Nickel-base alloys passive film formation

More recently, attention has been directed to the "ninth form of corrosion, biologically influenced corrosion, which includes studies on an area referred to as "ennoblement. The presence of biofilms on metals and alloys immersed in natural seawater produces a complex, heterogeneous chemistry along the metallic surface. It has usually been observed that passive alloys such as aluminum, stainless steels, nickel-base alloys, or titanium show an increase to more noble (electropositive) potentials or ennoblement of several hundred millivolts with exposure time in natural seawater, thus magnifying the potential differences that may exist between dissimilar alloys [26,55-64]. Ennoblement is likely caused by the formation of microbiological films, which increase the kinetics of the cathodic reaction [55-63],... 

The nickel base alloys are typically singlephase, multi-component and contain iron and chromium. Carbon is present in the range of 0.03-0.08 wt%. From several investigations, it is clear that chromium increases the corrosion resistance of nickel base alloys by virtue of formation of a passive film containing chromium. Inconel... 

The alloy composition (and microstructure) has strong effects on all the aspects of passivity that have been described above chemical composition and thickness of the passive film, electronic properties, structure, and kinetics of formation. The influence of alloyed elements on the electrochemical characteristics of passive systems can be seen in Fig. 3-16. This is the same current-potential curve as in Fig. 3-1, on which the two major effects of alloyed elements are indicated lowering of the dissolution current in the active region and at the active-passive transition, and broadening of the passive region. A third effect, not illustrated in Fig. 3-16 but which will be discussed later, is the improvement of the resistance of the alloy to passivity breakdown and localized corrosion. For iron-based alloys, these beneficial effects are obtained with chromium, molybdenum, nickel, and nitrogen. 

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