The influences of Mo addition and air exposure on the corrosion behavior of amorphous Fe-8Cr-13P-7C alloy in de-aerated 1 M HCl

The change in the corrosion behavior of amorphous Fe-8Cr-xMo-13P-7C (x=0, 2, 4, 6) alloys in de-aerated 1 M HCl with molybdenum content and pretreatment has been studied. Amorphous Fe-8Cr-13P-7C alloy shows active dissolution in de-aerated 1 M HCl. This alloy becomes corrosion-resistant with the addition of molybdenum. X-ray photo-electron spectroscopic analysis reveals that the chromium content of the films decreases continuously with immersion time despite an initial sharp increase in chromium concentration in the surface film. No enrichment of chromium is detected in the surface films on molybdenum-free and molybdenum-bearing alloys except for 6 at% molybdenum alloy after prolonged immersion. Molybdenum is deficient in the air-formed film but is concentrated in the films on molybdenum-containing alloys throughout the immersion experiment for 20 h. Molybdenum oxyhydroxide, rather than chromium oxyhydroxide, is considered to be responsible for the formation of the passive film on molybdenum-bearing alloys immersed in de-aerated 1 M HCl. The addition of 6 at% molybdenum is effective in maintaining metallic luster for prolonged immersion due to the formation of the passive film in which molybdenum and to some extent chromium are concentrated. Phosphorus is little involved in the air-formed films on all alloys but accumulates by immersion in the passive films on molybdenum-containing alloys up to its bulk composition, whereas the phosphorus content of the surface film on the molybdenum-free alloy is low. Air exposure for 24 h prior to polarization or immersion raises the initial open circuit potential of specimens and retards the enrichment of molybdenum in the passive film on the 6 at% molybdenum alloy in de-aerated 1 M HCl.