Discrimination by EIS of degradation mechanisms in lap joints of coated metal sheet

The information derived from impedance diagrams obtained by using electrochemical sensors specifically designed for lap joints testing of coated metallic sheets is analyzed. The electrochemical sensors were made by means of steel or zinc sheets, uncontaminated or contaminated with variable amounts of sodium chloride, and overcoated with organic coatings. Such sensors show the effect of the nature oft he metallic substrate, the underfilm saline contamination and the relative humidity of the environment outside the lap joint. The results show that the information obtained from electrochemical impedance spectroscopy (EIS) err:on the behavior within the lap joints can be assessed by the criteria traditionally used for plain painted metal surfaces. By modeling the data, it is possible to discriminate between effects due to the organic coating and to the corrosion reaction at the metal-coating interface.