Nickel pure, pitting corrosion

With respect to resistance to pitting corrosion, there is an increasing advantage to be obtained by increasing the nickel content up to 50%. There is little distinction between the Fe-50Ni alloy and pure nickel. Data on the corrosion of Fe-36Ni alloy at an industrial site in the USA are reported by La Que and Copson and at a European site by Evans. ... 

Nickel exhibits good corrosion resistance in neutral and alkaline solutions and in the atmosphere. However, it corrodes in strongly oxidizing acids. Chloride-containing environments may cause pitting corrosion, especially in commercial grade nickel that contains sulfide inclusions. Pure nickel is quite soft and is used mostly as a coating. 

Nickel is usually alloyed with elements including copper, chromium, molybdenum and then for strengthening and to improve corrosion resistance for specific applications. Nickel-copper alloys (and copper-nickel alloys see Section 53.5.4) are widely used for handling water. Pumps and valve bodies for fresh water, seawater and mildly acidic alkaline conditions are made from cast Ni-30% Cu type alloys. The wrought material is used for shafts and stems. In seawater contaminated with sulfide, these alloys are subject to pitting and corrosion fatigue. Ammonia contamination creates corrosion problems as for commercially pure nickel. 

The higher nickel content of alloy 825 compared to alloy 800 makes it resistant to chloride SCC. Addition of molybdenum and copper gives resistance to pitting and to corrosion in reducing acid environments, such as sulfuric and phosphoric acid solutions. Alloy 825 is resistant to pure sulfuric acid solutions up to 40% by weight at boiling temperatures and at all concentrations at a maximum temperature of 150°F (60°C). In dilute solutions, the presence of oxidizing salts such as cupric or ferric actually reduces the corrosion rates. It has limited use in hydrochloric or hydrofluoric acids. 

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