Abnormal Deposition and its prevention in galvanizing coating on high sillicon steel.

Abnormal Deposition with an irregular surface and an abnormally thick structure involving many cavities was obtained on high silicon steel having both an ordinary surface scale (Fe3O4 and α-Fe), and a subscale with fayalite (2FeO·SiO2). In abnormal deposition H2 gas evolved in large amounts during pickling, leaving a residual subscale on the steel surface, and that, in hot dip galvanizing, H2 gas released from the residual subscale was arrested in the deposits and ξ-phase precipitated directly and rapidly on the steel surface through cracks in the δ-phase layer induced by H2 gas evolution. It was found that abnormal deposition could be prevented by: 1. Completely removing subscale by pickling. 2. Descaling by a mechanical method, without H2 gas evolution. 3. Baking if residual subscale is still present after pickling.