Smooth steel strip is covered with a film of oxides. This deposit of rust is removed by means of hydrochloric acid in four different tanks, each with its own specific strength. The first tank has the weakest solution, and the last is the most powerful. In each tank is a squeegee roll, which prevents the acid from following along with the steel strip as it moves through. Used acid is treated and recycled in an acid regeneration plant. The fumes from the acid, meanwhile, are absorbed by water in a scrubber. The air emerging from this process is, therefore, free of acid vapour.