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 t ests. Microcracks formed in the as-produced delta (1) phase as a result of tensile residual stresses developed during processing act as nucleation sit es for cracks formed during bending. These cracks follow a Mode I path, nor mal to the substrate/coating interface, are arrested at the Gamma/delta (1) , interface and continue to propagate parallel to the coating/substrate int erface resulting in the delamination and failure of the coatings. Measureme nts of crack population followed by statistical analysis show that the dens ity of cracks formed during bending in the coating is independent of coatin g thickness and depends on the applied strain, indicating that crack densit y is not controlled by crack initiation, but by crack propagation. It is al so shown that the density of cracks that reach the surface of the coating d epends only on the thickness of the outer Zn-rich layers of the coating tha t act as barriers to crack propagation. (C) 2001 Elsevier Science B.V. All rights reserved.