Chromising of stainless steels by the use of the CVD-FBR technology

The use of fossil fuels containing small amounts of sulfur, vanadium, etc. may lead to degradation of the boilers and tubes commonly employed in power plants due to the well-known hot corrosion attack. Chromide coatings have been shown to extend the service life of the materials employed in such applications. Austenitic AISI 304 stainless steel may be the material of choice under certain conditions due to its ratio properties/cost. However, the amount of Cr of approximately 18 wt.% may be insufficient to form, grow and regenerate the protective scales formed upon exposure to the aggressive environment because of the presence of a relative high Ni amount. The use of higher alloyed steels on the contrary makes the prices rise due to the need of a higher Ni content so as to maintain the austenitic structure. Therefore, it could be of interest to increase the amount of Cr on the surface of relatively cheap materials by surface engineering. Either single Cr deposition or Al/Cr co-deposition has been the goal of different studies mainly on Armco iron, low alloy steels or in Ni-base alloys employing different coating techniques. However, to our belief, very little or no work has been performed on investigating the formation of pure chromide coatings on austenitic stainless using the chemical vapour deposition in fluidised bed reactors (CVD-FBR). Thus, in this work, we will be presenting the results obtained by applying this technique to a commercial austenitic 18Cr–8Ni stainless steel at temperatures ranging from 825 to 900 °C using different H2/HCl ratios so as to obtain the best coating quality (i.e. composition and morphology). Attack from HCl to the alloy will be hindered to some extent by increasing the H2 amount in the coating medium. The deposition rate will be shown to increase with temperature but the quality of the coatings will be somewhat inferior. A solution of compromise is, therefore, found to fulfil all the requirements.