Analysis of the Influence of Hydrogen on Pitting Corrosion and Stress Corrosion of Austenitic Stainless Steel in Chloride Environment

Abstract

Abstract In a study on the mechanism of pitting corrosion, a gas evolution in the pit has been observed. This gas has been gaschromatographically identified as hydrogen. The explanation of the evolution of hydrogen by the acidification of the pit electrolyte and the potential drop across the pit have been experimentally proved. By means of vacuum extraction experiments, the diffusion and dissolution of hydrogen in the metal has been shown. Experiments have been carried out to determine the effects of the dissolved hydrogen on the austenitic stainless steel. Replica techniques show that the hydrogen embrittles the metal around the pit. X-ray diffraction lines demonstrate that hydrogen causes phase transformations in the austenitic stainless steel. In the case of pitting corrosion, such transformations have not yet been confirmed. It is suggested that dissolved hydrogen can create new initiation places for pitting corrosion and in this way is responsible for the secondary pit initiation. The most important effect of hydrogen is connected with the internal stresses set up in the metal. There is no hydrogen development during the pitting corrosion of pure nickel. This leads to the conclusion that hydrogen is not of primary importance in the pitting corrosion process. A comparison is made between the pitting and the stress corrosion process. The electrochemical processes occurring in both localized corrosion forms are analogous. The hydrogen is responsible for the cracking in the stress corrosion process. Stress corrosion cracking is represented as a specific case of pitting corrosion.