Cu addition to the base alloy reduces the total amount of deleterious phases such as chromium nitride and sigma and chi phases. In particular, Cu addition to the base alloy results in pronounced suppression of the amount of sigma phase whereas it slightly facilitates the precipitation of chromium nitride and chi phase along the phase boundaries and within ferrite grains. During the initial stage of aging, the preferential precipitation of chromium nitride and chi phase seems to be closely associated with the retardation of the precipitation of the sigma phase. The preferential precipitation of chromium nitride and the chi phase inhibits the nucleation and growth of the sigma phase by depleting the Cr adjacent to the chromium nitride particles and depleting the Mo and W adjacent to the chi phase. Thus, the addition of Cu to the base alloy reduces its embrittlement owing to the delayed precipitation of these deleterious phases.

The effects of Cu addition on the microstructure and localized corrosion of hyper duplex stainless steels aged at 748 K were investigated using TEM analysis and electrochemical test. The addition of Cu to the base alloy facilitated the precipitation of a Cr-enriched ¡A-phase due to an increase in the Cr activity, thereby reducing the localized corrosion resistance. The localized corrosion was initiated at the Cr-depleted regions around the Cr-enriched ¡A-phase.