Copper pure, pitting corrosion

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. 

Figure 9-17. Schematic showing the corrosion of aluminum around an aluminum-copper intermetallic particle in an aluminum copper alloy with a copper content of 0.5-2%. The aluminum-copper particle, in the presence of pure aluminum, promotes the reduction of water (shown) or oxygen (not shown). Simultaneously, the reduction reaction causes the pure aluminum to oxidize and then dissolve. This localized corrosion (Al dissolution) results in the formation of pits.

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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