Abstract: A conversion coating process for pretreating an aluminum or aluminum alloy surface in order to increase overlying coating adhesion and diminish corrosion and abrasion. The process involves delivering a flow of pure steam produced from deionized feedwater into a chamber. The flow of pure steam is directed against a baffle within the chamber causing the flow to diffuse. That is, the flow of steam is diverted from its initial direction of travel and caused to swirl. The surface of the aluminum or aluminum alloy metal is subjected to the swirling flow of pure steam so as to generate an oxide layer on the metal. The swirling pure steam subjects the resulting oxide layer to a pressure across its face which prevents the crystallization of the oxide layer in a defined and orderly pattern. The workpiece surface is preferably preheated to a threshold temperature prior to delivering the steam.