Performance of Zn-Al Alloy Coated Steel on Automotive Exhaust Systems

Changes in the coating structure of four hot-dip coated steels (Type 1 aluminized steel, 55%Al-Zn alloy coated steel, 5%Al-Zn alloy coated steel and galvanized steel) due to heating and their subsequent corrosion resistance towards muffler condensate were investigated to evaluate their basic performance as automotive muffler materials. Test methods used included continuous or cyclic heating and a muffler condensate test. Zn-Al alloy coatings are converted into the mixture of locally formed Fe-Zn intermetallic compounds outbursts and relatively uniform Fe-Al intermetallic compounds layer by heating at temperatures above 400°C. The former compounds become the initiating spots for the local corrosion of steel substrate with muffler condensate. Micro crevices, formed on 55%Al-Zn alloy coating by cyclic heating at around 300°C, accelerate muffler condensate corrosion. Aluminum coating is not affected by heating below 550°C and thus the corrosion resistance remains the same as unheated; it tends to locally corrode to some extent, but the corrosion rate is lower than that of Zn-Al alloy coatings.