Theory of Electrochemical Corrosion

Three kinds of impinging jet, namely free jet, submerged jet and jet-in-slit were introduced for producing erosion-corrosion on the specimens of copper and copper alloys. The complete process of impingement attack was reproduced with submerged jet and jet-in-slit but not with free jet: the origin of this sort of erosion-corrosion is the separation of protective oxide layer from the metal surface due to shear force as well as turbulence force; free jet caused only the shear force. When the flow direction of test liquid was reversed in jet-in-slit the turbulence in the flow disappeared, and instead, characteristic flow velocity distribution or fixed vortex was produced on the specimen surface. In accordance with those the localized corrosion with the morphology similar to the differential flow-velocity corrosion or the horseshoe corrosion appeared. The theory of macro-cell corrosion renders a comprehensible rationale to the relationship between the morphology of the localized corrosion and the characteristic flow pattern of liquid as follows: a difference in the flow condition on a metal surface causes the difference in the anodic dissolution rate of the metal, which induces the formation of macro-cell of corrosion. Once a macro-cell is formed the corrosion rate, in particular, the metal dissolution rate at the macro-anode is accelerated through “macro-cell current effect” as well as “surface area ratio effect”. The measurement of macro-cell current was actually carried out during the progress of erosioncorrosion on a jet-in-slit specimen, and a clear difference was recognized in the behavior of anodic polarization curves at the corresponding locations. Thus, it was demonstrated that the erosion-corrosion on the copper alloy is electrochemical, localized corrosion.

Keywords: Free jet, submerged jet, jet-in-slit, impingement attack, shear force, turbulent force, fixed vortex, uniform corrosion, localized corrosion, micro-cell corrosion, macro-cell corrosion, polarization curve.