Optimization of conversion coatings: study of the influence of parameters with experimental designs

The effect of conversion treatment parameters on the characteristics of con version coatings was studied in order to determine the optimal elaboration conditions, using statistical experimental designs. These conversion coatin gs must be characterised by strong interfacial adhesion, high roughness and high real surface area, which were measured by an electrochemical method. The influence of all the elaboration factors (temperature, time and bath co mposition: sulphuric acid, thiosulphate (accelerator), propargyl alcohol (i nhibitor)) and also the interactions between these factors were evaluated, using 2(5-1) fractional factorial design. A polynomial model was deducted f rom statistical analysis of the experiments. The evaluation showed, by usin g a designed experimental procedure, that the most important factors were s ulphuric acid, thiosulphate and alcohol concentrations. But the growth of t he conversion coating is also influenced by the interaction effects between the bath compounds. With such optimised conditions, the real surface area of conversion coating obtained was about 200 m(2) m(-2). This conversion co ating modified by alumina deposit increases the thermal oxidation resistanc e of the Fe-C alloy. A discussion of the use of experimental designs for in creased understanding and optimisation of conversion coating process is als o given. (C) 2000 Elsevier Science B.V. All rights reserved.