Corrosion Behavior of 304 Stainless Steel in Borate Buffer Solution:Effects of pH & Chloride Ions

Abstract

The thin adherent oxide layer formed on the metal surface keeps the surface of metal protective from corrosion i.e. known as passivation.This oxide layer may break due to different types of environmental effects leading to rapid corrosion.Hence the study on breakdown of passivity should permit one for the interpretation of process parameter and environmental restriction for long time of serviceable of the metal.The 304 stainless steel is having the higher percentage of chromium making it corrosion proof literally. In this thesis the study is on the passivity breakdown of 304 stainless steel in deaerated borate buffer solution as a function of pH and Cl ions.The pitting corrosion studies of 304 SS is carried out in borate buffer solution at varying pH(8.3,9.3,10.3)in the presence of varying chloride ions (0.1M, 0.5M, 1M).Corrosion studies were performed by potentiodynamic scans,microscopy techniques and point defect model.The study of the passivity and pitting can be evaluated through a point defect model.As increasing the pH of borate buffer solution, the breakdown potential increases i.e. the pitting tendency decreases whereas it got decreased with increasing chloride ion concentration. The value of breakdown potential was also found to increase with the potential scan rate.After obtaining the potentiodynamic curve pitting studies of the surface is done by optical microscope.The microstructure is shown in above figure at 1M chloride ion and at varying pH.The probability distribution in breakdown potential is the confirmation with the analytical prediction of the breakdown potential distribution obtained from the point defect model.