Corrosion Resistance of 90-10 and 70-30 Copper-Nickels
90-10 and 70-30 copper-nickels were originally developed for naval condensers and piping. Today, copper-nickels have an established reputation for handling seawater in a wide range of conditions and applications.
Their corrosion resistance is achieved by the formation of a complex surface film which develops by an interaction with the seawater itself, thereby protecting the metal below it. General corrosion rates for 90-10 and 70-30 Cu-Ni alloys in seawater range between 0.025 and 0.0025 mm/yr. For a majority of applications, these rates enable the alloys to last the service lifetime.
Of the wrought copper alloys, copper-nickels have the best resistance to seawater flow velocity. As with all copper alloys, it is important that flow rates remain below the recommended design velocity to avoid erosion corrosion. In defined conditions of pipework, the maximum flow rate for 90-10 Cu-Ni is normally about 3.5m/s. In more open structures, the hydrodynamic conditions are different and the flow rate before the initiation of erosion can be much higher.
Copper-nickels are not sensitive to chloride attack as nickel-containing stainless steels might be. They have high resistance to chloride pitting, crevice corrosion, stress corrosion cracking, even at elevated temperatures. Ammonia stress corrosion cracking, which copper-zinc brass alloys can be susceptible to, is not found with copper-nickels in seawater.
Exposure to sulfides should be restricted, particularly while the protective oxide film is maturing.
Copper-nickel alloys fall in the middle of the galvanic series. As with all bimetallic coupling, unfavorable galvanic combinations should be avoided or standard preventative measures taken.