Solution Conductivity Influence on Pitting Corrosion Studies by SVET

The controversial statements about the solution conductivity changes occurring above the pits appeared on corroded Al alloy surface were verified. Original local conductometric measuring technique was employed together with SVET to evaluate the solution conductivity influence on the SVET results, completely dependent on the solution conductivity (IRsolution ohmic drop). Bare and chromated AA 7075-T6 aerospace aluminum alloy specimens were employed in 5 % NaCl solutions for the conductometric/SVET measurements. Artificial "pits" (Pt spots) with known dimensions were used for the SVET measuring instrument calibration. Solution conductivity variations of about 4.8 % above the non-chromated specimens' surfaces were registered at a distance of 30 µm 5 minutes after the specimens' immersion, against less than 1% changes above the chromated specimens at same conditions. A special noise reduction technique was applied allowing distinguishing of the small changes of the SVET signal caused by the conductivity influence.