Abstract
Alternate immersion cyclic exposure of prime coated and scribed steel sheet resulted in lifting of the coating at the edges of the scribe. A thick oxide corrosion product film grew in the scribe and under the adjacent coating. Continuous immersion did not produce thick oxide films or lifting of the coating. A phosphate pretreatment limited growth of the oxide to an area within the scribe and prevented attack at the coating/metal interface in cyclic exposures. Polarization resistance, measured during the immersion segment of the cycle, was higher for phosphated specimens, reflecting the lower extent of corrosion at the scribe edges. However, sensitivity was not high enough to produce any advantage over simple visual examination and measurement. In the presence of thick oxide films in the scribe, potential changes were very slow, and a modified procedure was necessary to measure polarization resistance. Galvanized and Zincrometal treated specimens were resistant to lifting of the primer coat during cyclic exposure, and phosphate pretreatment had little effect. In these cases, polarization resistance to some extent measured galvanic dissolution of zinc and polarization resistance values were comparatively low.
