CORROSION BEHAVIOR OF ANNEALED AND HARD-DRAWN COPPER IN SOFT TAP WATER

The corrosion behavior of annealed and hard-drawn copper pipes was studied in soft tap water at room temperature. The work was carried out under stagnant and flow conditions on short- and long-term bases using electrochemical techniques. As-received copper pipes did not suffer from pitting regardless of whether the water was flowing or stagnant. The pipes experienced only mild general corrosion, which was more apparent in the hard-drawn copper. However, the pipes suffered from pitting when their initial air-formed oxide film was removed. The pitting attack occurred only when an external potential was applied and the water was stagnant. When general corrosion occurred, the protective oxide film thickened but did not separate from the metal surface. When pitting occurred, a thin and translucent cuprous oxide scale was formed. This scale separated from the metal surface, creating the occluded cell required for pitting initiation. The number of pits was similar to 7 pits/cm(2) to 10 pits/cm(2) for both types of copper and remained almost unchanged with polarization time. The average pit diameter was larger in the annealed copper than in the hard-drawn, but the average pit depth was almost equal in both types. Impedance measurements revealed that a mixture of capacitive and faradaic behaviors existed during the developmental stages of pitting, with an increase in the faradaic component as pitting progressed. Thus, the growth of pitting appeared to be dependent an mass transport through the oxide scale.