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

In order to determine the effect of treatment baths nature on the character istics of conversion coatings modified by electrolytic alumina layers, diff erent conversion/alumina coatings have been studied for a Fe-17%Cr ferritic stainless steel, Results for conversion coatings prepared in nitric or pho sphoric 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 coati ngs more or less protect the Fe-17%Cr alloy against oxidation at high tempe rature (1000 degrees C), The extent of their beneficial effect depends on t he conversion treatment procedure: conversion coatings obtained in nitric a cid modified by alumina deposit is more effective by reducing the oxidation parabolic rate constant, equivalent to the standard coating obtained in su lphuric acid. The electrochemical behavior, chemical composition and oxidat ion kinetics of the various coatings are analyzed. (C) 2000 Elsevier Scienc e Ltd, All rights reserved.