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 microscopy, and electrochemical polarization measurements. Corrosive attack occurs very early leading to general corrosion and pit nucleation, coalescence, 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 Cu(OH)Cl. The x-ray measurements revealed several significant findings: (i) the Cu(111) plane corrodes preferentially, (ii) Cu2O forms very early under open-circuit conditions and grows progressively with time. Conversely Cu(OH)Cl was detected microscopically on the corroded surface after 9 days and by x-ray diffraction after much longer times, (iii) CuCl forms only at anodic potentials, considerably higher than the free corrosion potential. Indeed, it was not detected under free corrosion conditions, and (iv) the composition of the corrosion products which were retained on the alloy surface is quite different from those which spalled off the corroding surface.