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.