

Effects of the solution temperature and the pH on the electrochemical properties of the surface oxide films formed on Alloy 600

The surface oxide films on Alloy 600 have been investigated as a function of the solution temperature and the pH by using a cyclic voltammetry, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and a depth profiling by Auger electron spectroscopy (AES). H₃BO₃, Na₂SO₄ and NaOH aqueous solutions with temperatures in the range of 30–300 °C were used as the test solutions. As the solution temperature of the 0.5 M H₃BO₃ increased, the thickness of the passive film increased but the resistance of the passive film was diminished, which is coincident with a solution temperature dependency of the passive current in the potentiodynamic curve. The inner oxide film on Alloy 600 was distinguishable from the Cr-rich outer oxide film above 100 °C. From the Mott–Schottky relation, the oxide formed in 0.5 M H₃BO₃ at 300 °C showed a p-type semiconductor property, accompanied by a Cr-rich oxide film throughout the whole oxide film unlike the n-type oxide films up to 250 °C. The oxide resistance of the passive film decreased in the order of 0.5 M H₃BO₃, 0.1 M NaOH and 0.5 M Na₂SO₄, which is consistent with the pH dependency of the passive current. Ni-rich oxide films of a p-type were formed in the 0.5 M Na₂SO₄ or 0.1 M NaOH.     

轧制纳米块体304不锈钢腐蚀行为的研究Ⅱ钝化膜保护性能

采用动电位极化曲线和Mott-Schottky分析等电化学测试手段,探讨了轧制纳米块体304不锈钢与普通304不锈钢在0.05mol/L H2SO4＋0.05mol/L Na2SO4溶液中钝化膜的保护性能;运用点缺陷（PDM）模型,分析了不同电位下在0.05mol/L H2SO4＋0.25mol/L Na2SO4溶液中两种材料形成钝化膜的半导体性质,阐述了导致两种钝化膜保护性能差异的根本原因.结果表明：两种材料表面钝化膜都具有n型半导体特征,氧空穴作为主要的载流子参与钝化膜的形成和溶解过程;钝化膜中载流子密度与钝化膜的形成电位之间满足幂指数关系,载流子在两种材料表面的钝化膜中的扩散系数非常接近,说明两种钝化膜遵从相似的形成和溶解机制,但轧制纳米块体304不锈钢中的载流子密度小于普通304不锈钢钝化膜中的载流子密度,从而使其钝化膜具有更好的保护性.     

The inhibition activity of ascorbic acid towards corrosion of steel in alkaline media containing chloride ions

The activity of ascorbic acid towards steel corrosion in saturated Ca(OH)₂ solution containing chloride ions was investigated in this study. Concentration and time dependence of the protective properties of the passive film were acquired by electrochemical impedance spectroscopy. The best inhibitive performance, i.e. the longest pitting initiation time was obtained in the presence of 10⁻³ M ascorbic acid, while both lower and higher concentrations showed shortening of the pitting-free period. The overall behaviour of ascorbic acid was attributed to its ability to form chelates of various solubility having various metal/ligand ratios and oxidation states of the chelated iron. The assumption of ascorbic acid assisted reductive dissolution of the passive layer at higher inhibitor concentrations was confirmed by cyclic voltammetry and ATR FTIR spectroscopy. It is proposed that the overall inhibitive effect at lower concentrations is due to the formation of insoluble surface chelates and the effective blocking of the Cl⁻ adsorption at the surface of passive film. A pronounced inhibitive effect observed after the pitting had initiated was ascribed to the formation of a resistive film at the pitted area.     

Corrosion protection of Zn-phosphate containing water borne dispersion coatings on steel: Part 1: Design and analysis of model water based latex films on iron substrates

Model thin film coated substrates were developed to investigate the functionality of Zn-phosphates, the polymeric binder and emulsifiers during the interface formation on iron substrates. By means of grazing incidence FTIR-spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy, it was observed that ultra-thin films of zinc phosphates and hydroxides, with a thickness of less than 2 nm, adsorb on the iron surface during the formation of the latex coating/metal interface. Moreover, film formation conditions were developed for the deposition of hexagonal closed packed defect-free-monolayers of latex particles over large areas. This allowed the application of time of flight SIMS for the analysis of the monolayer-metal interface and the study of its morphology before and after film annealing. After annealing, the monolayer film showed an enrichment of surfactants at the particle boundaries and at the interface to the phosphate and hydroxide covered metal substrate.     

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.     

Electrochemical and XPS studies of the passive film formed on stainless steels in borate buffer and chloride solutions

The passivity of AISI 304L and AISI 316L stainless steels in a borate buffer solution, with and without the addition of chloride ions, was studied using cyclic voltammetry and potentiodynamic measurements. The passive layers formed by electrochemical oxidation at different passivation potentials on both the stainless steels were studied by X-ray photoelectron spectroscopy, their compositions were analysed as a function of depth, and the cationic fraction of the passive film was determined. The passive films established on the two stainless steels in the borate buffer solution at pH = 9.3 contained the oxides of two main elements, i.e., Fe and Cr. The oxides of the alloying elements Ni and, optionally, Mo, also contribute to the passive layer. In the presence of chloride ions a strong chromium enrichment was observed in the passive layers.     

A two step anodizing process of aluminium as a means for improving the chemical and physical properties of oxide films

An anodizing process was developed to form corrosion resistant and hard oxide films on aluminium. The process consists of two steps: first the formation of chromate/phosphate treated layer on the surface of aluminium and secondly anodizing in a sulphuric acid solution. The anodic oxide films formed by the present process contain Cr(III) and phosphate species mostly in the outer part of the porous layer. The films formed by the present process provided a better corrosion resistance to the substrate aluminium from pitting in a chloride medium than the films formed by conventional anodizing and sealed in a boiling chromate solution. Further, Vickers hardness on the cross section of film increased compared with the films formed by conventional anodizing. This two step process can be developed to form other composite oxide films by using different treatments for the first step.     

In-situ thermal evolution of Ni/Ti multilayer thin films

Nickel and titanium alternated nanolayers were deposited from Ti and Ni targets. The multilayer thin films were designed in order to have equiatomic overall chemical composition with a period from 5 to 70 nm. The chemical composition, morphology, structure and phase transformation behaviour were studied. The surface and cross-section morphology of the as-deposited thin films was analysed by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The Ni/Ti thin films present in their surface nanograins and for higher periods in cross section it was possible to distinguish the alternated layers and measure their thickness. The structural evolution with temperature was analysed by in-situ hot stage X-ray diffraction (XRD). The as-deposited multilayer thin films exhibit a tendency to a transitory disorder as the period decreases. For the smaller periods a disordered phase forms during the deposition process, while in Ni/Ti multilayers with higher periods this phase is only observed during annealing. By increasing the temperature an exothermic reaction occurs with the formation of the B2-NiTi austenitic phase. In spite of moderate enthalpy of mixing, the multilayers with intermediate modulation period of Ni/Ti films show potential to be used for joining purposes.     

Influence of dissolved oxygen on oxide films of Alloy 690TT with different surface status in simulated primary water

Oxidation of Alloy 690 with different surface conditions were performed in oxygenated primary water containing B and Li at 325 °C and 15.6 MPa for 2 months. Oxide scales were analyzed using various methods. Results showed triple-layer oxide films were formed: the outmost layer with dispersed big oxide particles; the intermediated layer with loose sheet-like or needle-like oxides and the inner layer with incompact cellular oxides, which were lack of protectiveness. Oxide morphologies were affected by the surface status of samples. Dissolved oxygen increased the intergranular attack (IGA) sensitivity of Alloy 690TT. Growth mechanism of the oxide film was discussed.     

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.