Intermetallic Growth in Hot Dip Aluminized SteelTiffany Sanders - Purdue Graduate Student
Hot dip aluminizing (HDA) is commonly used in automotive exhaust systems and drainage pipe to enhance corrosion resistance of mild steel. There are two types of HDA commercially available. Type 1 wherein the coating contains 5 to 11% silicon to promote better adherence and ductility, and Type 2 which is commercially pure aluminum.
The process to make HDA is fairly straightforward. The three steps are a surface treatment given to the steel, dipping in molten coating, and then cooling. In addition to the pure coating there are two predominant intermetallic phases formed. FeAl3 and Fe2Al5. When the "pure form" HDA is annealed at 600 C for 10 minutes, both FeAl3 and Fe2Al5 are observed at the interface). When annealed for 30 minutes faint Al is observed in the steel but the intermetallics were not characterized.
As received commercial (type 2) HDA is about a 25-30 micro-meters layer on the steel. During short anneals (2 - 16 minutes) little change was observed in layer characterization. With long anneals the FeAl3 layer was observed. Apparently the silicon addition limited the diffusion. With a 32 minute anneal three zones were identified: Al-Si coating, Intermetallic compounds, and substrate. The middle layer was studied and found to contain Al8Fe2Si and Al27Fe with about 10% Silicon. There was a continuous formation of the Intermetallic layer during annealing of the Type 2 commercial HDA.
In the future we want to further characterize layers by x-ray diffraction and obtain concentration profiles.