73 Aqueous Corrosion

There are various forms of corrosion that can be classified into different categories. Although these differ in nature, they are also interrelated. These are:

• Uniform or general corrosion

• Galvanic corrosion

• Localized corrosion (pitting, crevice, or underdeposit corrosion)

• Inter granular corrosion

• Environmentally assisted cracking (hydrogen embrittlement, chloride stress corrosion cracking, sulfide stress cracking, corrosion fatigue, liquid metal embrittlement)

• Selective leaching

• Erosion corrosion

A detailed description of these are adequately covered in the open literature [6-8].

TABLE 7.2 Alloying Elements in Aqueous Corrosion-Resistant Nickel Alloys

Alloying Elements

Main Feature

Other Benefits and Corrosion Resistance Improvements

Cr Mo

Provides matrix for metallurgical compatibility to various alloying elements. Improves thermal stability and fabricability Provides resistance to oxidizing media.

Provides resistance to reducing media

Behaves similar to Mo but less effective. Detrimental to thermal stability.

Austenitic stabilizer—economical substitute for nickel.

Improves resistance to seawater.

Enhances corrosion in mild reducing media. Alkali media improves chloride SCC

Enhances localized corrosion resistance.

Enhances localized corrosion resistance and chloride SCC. Provides solid solution strengthening.

Provides solid solution strengthening.

Enhances localized corrosion resistance, thermal stability, and mechanical properties.

Enhances resistance to H2SO4 and HF containing acid environments.

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