Effect of inter-critical annealing atmosphere on microstructure and subsequent corrosion behavior of hot-dip galvanized Mn containing high-strength steel

Abstract

A systematic study is performed on the development of hot-dip galvanized coatings on an Mn-containing high-strength steel sheet by varying the dew point (- 50, - 10, and + 10 degrees C) during inter-critical annealing of the steel strip at 800 degrees C. It also studies the effect of dew point on the corrosion behavior of the coatings in freely aerated 3.5 wt% NaCl solution. The reducing gas atmosphere consists of 95% N-2 and 5% H-2, where inter-critical annealing is carried out. Surface oxidation of the steel has a strong effect on the development of sound coating. A defect-free adherent galvanized coating is obtained on the annealed steel surface at a fixed dew point of + 10 degrees C, and it is attributed to the fine and continuous compact Fe-Al crystals compared to galvanized coatings produced at other dew points as well as the highest atomically dense (0002) basal plane. This also leads to the lowest corrosion rate (similar to 0.164 mm y(-1), where mm and y stand for millimeter and year, respectively) of the galvanized coating produced at a dew point of + 10 degrees C when compared with galvanized coatings produced at dew points of - 50 degrees C (similar to 0.279 mm y(-1)) and - 10 degrees C (similar to 0.259 mm y(-1)). The lowest corrosion rate of the galvanized specimen developed at + 10 degrees C dew point can be attributed to the uniform and defect-free coating surface, together with the dominance of the more atomically dense (0002) basal plane.