CORROSION OF ALPHA-AL BRONZE IN SALINE WATER

The corrosion behavior of alpha-Al-bronze (Cu-7%Al) was studied in 3.4 % NaCl solution for extended periods of time using x-ray diffraction, x-ray photoelectron spectroscopy, optical and scanning electron micros copy, and electrochemical polarization measurements. Corrosive attack occurs very early leading to general corrosion and pit nucleation, coa lescence, growth, and eventual merging of neighboring pits together to give multiple trough-like cavities. The corrosion products which form under free corrosion conditions appear as a duplex film with an inner adherent layer of Al2O3 and an outer layer of Cu2O, Cu2(OH)3Cl, and C u(OH)Cl. The x-ray measurements revealed several significant findings: (i) the Cu(111) plane corrodes preferentially, (ii) Cu2O forms very e arly under open-circuit conditions and grows progressively with time. Conversely Cu(OH)Cl was detected microscopically on the corroded surfa ce after 9 days and by x-ray diffraction after much longer times, (iii ) CuCl forms only at anodic potentials, considerably higher than the f ree corrosion potential. Indeed, it was not detected under free corros ion conditions, and (iv) the composition of the corrosion products whi ch were retained on the alloy surface is quite different from those wh ich spalled off the corroding surface.