Influence of the electrolyte composition and temperature on the transpassive dissolution of austenitic stainless steels in simulated bleaching solutions

The transpassive corrosion of highly alloyed austenitic stainless steels—UNS N08904, UNS S31254 and UNS S32654—was investigated at 20 and 70 °C in a range of simulated bleaching solutions with conventional and rotating ring-disc electrode voltammetry, as well as electrochemical impedance spectroscopy. The overall transpassive oxidation rate of UNS S32654 was found to be much higher than that of the other two alloys. The general features of the impedance spectra demonstrate that transpassive dissolution is favoured for UNS S32654 and secondary passivation predominates for the two other steels. The addition of oxalic acid resulted in a significant increase of the transpassive oxidation rate at both temperatures. At room temperature, the addition of diethylenetriaminopentaacetic acid (DTPA) led to a decrease of the transpassive oxidation rate, especially at pH 3. Conversely, the addition of DTPA to the pH 3 solution at 70 °C has been found to increase the transpassive oxidation rate. A kinetic model of the process is proposed, featuring a two-step transpassive dissolution of Cr via a Cr(VI) intermediate species and taking into account the dissolution of Fe(III) through the anodic film. The model has been found to be in quantitative agreement with the steady-state current versus potential curves and the impedance spectra. The kinetic parameters of transpassive dissolution have been determined and the relevance of their values is discussed.     

Electrochemical corrosion characteristics of type 316 stainless steel in simulated anode environment for PEMFC

The corrosion behavior of type 316 stainless steel in simulated anode environment for proton exchange membrane fuel cell (PEMFC), i.e., dilute hydrochloric acid solutions bubbled with pure hydrogen gas at 80 °C, was investigated by using electrochemical measurement techniques. The main purpose is to offer some fundamental information for the use of stainless steels as bipolar plate material for PEMFC. Both polarization curve and electrochemical impedance spectroscopy (EIS) measurements illustrate that 316 stainless steel cannot passivate spontaneously in the simulated environments. The absorbed (and/or adsorbed) hydrogen atoms from cathodic corrosion reactions on the steel surface may deteriorate the passivity and corrosion resistance. The oxidation of these hydrogen atoms gives rise to a second current peak in the anodic polarization curve, and the current increases with immersion time. EIS spectra also reveal that a porous corrosion product layer formed on the steel surface during the active dissolution in the test solutions. 316 stainless steel exhibits the similar corrosion behavior in sulfate ions containing dilute hydrochloric acid solution.     

Effects of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels in acidic chloride solutions

The role of nitrogen on the passivation of nickel-free high nitrogen and manganese stainless steels was investigated in 0.5 M H₂SO₄, 3.5% NaCl and 0.5 M H₂SO₄ + 0.5 M NaCl solutions using potentiodynamic polarization, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy techniques. The passive film stability was enhanced in 0.5 M H₂SO₄ and the pitting resistance was improved in 3.5% NaCl solution by more nitrogen addition. The influence of nitrogen extended the whole anodic polarization region in 0.5 M H₂SO₄ + 0.5 M NaCl solution, as demonstrated by the enhanced dissolution resistance, promoted adsorption and passivation process, improved film protection and pitting resistance with increasing nitrogen content. Possible mechanisms relating to the role of nitrogen in different potential regions were discussed.     

The corrosion-inhibiting effect of polypyrrole films doped with p-toluene-sulfonate,benzene-sulfonate or dodecyl-sulfate anions,as coating on stainless steel in NaCl aqueous solutions

This article presents a study of the conditions for electro-synthesis of polypyrrole (PPy) films on stainless steel, in the presence of the anions p-toluene-sulfonate (pTS), benzene-sulfonate (BS) or dodecyl-sulfate (DS). Cyclic voltammetry (CV) was used in the synthesis of the polypyrrole films on the stainless steel (SS). These polymeric films were characterized by IR and UV–vis spectroscopy and their morphology and thickness were analyzed by scanning electron microscopy (SEM). Their performance as protective films against corrosive processes presented by the SS/PPy-pTS, SS/PPy-BS or SS/PPy-DS systems was evaluated in 0.1 M NaCl aqueous solution. The study of the corrosion processes of the stainless steel/polymer systems was conducted through measurements of open circuit potential (E_OCP), polarization curves (PC) and electrochemical impedance spectroscopy (EIS). The results showed that the protective capacity of these polymeric systems on stainless steel, mainly with regard to pitting, depends on the nature of the anion dopant used during electro-synthesis of the PPy film. The best performance was seen with the dopants pTS and BS.     

Electrochemical study of effect of the concentration of azole derivatives on corrosion behavior of stainless steel in H2SO4

In order to improve corrosion resistance of the stainless steel structures exposed to acidic media, a variety of corrosion inhibitors particularly organic ones have been examined. In this work, the corrosion inhibition performance of two azole derivatives namely benzotriazole and benzothiazole on stainless steel in 1 M sulfuric acid was studied through taking advantage of electrochemical techniques as well as SEM surface analysis. Revealing effectiveness of the two inhibitors, the AC impedance spectra indicated no change in corrosion mechanism. The noise resistance and average current density as parameters extracted from electrochemical noise measurements revealed the direct proportion of inhibition function to the inhibitor concentration. In accordance with the polarization curves, benzotriazole and benzothiazole appeared to act as mixed type inhibitors. The adsorption of the two corrosion inhibitors was shown to obey Langmuir isotherm. Moreover, it was deduced from the isotherm that the type of adsorption can be physical and chemical in nature. The corrosion damage mitigation was also confirmed through SEM in the presence of benzothiazole.     

Effect of Al content on the corrosion behavior of Mg-Al alloys in aqueous solutions of different pH

The effect of systematic increase of Al content on the electrochemical behavior of the Mg-Al alloys in aqueous solutions of different pH was investigated. Different electrochemical methods such as open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy, EIS, were used to investigate the electrochemical behavior of the alloys in aqueous solutions. The results have shown that Mg-5Al is easily corroded due to the microgalvanic effect between α-phase and β-phase, its corrosion rate is even higher than that of Mg itself. The increase of Al content increases the corrosion resistance of the alloy due to the formation of the β-phase (Mg₁₇Al₁₂) together with the Mg α-phase. The ranking of the corrosion rate of these alloys was Mg-5Al > Mg > Mg-10Al ≅ Mg-15Al. The corrosion rates of the alloys in acidic solutions are pronouncedly high compared to those measured in neutral or basic solutions. The impedance measurements are in consistence with the polarization techniques and the impedance data were fitted to theoretical data obtained according to an equivalent circuit model describing the electrode/electrolyte interface.     

Corrosion behaviour of low-nickel austenitic stainless steels reinforcements: A comparative study in simulated pore solutions

The use of austenitic stainless steel reinforcements is one of the most reliable methods to assure the durability of reinforced concrete structures exposed to aggressive environments, but the initial cost of the material often limits its use in practice. Nickel is one of the alloying elements that raise most the cost of the stainless steels reinforcements. In this work the corrosion resistance of low-Ni 204Cu reinforcements is compared with that of more traditional austenitic stainless steels such as 304, 304L, 316, 316L and 316Ti. Polarization tests were carried out in simulated carbonated and non-carbonated pore solutions with different chloride concentrations. Results prove the very good corrosion behaviour of 204Cu stainless steel in carbonated media and in non-carbonated, chloride contaminated media. The corrosion resistance of low-Ni type 204Cu is only meaningfully lower than that of more traditional austenitic stainless steels in very highly aggressive solutions.     

Electrochemical synthesis of polyindole on 304-stainless steel in LiClO4–acetonitrile solution and its corrosion performance

The use of electrochemically synthesized polyindole (PIN) film was investigated for protective coating on 304-stainless steel (SS). Polyindole was deposited via anodic oxidation of the corresponding monomer, indole in acetonitrile (ACN) solution containing LiClO₄. It provided an adherent and stable coating on SS. The corrosion performance of coated and uncoated SS was investigated in 3.5% NaCl solution using electrochemical impedance spectroscopy (EIS), anodic polarization curves and open-circuit potential–time (E_ocp–t) diagrams. These tests demonstrated that PIN coating provided important barrier effect to SS for important immersion times in aggressive medium.     

Corrosion behaviour of new stainless steels reinforcing bars embedded in concrete

The activities concerned with the evaluation, repair and restoration of structures are estimated to amount to 35% of the total volume of the work in the building sector and this continues to increase. The corrosion of rebars in the reinforced concrete structures (RCS) is the main reason for their degradation, so the use of reinforcing stainless steels seems to be one of the possible solutions with most probabilities of solving this problem. In this work, in order to demonstrate the advantages of using reinforcing stainless steels, the corrosion behaviour of AISI 304 and 316 reinforcements embedded in concrete slabs (C35/45 and C60/70 concrete) with two chloride contents are compared with three low-cost and low-Ni austenoferritic stainless steels and with the conventional carbon steel. The lower chloride contamination selected in this research, was enough to cause the corrosion in the active state of the carbon steel reinforcements, whereas the highest one exceeded the expected contamination in the natural environments, including sea media. The metallic materials remaining in the passive state can be considered, from the point of view of corrosion resistance, adequate as reinforcements in the RCS.     

Threshold chloride concentration for stainless steels activation in concrete pore solutions

The threshold chloride concentration for stainless steels activation in concrete environments depends not only on chemical composition of the steel and on pH level of concrete pore solution but is also considerably affected by the superficial state of the steel. The presence of scales leads to significant decrease of corrosion resistance. This study evaluates the corrosion resistance of various stainless steels exposed to pore solutions of fresh concrete by means of electrochemical methods. All tested materials with bare surface, except for martensitic chromium steel FeCr12, proved to be completely resistant to fresh concrete pore solution containing chlorides. If the surface is scaled, the identically high resistance cannot be expected even for the most alloyed stainless steels.     

Electrochemical synthesis of poly(5-nitroindole) on 316L-stainless steel in LiClO4-acetonitrile solution and its corrosion performance

The preparation of poly(5-nitroindole) (P5NI) coating was achieved on 316L-stainless steel (SS). Poly(5-nitroindole) was deposited via anodic oxidation of the corresponding monomer in acetonitrile (ACN) solution containing LiClO₄. The influence of P5NI coating against SS corrosion was studied in 3.5% NaCl solution by electrochemical impedance spectroscopy (EIS), anodic polarization curves and the open circuit potential–time (E_ocp–t) diagrams. The results obtained suggest that P5NI coating forms a sacrificial layer but the efficiency against corrosion is limited with increasing time.     

Electrochemical behavior of magnesium alloys AZ91D,AZCe2, and AZLa1 in chloride and sulfate solutions

The influence of Cl⁻ and SO₄²⁻ on the electrochemical behavior of AZ91D, AZCe2, and AZLa1 was studied. For all alloys, there was a current plateau in the anodic polarization curves in Na₂SO₄ solutions. In 0.5% NaCl solution, there was a small current plateau, whereas there was none in the 3.5% and 5% NaCl solutions. This indicated that SO₄²⁻ is less aggressive than Cl⁻. The range of the current plateau decreased with increasing SO₄²⁻ concentration. For all alloys, the high frequency capacitive loop in the Nyquist plots decreased with increasing concentration consistent with the decrease in corrosion resistance with increasing Cl⁻ and SO₄²⁻ concentration.     

Development of an environmentally friendly protective coating for the depleted uranium-0.75 wt.% titanium alloy: Part V. Electrochemical impedance spectroscopy of the coating

Molybdenum oxide based conversion coatings have been formed on the surface of the depleted uranium-0.75 wt.% titanium alloy. Electrochemical impedance spectroscopy (EIS) measurements have been performed on the as-made and aged coatings and compared with the untreated depleted uranium (DU) alloy. The Nyquist and Bode plots of the as-made coating were similar to the untreated samples and contained capacitive and inductive loops. The aged coating exhibits significantly different behavior from the as-made coating and has been modeled with a four element equivalent circuit that contains a constant phase element (CPE).     

Local electrochemical impedance measurements on inclusion-containing stainless steels using microcapillary-based techniques

MnS inclusions are good precursor sites for pitting corrosion of stainless steel. The objective of this paper was to quantify the passive properties of resulfurized stainless steel after immersion in chloride media. This was done by combining microcapillary techniques with electrochemical impedance spectroscopy and numerical analysis (specific equivalent circuit). It was shown that sulfur species produced in the electrolyte during the dissolution of inclusions react with the native passive film to CrS and FeSO₄. Local electrochemical impedance spectroscopy measurements provided data describing the behaviour of the affected matrix at the microscale. For example, the value of the charge transfer and migration of point defects resistance decreases from 51,700 Ω cm², in sites free of any metallurgical heterogeneity down to 12,200 Ω cm², in sites containing a high density of inclusions. It was also shown that the integrity of the microcapillary can be altered by the presence of high quantity of sulfur in the electrolyte. Local impedance data allowed the detection of such problems.     

Stainless steel as new substrate for coil coating

Increasing needs of very high resistance to cosmetic corrosion, of more extended service life and reduced maintenance costs for infrastructures, civil and industrial buildings open new fields of application for coil coated stainless steel. This paper describes the adhesion and corrosion properties of new coil coated stainless steel materials produced in industrial coil coating lines. The use of an electrochemical test (electrochemical impedance spectroscopy) can give detailed information on the reactivity of the system and allow the performance of different substrates (AISI 409, 430, 316 and 304) coated with different polymers (polyvinylidene and polyester) to be compared. The results obtained show the interesting properties of this new class of coil coated products. The materials were tested for a long time (about 200 days) in an aggressive environment (3.5% sodium chloride solution) also in the presence of macrodefects. In particular, VIVINOX 430, 304 and 316 revealed no reactivity, corrosion or disbonding, thus supporting the expectancy of very long trouble free exposure also in very aggressive natural environments. (VIVINOX is the brand name of the AST (Acciali Speciali Terni) line of coil coated stainless steel.)     

不锈钢在湿法磷酸中的应用

介绍湿法磷酸生产中杂质、操作条件、酸浓度、温度、搅拌、有饥物等对腐蚀的影响和材料选择的基本要素，并列出多种不锈钢的主要化学成份和几种工艺流程的材料选用表。     

Methacryloxypropyltrimethoxysilane films on aluminium: Electrochemical characteristics, adhesion and morphology

The electrochemical characteristics, adhesion and morphology of methacryloxypropyltrimethoxysilane (MAPT) films on aluminium were investigated during exposure to 3 wt.% NaCl. The MAPT films were deposited on aluminium surface from 2 to 5 vol.% methacryloxypropyltrimethoxysilane solutions, with the aim to investigate the influence of deposition parameters (silane solution concentration and curing time) on electrochemical characteristics, adhesion and morphology of MAPT films on aluminium.Using electrochemical impedance spectroscopy (EIS), potential–time measurements, adhesion measurements and optical microscopy coupled with image analysis, it was shown that films deposited from 5 vol.% solution exhibited better corrosion stability and adhesion, as well as lower porosity comparing to 2 vol.% solution and improved the corrosion protection of aluminium substrate, while the curing time had no influence on these characteristics.     

Stability of spontaneous passive films on high strength Mo-containing stainless steels in aqueous solutions

The stability of naturally grown passive films on some Mo-containing stainless steel specimens was examined in aerated and deaerated universal buffer solutions with different pH (2–12) as well as in sulphate and chloride solutions. Open circuit potential (E _oc) and electrochemical impedance spectroscopy (EIS) were used as measuring techniques. In all cases, E _oc shifts towards less negative values with time until the potential reaches its steady-state (E _ss) value. The E _ss value is found to be more positive with decrease in solution pH or increase in Mo content in the alloy and becomes less positive in deaerated buffer solutions. Also, the thickening rate of the outer layer for the duplex passive film increases with increasing extent of Mo in the steel substrate or pH of the test solution. For a given alloy, E _ss decreases linearly with the anion concentration (C), and is always more positive in Cl⁻ than in SO₄²⁻ media for C ≥ 0.05 M. Analysis of the EIS data showed that the total resistance (R _T) of the passive film has higher values in aerated solutions, and is generally lower in basic solutions. This indicates that lower solution pH favours the formation of oxide films offering better protection. Furthermore, the higher values of R _T in Na₂SO₄ solutions suggest the formation of more stable passive films in sulphate than in chloride solutions. This is discussed on the basis of the relative degree of anion incorporation into the passive films.     

Electrochemical potential noise of 321 stainless steel stressed under constant strain rate testing conditions

An investigation was carried out on the features of potential electrochemical potential noise of stressed AISI321 stainless steel in solution 0.5 mol/L Na₂SO₄ + 5 × 10⁻³ mol/L H₂SO₄ under constant-strain-rate-testing (CSRT) conditions. The results showed that the strained steel exhibited a white noise feature at low frequencies and the amplitude of potential fluctuation depended on elongation of the steel. Power spectral density (PSD) of the noise increased with increasing strain level. The noise level in the elastic region of the steel was relatively low, which increased with elongation. After the steel yielded, the electrochemical noise level became higher but it increased less significantly with increasing strain. In the fracture region, the potential noise reached the maximum level. In addition to the dependence of electrochemical noise on strain level, it was also found that the electrochemical noise level increased with increasing strain rate. To interpret the generation mechanism of electrochemical noise, a simple model was proposed based on an assumption that strain results in breakdown and repairing of the passive film on the steel. With this model, the dependence of electrochemical potential noise and its power spectral density on strain level and strain rate can be successfully explained.