Effect of treatment baths nature on the characteristics of conversion coatings modified by electrolytic alumina deposits

Abstract In order to determine the effect of treatment baths nature on the characteristics of conversion coatings modified by electrolytic alumina layers, different conversion/alumina coatings have been studied for a Fe-17%Cr ferritic stainless steel. Results for conversion coatings prepared in nitric or phosphoric acid modified by alumina are presented. The proposed process leads to the formation of a coat characterized by good adhesion to the substrate and with low concentration of sulphur at the surface. All the studied coatings more or less protect the Fe-17%Cr alloy against oxidation at high temperature (1000°C). The extent of their beneficial effect depends on the conversion treatment procedure: conversion coatings obtained in nitric acid modified by alumina deposit is more effective by reducing the oxidation parabolic rate constant, equivalent to the standard coating obtained in sulphuric acid. The electrochemical behavior, chemical composition and oxidation kinetics of the various coatings are analyzed.