Stress corrosion cracking of structural steels immersed in hot-dip galvanizing baths

摘要

Failures during hot-dip galvanizing are occasionally, but they are important because of the high responsibility of the evolved structures. Traditionally two processes have been related with this phenomenon: liquid metal assisted cracking and hydrogen embrittlement. Hydrogen influence is discarded after the hydrogen concentration tests performed in this paper. Besides, crack characterization tests on steel CT specimens which are immersed in liquid galvanization bath with Zn, 1.1 percent Sn and 0.1 percent Bi at 450 deg C are detailed. The main obtained results were three: 1. The value of the threshold stress intensity factor, K_(th), which is the lowest value to produce cracking; 2. The subcritical crack propagation rate (da/dt)_(II); and 3. The failure assessment diagram (FAD) of the steel for two different load conditions. Microscopy revealed intergranular propagation and the existence of an intermetallic compound, the FeSn, at the crack tips. As a final result, the authors propose the key points of a qualitative failure model which consists of a first step of accumulation of Sn and Pb in crack tips, a second step of production of FeSn compound, and a final step of FeSn cracking due to accumulated stresses.