The high temperature oxidation protection for molybdenum by siliconizing, chromizing or Cr, Ni and Co electro-deposited coating, etc. was investigated and it was recognized that Cr and Ni double layer deposited coating was the most adequate among the oxidation protective coatings for molybdenum, but the oxidation resistance life of molybdenum at 1000°C in air obtained by this coating was up to 300hrs.
Therefore, investigating the cause of destruction of this coating at high temperature oxidation, a considerable effort was exerted to obtain Cr-Ni coating with longer oxidation resistance life by improving the electro-deposition process, that is, anodic treatment for molybdenum surface before chromium deposition and expelling absorbed hydrogen in Cr-deposited layer which are the cause of blistering and spalling of Ni layer.
By this improvement, the double deposited layer showing oxidation resistance life for maximum 700hrs, at 1000°C could be obtained when Cr layer was 0.025mm and that of Ni 0.150mm thick.
The mechanism of oxidation of this coating was also investigated. The oxidation resistance of this coating mainly owes to the outer Ni layer and inner Cr layer has nothing directly to do with the oxidation resistance. The rate of weight gain by oxidation is slower than that of pure nickel because of the diffusion of chromium into nickel layer by heating. It was discerned by X-ray diffraction and qualitative analysis that the oxidation product on coated surface was not nickel oxide (NiO) but solid solution with face centered cubic lattice in which Cr, Ni and O atoms were arranged irregularly.