USE OF IN-SITU ATOMIC-FORCE MICROSCOPY TO IMAGE CORROSION AT INCLUSIONS

In situ atomic force microscopy was used in conjunction with microlith ography and scanning Auger electron spectroscopy to monitor localized corrosion near iron-rich inclusions in Al-6061-T6 immersed in 0.6 M Na Cl and also sulfur-rich inclusions in 304 stainless steel (SS-304) in 0.5M NaCl. The local rate of aluminum corrosion was found to depend on the shape of the nearby iron-rich inclusion. At the corrosion potenti al, trenches were observed to form in the aluminum host matrix adjacen t to the inclusions, and the corrosion sites gradually evolved into ci rcular shapes owing to dissolution. During the dissolution process, th e width of the dissolution area was an order of magnitude greater than the depth. Application of a 400 mV cathodic overpotential prevented c orrosion initiation, while application of a 500 mV cathodic overpotent ial greatly accelerated the dissolution rate in comparison with that a t the rest potential. On SS-304, exposure to 0.5M NaCl was accompanied by formation of deposits, which decorated the inclusion surface as we ll as the surrounding area up to four times the radius of the original inclusion.