Microelectrochemical studies of pit initiation on high purity and ultra high purity aluminum

The onset of pitting on high purity and ultra high purity aluminum has been studied in the micrometer range by using a microelectrochemical cell in order to evaluate influences that induce localized corrosion. Microelectrochemical tests on polished samples of Al 99.999 and Al 99.9999 in 1 M NaCl showed that pitting at large areas occurred at several hundreds mV more negative than at small areas, suggesting that even very pure Al contains weak points. On rough surfaces weak points are more "activated" than on smooth surfaces. Locally measured pitting potentials of polished samples were shifted to more positive values than those measured on ground samples. Larger areas contain always small scratches, even on polished samples. Hence, the surface roughness showed a minor influence on the onset of pitting on larger areas. Microelectrochemical measurements performed on pure Al 99.99 with small amounts of copper showed that a copper content below 30 ppm is too small to have a beneficial or detrimental effect, whereas a copper content above 300 ppm probably leads to the formation of small inclusions or precipitations that acted as preferential corrosion initiation sites. A copper content around 100 ppm showed a beneficial effect on very small areas but not on larger ones.