| Corrosion of metals is a multi-billion dollar a year problem in the US. Until recent years, corrosion research has been carried out in bulk conditions; that is, the whole surface of the metal is subjected to the aggressive environment and monitored during attack. While these experiments provide vital information on the propagation step of corrosion, the initiation steps leading to breakdown are not fully investigated. To better understand the initiation steps, it is essential to understand the influences acting locally on the electrode.; This research discusses the use of the scanning electrochemical microscope (SECM) as a tool to probe pitting-type corrosion on iron, stainless steel, and aluminum. Pit initiation is carried out through the electrogeneration of aggressive Cl- ions from the reduction of trichloroacetic acid. The SECM was used to break down a forming passive layer on iron in pH 6.0 phosphate-citrate buffer solution by generation of Cl- ions at the SECM tip. In addition, the SECM tip was used to detect the products of corrosion and to observe precursor events prior to actual breakdown. The probability of initiation of pitting corrosion was directly proportional to the position of the tip with respect to grain boundaries on the metal surface. A time dependence of passive film growth was also found. In areas devoid of defects or grain structures, a critical time of 1 second was needed to completely protect the surface from Cl- ion attack.; Further studies to better understand the passive film were carried out using aluminum. The SECM was used to electrogenerate OH- ions from the reduction of water to dissolve the aluminum oxide film. Once the oxide was dissolved from the surface, the film was allowed to repair at a different potential. The film was also altered by the addition on ions to the film repair solution that would incorporate in the repaired film. The reactivity of the newly repaired films was studied using the feedback mode of the SECM. It was found that the areas in which the oxide film was removed and allowed to repair were more reactive and therefore, more susceptible to corrosion initiation. |
