Localised Corrosion

Copper-nickel alloys also have good inherent resistance to chloride pitting and crevice corrosion. In fact, crevice corrosion seldom occurs and is therefore not well documented. The mechanism is a metal ion concentration cell type and is different to that occurring in stainless steel as any corrosion occurs outside the crevice. Copper ions, which are released by surface reactions within the crevice, are not swept away and concentrate there. The area within the crevice becomes more noble than either the mouth of the crevice or the adjacent exposed region. The resulting corrosion occurs adjacent to the crevice, and tends to be shallow in nature.

Copper-nickel alloys are not susceptible to chloride or sulfide stress corrosion cracking or hydrogen embrittlement, and unlike brass have not been found to suffer cracking due to ammonia in seawater service. The presence of ammonia may however cause higher corrosion rates. Ammonia may be released as a result of decaying matter on the seawater side of condenser tubes or may be present on the steam-condensate side when certain water treatment chemical are used.

In the latter situation, ammonia concentrations may reach high levels in the air removal sections of condensers. The presence of ammonia can lead to corrosion. However, the 70-30 copper-nickel alloy displays lower corrosion in ammonia than the 90-10 alloy and the rates of both are lower than in those observed in other copper based alloys. Copper-nickel tubing is resistant to chlorination at normal dosing levels used to control biofouling. Excessive chlorination, however, can be detrimental.

While dealloying can occur in copper alloys, it is not a common occurrence in copper-nickel alloys. Denickelification or hot spot corrosion has been encountered occa-sionally in the 70-30 copper-nickel alloy in refinery overhead condenser service, where hydrocarbon streams condense at temperatures above 150°C. This appears to be due to thermo-galvanic effects resulting from the occurrence of local hot spots. The solution has been to remove deposits which lead to the hot spots either by more frequent cleaning or by increasing flow rates. Hot spot type corrosion can occur at near ambient temperature seawater if ammonia is present. Again, prevention requires avoidance of slow flow and dead areas; dosing the cooling water with ferrous ions is also advantageous.

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