Coal ash can severely attack alloy materials at high temperature. To understand coal ash corrosion, a critical study is needed to determine the effect of the elements in ash on corrosion and find an approach to reduce the corrosion rates in an ash environment at high temperature. Chlorine is a harmful element in coal. The role of chlorine in ash corrosion is investigated in simulated oxy-fuel environments. Long time tests up to 3000 hours were performed on various structural alloys at 750�C. Detailed results are presented on weight change, scale thickness, internal penetration, microstructural characteristics of corrosion products, and the cracking of scales for the alloys after exposure at 750�C for 3000h. In addition, we performed synchrotron nanobeam Furthermore X-ray analysis to study the phase and chemical composition of the oxide layers on the alloy surface. Results from these studies are used to address the role of chlorine in ash in the long-term corrosion performance of alloys. Nanobeam X-ray and SEM analyses indicate that chlorine can modify the diffusion mechanism near alloy surface, and increase corrosion rate.