Conversion coating is commonly used as treatment to improve the adherence of ceramics films. The conversion coating properties depend on the structure of alloy as well as on the treatment parameters. These conversion coatings must be characterized 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 composition: sulphuric acid, thiosulphate as accelerator, propargyl alcohol as inhibitor, and surface state) and also the interactions between these factors were evaluated, using statistical experimental design. The specific surface area and optical factor (α) correspond to the quantitative responses. The evaluation showed, by using a designed experimental procedure, that the most important factor was “surface state.” Sanded surface allows the formation of conversion coating with high real surface area. A further aim was to optimise two parameters: treatment time and temperature using Doehlert shell design and simplex method. The growth of the conversion coating is also influenced by treatment time and temperature. With such optimized conditions, the real surface area of conversion coating obtained was about 235？m2/m2. 1. Introduction Coatings have been developed from various materials using several deposition methods [1–3]. Electrochemical deposition is an interesting technique to obtain corrosion protection coatings, but the problem for such coatings is adhesion. In previous papers [4–6], we described an original method to strengthen the interface between ceramic layer and stainless steel or super alloy substrate. This method involves three steps. In the first, the metal surface is modified by a conversion treatment in an acid bath with S2？ and acetylenic alcohol as additions, allowing the control of the conversion coating growth [6, 7]. This pretreatment of the surface leads to a conversion coating which is very adherent, with a particular morphology, with micropores that allow deposition during the second step and contribute to the “anchoring” of the ceramic layer. In the second step, a refractory character is conferred to the surface by a cathodic treatment in a suitable bath, which induces the deposition of oxides or hydroxides with varying degrees of hydration. In the third step, a thermal treatment leads to ceramic oxides and stabilized the coating. So, to strengthen the interface between ceramic and substrate, a specific pretreatment of the metal surface is proposed so as to form a conversion coating. The
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