Intermetallic phases formed during galfanizing

Abstract

Galfan (5 wt% Al-Zn plus 0.05 wt% mischmetal or 0.1 wt% Mg) is a coating which in recent years has undergone many industrial studies and trials. The aim with Galfan is to produce a coating which has no intermetallic phases in the coating but this condition is not always achieved. Though it is important, the nature of the intermetallic phases and the mechanisms of their formation and growth during Galfanizing are far from fully understood. The overall objective of this investigation was to identify the intermetallic phases which form and grow during Galfanizing. At 450°C, the formation of the intermetallic phases was in the form of a local outburst and the intermetallic phases then grew in a breakaway morphology. The previously reported structure and composition of the intermetallic phases were found to be either incorrect or incomplete. The intermetallic local outburst was determined to be mainly Fe2Al5-Znx with the outer part next to the eutectic being FeAl3-Znx. The breakaway phase was FeAl3-Znx and another phase next to the interface was Fe2Al3-Znx. The intermetallic growth is characterized by the growth of Fe2Al3-Znx towards the substrate with a <001> growth direction, following a reaction path of α (substrate) - Fe2Al5-Znx - FeAl3-Znx - Galfan (melt). When the dipping temperature was increased, the rates of intermetallic formation and growth were greatly increased. The intermetallic morphology was shown to change from predominantly breakaway at 450ºC to predominantly layered at temperatures higher than 490ºC resulting from a fast growth of Fe2Al5-Znx. The degree of the preferred orientation of Fe2Al5-Znx, increased at higher dipping temperatures. It was shown that the lattice parameters and therefore the interplanar spacings of Fe2Al5-Znx formed during Galfanizing were dependent on the dipping temperatures. The same reaction path as that described for 450ºC was also followed at higher temperatures. However, when the substrate is dipped at high temperatures and when the Fe2Al5-Znx grows to a considerably thickness, the limited diffusion rate of aluminium across a thick Fe2Al5-Znx layer resulted in the formation of FeZn10-Aly. When silicon was present in the substrate, the time for the intermetallic formation was much lengthened and the growth of Fe2Al5-Znx and therefore the whole intermetallic layer became considerably slower. When silicon was present in the melt, the intermetallic phases did not grow to any noticeable extent, at 450ºC and 470ºC. However, at 490ºC and above though the nucleation time of the intermetallic was also delayed, once nucleated the growth was fast resulting in a highly localized growth. It was found that the existing data on the interplanar spacings of Fe2Al5-znx were incorrect. The two most intense peaks were shown to be due to diffraction from 221 and 311 rather than from 002 and L30. A new list of interplanar spacings was proposed. It was shown that the lattice parameters and therefore interplanar spacings of Fe2Al5-Znx formed during galvanizing were dependent on the aluminium content of the bath. The compositions of intermetallic phases determined in this study were not, consistent with those expected from previously published ternary phase diagrams. At 450ºC, the solubility of zinc was found to be 5.3 wt% in FeAl3, 20.9 wt% in Fe2Al5, and 2.0 wt% in FeAl at 450ºC. Based on these results, a new version of the Fe-Al-Zn ternary phase diagram at 450ºC was proposed.