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.