Abstract
Reliability of the paint coatings on zinc plated steels is not sufficient enough for corrosion prevention, because in spite of the initial good adhesion of the coatings to the base metals, peeling might take place with the lapse of time. This study has been made, therefore, to elucidate the causes for the adhesion failure and to find out the methods to prevent the deterioration of the coatings. Five different paints classified according to the kinds of resins contained were directly applied, with and without zinc chromate corrosion prevention pigment, to the surfaces of electroplated zinc plates, fresh and stored, and hot-dipped plates, respectively. Each specimen was subjected to the salt spray test to accelerate the adhesion failure of the coatings, while the permeability of water for clear paint films was measured separately. Results indicate that water which can pass through the coatings reacts with ZnO and Zn(OH)2 on zinc metal surfaces to cause liberation of hydrogen and subsequent pin-hole formation in the coatings. Peeling also takes place because the corrosion products push the coatings upward as the corrosion products grow. It is shown that the deterioration of the coatings is accelerated with increasing thickness of the natural oxide layers on zinc surfaces, while zinc chromate can retard the growth of Zn(OH)2 under the coatings through which adequate amount of water can pass.
