PrOBOx®, the oxidation technology

High-strength steels contain alloying elements that form oxides during annealing. Such oxides cause wetting and adhesion problems during the subsequent galvanizing process. The PrOBOx® technology allows to solve these adhesion problems. The result is an iron layer of good wettability, forming a highly suitable substrate for a high-grade zinc coating.

The PrOBOx® technology has proven to be an ideal hot-dip galvanizing process for high-strength steels with high contents of silicon and manganese. It prevents wetting problems as known from conventional hot-dip galvanizing by the use of a special oxidation and reduction process. Controlled oxidation takes place on the strip surface at a defined temperature during annealing: a uniform and extremely thin iron-oxide layer develops on the strip surface. This layer prevents further alloying elements from migrating towards the surface and undesired oxides from forming on the strip surface. This oxide is subsequently reduced in the radiant tube furnace prior to galvanizing, forming a homogeneous and slightly porous iron layer. As the strip runs through the zinc bath, a very thin iron-aluminum layer will form on the porous iron layer. This intermediate layer is required in the galvanizing process as it serves as an adhesion promoter. Thus, during the subsequent coating process, a uniform zinc layer will form on the strip, coming up to highest surface finishing standards.

Special features of the PrOBOx® technology include a continuous control of the oxide thickness and a maximal oxygen pick-up by the strip, avoiding oxygen contamination of the furnace atmosphere.

The system can be implemented in the heating zone of a galvanizing furnace in two basic designs: either as a vertical PrOBOx® chamber or as a process chamber incorporating a deflector roll. The technology has been implemented, for example, in new galvanizing lines in USA and China.