Current transients of passive iron observed during micro-indentation in pH 8.4 borate buffer solution

Micro-indentation test of passive iron electrode in deaerated pH 8.4 borate buffer solution was carried out to investigate the rupture and repair of passive film. During driving a conical diamond micro-indenter with a load of 0.1 N order downward to the electrode and driving upward from the electrode, a couple of anodic current peaks were observed. The first current peak spiked during loading was responsible for partial exposure of iron substrate to the solution due to rupture of the passive film when the indenter tip contacted. The second peak emerged during unloading when elastic deformation recovered, which was ascribed to repair at the ruptured sites. The second peak current was larger than the first one. Both peaks were sensitively influenced by electrode potential or concentration of sulfate ions containing in solution. The model for a series of rupture and repair processes of the passive film by micro-indentation was proposed to discuss the current transients.     

不锈钢过钝化－二次钝化的研究

通过对３０４不锈钢在较高电位下过钝化和二次钝化的稳态过程和电化学交流阻抗（ＥＩＳ）的分析，认为不锈钢过钝化和二次钝化现象与膜／溶液（ｆ／ｓ）界面上的电化学反应有密切关系，并通过理论模型分析，解释了过钝化和二次过钝化膜在高电位下溶解速度高，而在开路自腐蚀电位下反而有较好的稳定性这一现象．     

Effect of N and C on stress corrosion cracking susceptibility of austenitic Fe18Cr10Mn-based stainless steels

Stress corrosion cracking (SCC) susceptibility of austenitic Fe18Cr10Mn alloys with 0.3N, 0.6N and 0.3N0.3C was investigated in aqueous chloride environment using a slow strain rate test method. The SCC susceptibility of Fe18Cr10Mn alloys in 2 M NaCl solution at 50 °C under constant anodic potential condition decreased with increase in N content from 0.3 to 0.6 wt%, and with addition of 0.3 wt% C to the Fe18Cr10Mn0.3N alloys. The present study strongly suggested that the beneficial effects of N and C on the SCC behavior of Fe18Cr10Mn alloys would be associated with the resistance to pitting corrosion initiation and the repassivation kinetics.     

Electrochemical study of the corrosion of different alloys exposed to deaerated 80 °C geothermal brines containing CO2

Corrosion of metallic engineering materials accounts for problems during geothermal operation in the Upper Rhine Graben (URG). Herein, we study the electrochemical behaviour of various metal alloys in an 80 °C simulated geothermal environment by using potentiodynamic polarisation and open-circuit potential measurements. Two different natural geothermal waters from URG geothermal sites were used for the experiments. The measurements reveal spontaneous passivation to be a key process for all alloys. This ennoblement protects more noble alloys from significant corrosion (e.g. titanium gr. 2, alloy 625) and brings less noble alloys to failure, mostly due to pitting corrosion (e.g. 316L).     

Electrochemical and corrosion behaviour of stainless steels 316L and 317L in chloridised ammonium sulphate solution

Abstract

The corrosion of stainless steel alloys 316L and 317L in chloridised ammonium sulphate solution at 100°C and varying pH was investigated. Alloy 316L was found to be an adequate material for use in naturally aerated solution within the pH range of 5·4–7·4. At low potentials (<0·23 V) and a high potential (0·75 V), alloy 317L shows better performance. It also exhibits better pitting resistance at high potential, however, in the potential range of 0·23–0·45 V, alloy 316L shows better corrosion resistance. The dissolution of the Mo oxide species is proposed to be the reason for the weak performance of alloy 317L in the potential range of 0·23–0·45 V.

On a examiné la corrosion des aciers inoxydables 316L et 317L dans une solution chlorée de sulfate d’ammonium à 100°C et à pH variable. On a trouvé que l’alliage 316L était un matériel adéquat pour utilisation dans une solution aérée naturellement dans la gamme de pH de 5·4 à 7·4. Aux faibles potentiels (<0·23 V) et à potentiel élevé (0·75 V), l’alliage 317L montrait une meilleure performance. Il exhibait également une meilleure résistance à la corrosion par piqûres à potentiel élevé. Cependant, dans la gamme de potentiel de 0·23–0·45 V, l’alliage 316L montrait une meilleure résistance à la corrosion. On propose la dissolution d’une espèce d’oxyde de Mo comme étant la raison pour la faible performance de l’alliage 317L dans la gamme de potentiel de 0·23–0·45 V.     

Influence of stress on passive behaviour of steel bars in concrete pore solution

The influence of stress on passive behaviour of steel bars in concrete pore solution was studied with electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The passive ability of steel decreased as the applied load increased and higher load had much greater influence on passivation than repeated loading of small magnitude. A micro-crack model was presented to explain the damage of passive layer by loads. Lower load caused micro-cracks in the passive film which might be completely recovered after unloading. Under higher load more micro-cracks were produced in the passive film and some may penetrate the film, leading to irreversible damages.     

Tribocorrosion of 316L stainless steel and TA6V4 alloy in H2SO4 media

Tribocorrosion of stainless steel 316L and titanium alloy TA6V4 has been conducted in a sulphuric acid solution using an apparatus designed and built for evaluating the joint action of corrosion and wear. The material electrochemical and wear behaviours have been investigated during friction tests under electrochemical potential control. The specimens have been submitted to friction against an alumina ball under cathodic, free and anodic potentials. The friction coefficient, the wear rate and the current density were measured and the obtained results were discussed in terms of passivating film stability and repassivation kinetics.     

A study of the potentials achieved during mechanical abrasion and the repassivation rate of titanium and Ti6Al4V in inorganic buffer solutions and bovine serum

Titanium alloys in orthopaedic implants are susceptible to mechanical disruption of the passive film (fretting corrosion). To study this effect, open-circuit potential (ocp) measurements before, during and after mechanical disruption of the passive film in a tribo-electrochemical cell on commercial pure titanium and Ti6Al4V alloy in inorganic buffer solutions in the pH range from 2.0 to 12.0 and calf bovine serum at pH 4.0 and 7.0 are reported. Additionally, the effect of pH, electrolyte and sample composition on the repassivation rate has been investigated. The potentials achieved during the abrasion of Ti6Al4V are the same as those characterizing pure titanium, which indicates that the corrosion current of both materials in the active state is due to the oxidation of titanium. However, commercial pure titanium displays a tendency to repassivate faster than Ti6Al4V in inorganic buffer solutions thanks to the lower critical current density and the higher catalytic activity towards the hydrogen evolution reaction observed on the pure metal in comparison with the alloy.Proteinaceous solutions like bovine serum, significantly slow down the anodic dissolution and the cathodic reactions both on titanium and the alloy. However, the repassivation rate of the Ti6Al4V is not affected by serum, while that of cp titanium significantly decreases both at pH 4.0 and 7.0.     

A model for an anodic dissolution cell in connection to its dimensions for stress corrosion cracking

The anodic dissolution is modelled for pure iron for conditions simulating those of low alloy steels in high temperature water undergoing stress corrosion cracking. The dissolution is assumed to be initiated by the rupture of the protective oxide film and arrested by the repassivation. The goal of the model is to identify the relevant parameters of the anodic dissolution by a parameter sensitivity study. The cell is three-dimensional for taking into account the cathode to anode area ratio, at the price of the assumption of homogeneity of the electrolyte, and is assumed to be electrically uncoupled from the rest of the crack. The model uses the Butler-Volmer, the Ohm law, the charge conservation, and takes into account the passive current. The main results include the limited extent of the cell along the crack, depending on the values of the parameters, and non-linear and coupled effects for the occurrence and amplitude of the dissolution with as critical parameters the composition of the electrolyte, the exchange current densities and the anode dimensions.     

Repassivation behavior of alloy 690TT in simulated primary water at different temperatures

The repassivation behavior of Alloy 690 T T in simulated primary water at different temperatures was investigated by the rapid scratching electrode technique together with electrochemical measurements.The results showed that the repassivation process had three stages: the initial stage conformed to the place exchange model, the final stage conformed to the high field ion conduction model and in between there was a transition stage. At the initial stage, when the repassivation process of alloys was controlled by the place exchange model, anodic dissolution of substrate was dominated;after the film coverage rate was more than 0.99, the repassivation process of alloys was controlled by high field ion conduction model. Increasing the temperature resulted in a reduction of the repassivation rate and protectiveness of the passive film. The correlations among several mechanisms describing the repassivation behavior of alloys were discussed.