Nickel alloys hydrates

Passive films formed in aqueous solutions consist of an oxide or a mixture of oxides, usually in hydrated form. The oxide formed on some metals (e.g., Al, Ti, Ta, Nb) is an electronic insulator, while on other metals the passivating oxide film behaves like a semiconductor. Nickel, chromium, and their alloys with iron (notably the various kinds of stainless steel) can be readily passivated and, in fact, tend to be spontaneously passivated upon contact with water or moist air. It should be noted that passivation does not occur when chloride ions are introduced into the solution indeed a preexisting passive film may be destroyed. Many other ions are detrimental to passivity, such as Br, I, SO, and CIO, but chloride is the worst offender, because of its... 

Froment and co-workers " have employed REFLEXAFS (vide supra) for studying passive films on iron and nickel. Their early studies were concerned with demonstrating the applicability of the REFLEXAFS technique to electrochemical systems. Most recently, they have used this technique to study the structure of passive films on Ni and on Ni-Mo alloy electrodes. For the Ni electrodes, they performed studies after reduction at — 700 mV (vs. saturated mercurous sulphate electrode) as well as in the passive (-l-3(X)mV) and transpassive (-1-800 mV) regions. The Fourier transforms for the films in the passive region have a Ni—O peak at a distance that corresponds closely to that in bulk nickel oxide. However, no Ni-Ni interactions were observed. These investigators interpreted these results as consistent with a model that postulates an amorphous hydrated polymeric oxide. ... 

The passive film formed on austenitic stainless steel is duplex in nature, consisting of an inner barrier oxide film and an outer deposit of hydroxide or salt film. Passivation takes place by the rapid formation of surface-absorbed hydrated complexes of metals that are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that rapidly deprotonates to form an insoluble surface oxide film. The three most commonly used austenite stabilizers—nickel, manganese, and nitrogen—all contribute to the passivity. Chromium, a major alloying ingredient, is in itself very corrosion resistant and is foimd in greater abundance in the passive film than iron, which is the major element in the alloy. 

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