Abstract
Metallographic examination of anodically polarized nickel and chromium after long-time exposure at selected active, passive and transpassive potentials in H2-saturated, IN H2SO4 at 25 C (77 F) revealed significant differences in surface topography. Etch figures, present only in the trans-passive state, supported the belief that “active patches” are generated in passive films of these metals at potentials more noble than the passive region. The difference in nature of nickel surface in the early trans-passive region and visible oxygen evolution region is discussed.
Effect of temperature on passive nickel (i.e., nickel in passive potential region) over the range 25 to 95 C (77 to 203 F) was determined for H2-saturated 0.5, 1, 5 and ION H2SO4 (pH = 0.85 to −1.05). Apparent activation energies for dissolution of passive nickel were determined. Inflection temperature (Ti) above which “active patches” apparently are formed in passive film was dependent on acid concentration according to the expression 1/Ti × 103 = −0.11 pH+ 3.04 over the pH range investigated and was independent of the direction from which the temperature was approached. The change in apparent activation energy (ΔQ) was a function of direction from which the temperature was approached.
Measurements in which temperature was increased stepwise showed that ΔQ is related to pH by the expression ΔQ (kg - cal/mole) = 15,4 pH - 4.7 for acid concentrations studied. Because Ti and ΔQ are dependent on acid concentration, Seeger's activation energy for crystallization of a very thin pseudomorphic film and Kramer's exo-electron emission temperature which are independent of concentration cannot be used to explain fully generation of “active patches” in a passive film as proposed by previous investigators.
Effects of W Addition on the Electrochemical Behaviour and Passive Film Properties of Fe-Based Amorphous Alloys in Acetic Acid Solution
