Improving corrosion stability of ZnAlMg by alloying for protection of car bodies

Aluminium and magnesium are known for their ability to improve corrosion performance of zinc coatings used for steel protection in automotive applications. To investigate the inhibiting properties of other elements, series of model ZnX, ZnAlX, ZnMgX and ZnAlMgX alloys containing 0.2-2 wt% of titanium, mischmetal (mixture of cerium and other lanthanides), zirconium, molybdenum, chromium, boron, gallium, indium, copper, nickel, calcium, manganese and silicon were prepared and their corrosion performance in a cyclic accelerated test and at a marine field site and the ability to provide galvanic protection to steel in defects were characterized. On openly exposed surfaces, none of the investigated elements showed stronger inhibiting effect on atmospheric corrosion than Al and Mg. When exposed to marine climate, it was beneficial to combine Al and Mg. The corrosion stability of ZnAlMg was further improved by addition of a fourth element. Quaternary ZnAlMgX alloys outperformed binary ZnX and ternary ZnAlX and ZnMgX alloys. In average, mass loss was 4-fold higher in confined zones simulating hem flanges. Strong inhibition with Mg and detrimental effects of Al on corrosion in confined zones was found. Several quaternary ZnAlMgX alloys with improved corrosion stability in both open and confined configurations were identified.