Duplex conversion — alumina coating on stainless steel for high temperature applications

The optimization of conversion coatings on stainless steel in nitric acid solution is obtained using statistical experimental designs. These conversion coatings must present strong interfacial adhesion and must be very porous, in order to allow the deposit and the anchoring of the electrolytic ceramic layer. An electrochemical method was used to estimate the specific area of the coating. The influence of all elaboration factors (temperature, time and bath composition: acid, accelerators, inhibitors) and the interactions between these factors were evaluated, using 2(5-1) and 2(4-1) fractional factorial design. Doehlert uniform shell design was used in order to optimize the conversion coating. The most important factors were nitric acid and thiosulphate concentrations. With such optimized conditions, the real surface area obtained was about 100 m(2)m(-2). This conversion coating modified by ceramic deposits leads to the formation of a coat characterized by strong interfacial adhesion with the steel and by an efficient thermal barrier character. A discussion of the use of experimental designs for the optimization of conversion coating processes is also given. (C) 1998 Elsevier Science S.A. All rights reserved.