Behavior of Galvalume coated sheet steel at elevated temperatures in O2 and O2/H2O atmospheres

Galvalume (trademark of Bethlehem Steel Corp.) sheet steel samples were heated in pure oxygen and 97% O₂/3% H₂O atmospheres at temperatures ranging from 300 to 670°C. Time at a particular temperature was varied but did not exceed 48 hr. Above 480°C, the Galvalume coating became rapidly alloyed with iron to form Al₁₃Fe₄, and zinc was lost by vaporization. The Zn content dropped to about 15 wt%. The time required to fully alloy the overlay at 490°C was less than 4 hr. Below 480°C, only minor microstructural changes occurred, and coating integrity was maintained. No differences in behavior were observed by the addition of 3% water vapor to the gas stream.