Cracking mechanisms in a hot-dip zinc coated steel

Hot-dip galvanizing is one of the most used methods to apply zinc-based coatings on steels in order to provide sacrificial protection against corrosion over all the steel surface. The efficiency of the protection is affected by the coating mechanical response to loading: cracking and coatings delamination during forming and/or during service could decrease the corrosion resistance and could allow the contact between the environment and the substrate (the steel). The aim of this work is the analysis of the hot dip zinc coated steel plates mechanical properties by means of a non-standardized bending test performed minimizing both the bending moment differences along the bending axis and the interactions between the clamping system and the specimen coating. Bending tests are performed both on non-coated and on hot dip zinc coated plates, correlating the measured variables (applied load and crosshead displacement) with the bending moment and the specimen bending angle. Tests are characterised by a good repeatability. Results show that the main damaging mechanisms depend on the different mechanical behaviour of the intermetallic phases and on their thickness. For all the investigated coating conditions, radial cracks are observed. They initiate corresponding to the Λ phase and propagate up to the Ζ-η interface. The coating thickness increase implies both an increase of the importance of the cracks in δ and Ζ phases and the presence of cracks at Ζ-δ interfaces. As a consequence, the increase of coating thickness implies an increase of the susceptibility to a coating-steel debounding damage mechanism, with a consequent loose of the coating adhesion and a decreasing of the capability of the zinc coating to improve the steel corrosion resistance.