High-temperature batch hot-dip galvanizing. Part 2. Comparison of coatings formed in the temperature range 520–555 °C
By Verma, A.R.B. & van Ooij, W.J.
Published in Surface and Coatings Technology
1997
Abstract
Batch high temperature hot-dip galvanizing (HT-HDG) samples were prepared at temperatures in the range 520–555 °C in steps of 5 °C. The coating thickness, phase structure and distribution, behavior of weldlines, and corrosion in the unpainted state as a function of temperature were evaluated. The results were compared with samples prepared at 455 °C which is the conventional galvanizing temperature. Coatings were found to be fully alloyed for the entire temperature range of 520–555 °C. The surface of the coatings in the entire temperature range had a microporous character, consisting of small monoclinic crystals of the ζ-alloy embedded in a solid solution of the S and ζ-alloy. However, the coatings at the lower end of the temperature range had a well defined ζ layer on top of a 5 layer. This coating changed to primarily the δ-alloy at the higher end of the temperature range. As the amount of ζ was much reduced at higher temperatures, the coating thickness changed from around 80 urn at 520 °C to around 20 μm at 555 °C with a maximum of 100 μm at 530 °C. At weldlines, the characteristic outburst reaction linked with high silicon steels was observed at 455 °C. This problem was found less severe for HT-HDG coatings in the entire temperature range. DC electrochemical corrosion testing of the samples revealed that the corrosion rate was essentially constant in the entire temperature range of 455–555 °C.