Cracking mechanisms in high temperature hot-dip galvanized coatings

The cracking behavior of hot-dip galvanized coatings produced by immersion in a pure zinc bath at 560 degreesC is investigated after 3-point bending tests. Microcracks formed in the as-produced delta (1) phase as a result of tensile residual stresses developed during processing act as nucleation sites for cracks formed during bending. These cracks follow a Mode I path, normal to the substrate/coating interface, are arrested at the Gamma/delta (1), interface and continue to propagate parallel to the coating/substrate interface resulting in the delamination and failure of the coatings. Measurements of crack population followed by statistical analysis show that the density of cracks formed during bending in the coating is independent of coating thickness and depends on the applied strain, indicating that crack density is not controlled by crack initiation, but by crack propagation. It is also shown that the density of cracks that reach the surface of the coating depends only on the thickness of the outer Zn-rich layers of the coating that act as barriers to crack propagation. (C) 2001 Elsevier Science B.V. All rights reserved.