Oxidation kinetics of Cr-coated zirconium alloy: Effect of coating thickness and microstructure

Cr coatings with the thickness of 4.5-9.0 pm and dense/columnar microstructure were deposited onto Zr alloy by cooled or hot target magnetron sputtering. Steam oxidation tests were performed under temperature ramp from 500 to 1200 degrees C and isothermal treatment at 900-1200 degrees C for 10 - 30 min. The measurements of mass gain showed different oxidation kinetics depending on microstructure and thickness of the as-deposited Cr coatings. The dense microstructure is favorable to prevent alloy oxidation as long as the Cr layer is intact. The higher activation energy of 202 kJ/mol is observed for the dense 4.5 pm-thick Cr coating while thicker columnar coatings have 177-183 kJ/mol. The time of transition from protective to non-protective behavior increases with coating thickness. It was shown that the 9 pm-thick Cr coating with columnar microstructure better protects the zirconium alloy from oxidation at 1200 degrees C for 10 min in comparison with thinner coatings. The fast interdiffusion of Cr and Zr at coating/alloy interface significantly affects the oxidation kinetics of Cr-coated zirconium alloy at temperatures above 1100 degrees C and long oxidation time.