Abstract
The pitting corrosion of aluminum was investigated by autoradiography with 32P, 35S, 55Fe and 55Fe-59Fe mixture.
In the first experiment, commercially pure aluminum plate specimens were immersed in approximately neutral solutions containing 5ppm of phosphate labelled with 32P and 5ppm of chloride, and in those containing 50ppm of sulfate labelled with 35S and 5ppm of chloride. The test solutions had been chlorinated to a free chlorine content of 2-3ppm to induce pitting. After corrosion tests, thin collodion films were coated on the Specimens and autoradiographs were made by means of a contact film method. The distribution of blackening spots of the autoradiographs coincided accurately with that of pits formed on the specimens. Degrees of blackenings increased with pit growth. From these, it can be considered that phosphate and sulfate ions are concentrated radidly into the inside of the pits at the initiation stage of pitting and are adsorbed or precipitated with corrosion products such as aluminum hydroxide around the pits as the pits grow.
In the second experiment, radioactive iron was introduced into high purity aluminum, and then Al-Fe and Al-Fe-Si alloy plate specimens, labelled with 55Fe or 55Fe-59Fe mixture were prepared. The same preparation procedure as that in the first experiment was applied in this experiment. In addition, the newly developed Auger electronmicroautoradiography was used together with the stripping film method. Relatively large pits on the surface of the Al-0.4%Fe-0.1%Si alloy specimens and blackening images of autoradiographs coincided accurately. Precipitation of iron always occurred around each pit and progressed with the lapse of corrosion time.
Furthermore, the results of the identical experiments, carried out for Al-0.045%Fe, Al-0.08%Fe and Al-0.08%Fe-0.02%Si alloy specimens, showed that many tiny and rather large blackening spots of microautoradiographs, observed with a high magnification microscope, were located in the vicinity and the center of micropits. From these, it can be considered that micropits are formed by the local penetrative attack which takes place at the areas neighbouring intermetallic compounds such as FeAl3 and FeSi2Al4, and that iron ions from these compounds in the central parts of micropits are reduced at local cathodic parts around micropits and precipitate there.
