Passivation and corrosion of the metallic high performance materials Alloy 33 and MC-Alloy in different environments

The chemical composition and the thickness of surface layers on MC-Alloy and Alloy 33 samples after exposure to air and to different liquid environments were investigated using angular resolved X-ray photoelectron spectroscopy in combination with scanning electron microscopy. Corroding media included water, 96% sulphuric acid and 50% sodium hydroxide. Corrosion was only observed in sulphuric acid at elevated temperatures. Even in the alkaline medium the alloying element nickel did not contribute to the passive layer which consisted mostly of chromium(III)-oxide and in part of chromium(III)-sulphate after exposure to sulphuric acid. To understand this finding samples made from LC-Ni99 were studied, too. The composition of the surface under corrosive conditions allows to rationalize the different stability of the two alloys in hot sulphuric acid.