Corrosion resistance of hot-dip Zn-6%Al-3%Mg alloy coated steel sheet used in automotive parts

Abstract

For the purpose of applying hot-dip Zn-6%Al-3%Mg alloy
coated steel sheet (“Zn-Al-Mg”) to automotive parts, we
compared and investigated the corrosion resistance of Zn-
Al-Mg and ordinary materials treated using a conventional
rustproofing method (“post-Zn-coated material”) exposed
to accelerated corrosion test environments. We also
collected automotive parts made from Zn-Al-Mg from
vehicles that had been driven for three to five years in
Canada to examine corrosion resistance capabilities when
exposed to actual vehicle environment conditions. We
found that Zn-Al-Mg exhibited better corrosion resistance
than post-Zn-coated material, even at portions where the
steel substrate was exposed (along cut edges and in bent
or spot-welded portions). Such Portions of the Zn-Al-Mg
were observed as being covered by fine and dense Zn
corrosion products containing Mg, which suppresses
cathode reactions (dissolved oxygen reduction reactions).
As a result, elution of the coating layer around such portions
was suppressed and favorable corrosion resistance
maintained. The flat and bent portions of automotive parts
made of Zn-Al-Mg collected from actual vehicles were
covered by fine and smooth corrosion products, with only
little corrosion being observed. It was thus confirmed that
Zn-Al-Mg also exhibits excellent corrosion resistance in an
actual vehicle environment.