EIS and SVET assessment of corrosion resistance of thin Zn-55% Al-rich primers: Effect of immersion and of controlled deformation

The effect of biaxial strain on the electrochemical behaviour of a thin weldable primer containing zinc-aluminium particles applied to galvanized steel was studied by assessing both the integral behavior of the system and the micro-currents on the heterogeneous surface. The ionic currents near the surface were measured by the SVET and revealed to be sensitive to the shape and lifetime of micro-active areas of the coating. The EIS data on intact coatings revealed that immersion time caused a drop of the low frequency resistance while the capacitive slope remained constant; on elongated samples, a similar trend in low frequency was observed along with an increase of the capacitance. A model is proposed, based upon an electrical equivalent electric circuit that includes distinct elements mimicking the electrochemical behaviour of the exposed and embedded metal particles and for the polymer phase.