Chromium-nickel alloys corrosion characteristics

Water environments can also have a variety of compositions and corrosion characteristics. Freshwater normally contains dissolved oxygen as well as minerals, several of which account for hardness. Seawater contains approximately 3.5% salt (predominantly sodium chloride), as well as some minerals and organic matter. Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper-nickel alloys, and nickel-chromium-molybdenum alloys are highly corrosion resistant in seawater. 

Ferro-alloys Master alloys containing a significant amount of bon and a few elements more or less soluble in molten bon which improve properties of bon and steels. As additives they give bon and steel better characteristics (increased tensile sbength, wear resistance, corrosion resistance, etc.). For master alloy production carbothermic processes are used for large-scale ferro-sihcon, ferro-chromium, ferro-tungsten, ferro-manganese, ferro-nickel and metallothermic processes (mainly alumino and sihco-thermic) for ferro-titanium, ferro-vanadium, ferro-molybdenum, ferro-boron. 

The engineering properties of electroless nickel have been summarized (28). The Ni—P alloy has good corrosion resistance, lubricity, and especially high hardness. This alloy can be heat-treated to a hardness equivalent to electrolytic hard chromium [7440-47-3] (Table 2), and the lubricity is also comparable. The wear characteristics are extremely good, especially with composites of electroless nickel and silicon carbide or fluorochloropolymers. Thus the main applications for electroless nickel are in replacement of hard chromium (29,30). 

Active-passive behavior is dependent on the material-corrodent combination and is a function of the anodic or cathodic polarization effects, which occur in that specific combination. In most situations where active-passive behavior occurs, there is a thin layer at the metal surface that is more resistant to the environment than the underlying metal. In stainless steels, this layer is composed of various chromium and/or nickel oxides, which exhibit substantially different electrochemical characteristics than the underlying alloy. If this resistant, or passive, layer is damaged while in an aggressive environment, active corrosion of the freshly exposed surface will occur. The damage to... 

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. 

Alloying is also used to prevent corrosion. Stainless steel contains chromium and nickel, both of which form oxide coatings that change steel s rednction potential to one characteristic of the noble metals. In addition, a new technology is now being developed... 

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