Effect of Heat Treatment on Corrosion Resistance of Cu–Ni–P Alloy Tube

Abstract

Since the soaring price of copper during these years, demands to increase material strength and lower material costs have been driving manufacturers to develop high strength copper alloy tube with thinner wall–thickness for heat exchangers. It has been confirmed that the strength of Cu–Ni–P alloy is increased by precipitation strengthening even after brazing. Corrosion resistance evaluation is also necessary for the development of copper alloy tubes for air conditioners. The main corrosion problem of copper tube for air conditioner is ant’s nest corrosion. It is also known that stress corrosion cracking (SCC) occurs in phosphorus deoxidized copper, which is classified as pure copper alloy, when tensile stress is applied in ammonia–gas atmosphere. Since the precipitation state of Cu–Ni–P alloy differs depending on the heat treatment conditions, it is important to focus the corrosion resistance when the cooling rate after heating is substantially varied. In this study, we investigated the effects of heat treatment conditions on ant’s nest corrosion vulnerability and SCC susceptibility using a Cu–0.93mass% Ni–0.24mass% P copper alloy tube. Ant’s nest corrosion test found that the corrosion morphology of furnace cooled material, air cooled material, and water quenched material exhibited a hemispherical shape, showing immunity to ant’s nest corrosion. These were considered to be the effect of high phosphorus content in the alloy. Stress corrosion cracking test under tensile stress in ammonia–gas atmosphere revealed that grain boundary cracking was observed in all of the furnace–cooled, air–cooled, and water–cooled material. Above all, water quenched material achieved highest SCC resistance, which had withstood more than 10 times longer than furnace cooled material.