High purity aluminium and magnesium were exposed in the field. The meteorological data showed that the dew point remained at a constant level in the short period. The time dependence of corrosion mass loss of aluminium followed a logarithmic law, and that of magnesium did a linear law. The constant dew point corrosion test (CDPCT) was employed to investigate the effect of chloride deposition rates and dew points. Aluminium was insensitive to the change of chloride deposition rates and dew point. Magnesium corroded more in higher chloride deposition rates and lower dew points. The corroded surface area of aluminium slightly changed with exposure time, and that of magnesium increased gradually.