Corrosive delamination and ion transport along stretch-formed thin conversion films on galvanized steel

Microdefects on thin conversion film coated Zn hot-dip galvanized steel (HDG) sheets were generated by stretch-forming and verified by cyclic voltammetry, which revealed higher anodic and cathodic current density levels on the pre-damaged samples. The data were compared to the kinetics of electrochemically determined ion transport processes along strained conversion film covered substrate surfaces in humid air and along uniaxially stretched epoxy/conversion layer/zinc interfaces exposed to the same corrosive environment. Analysis with a Scanning Kelvin Probe indicated increased driving forces for ion transport, but verified accelerated kinetics of cathodic delamination only in some areas of the stretch-formed epoxy coated substrates. This finding reflected a rather insignificant macroscopic acceleration of corrosion processes for samples that were strained by up to 15%. Similarly, accelerated oxygen reduction driven electrolyte spreading on bare conversion film covered HDG surfaces was verifiable only on a μm scale during initial process stages. This confirmed that stretching-induced defect formation increased the electrochemical activity of the substrates, but neither effectively nor significantly promoted the kinetics of ion transport along the sample surfaces in the present case. As a result, stable epoxy/conversion layer/zinc interfaces were maintained.