Hot dip coating of ferrous metal with aluminium-zinc alloy - using protective atmos. of hydrogen and nitrogen with very low dew point

摘要

Parent Patent described the coating of a ferrous metal profile with a molten alloy of Al and Zn, effected in a hood contg. hot gas of compsn. by vol. 20% min. H2, balance N2 and dew point (d.pt) max. -18 degrees C. In this addn. the profile is fed continuously through a protective hood and into a bath contg. 25-70 wt % Al, rest Zn, the surface of the bath being covered by gas fed into the hood at min. 400 degrees C, and contg. by vol. min. 6% B2, pref. 615% H2, balance N2 and max. d.pt. -73 degrees C. Part of the gas circulates near the inlet for the profile, and part flows in counter-current to the profile. Only 6-15% H2 is needed to protect the ferrous profiles from oxidn. when the d.pt. is below -73 degrees C.