Effect of Ti, Mo and Al Addition on the Sulfidation Behavior of an Ni-20Cr-13.5Co Alloy

Effect of Ti, Mo and/or Al addition on the sulfidation behavior of an Ni-20Cr-13.5Co alloy was investigated in H2-H2S mixtures with sulfur partial pressures of 10-7, 10-5.5 and 10-4Pa at 873K. A four-layers scale was formed on the surface of the alloys at the sulfur partial pressure of 10-4Pa, while at the 10-5.5Pa-sulfur pressure a triplex scale was formed. The sulfidation resistance of the Ni-20Cr-13.5Co alloy was improved by adding of Mo or Al, especially Al, in those atmospheres. Outer sulfidation layer of Cr3S4, thin inner sulfidation layer and internal sulfidation were observed at the sulfur partial pressure of 10-7Pa. As the atomic ratio of Al to Ti in the alloy was larger than 1 on the alloys added Ti, Mo and Al all together, the thickness of the internal sulfidation layer greatly got thinner in this atmosphere.