Corrosion at cut edges is the most important failure mechanism of organic-coated, profiled galvanized steel architectural claddings. Currently, edge corrosion is generally controlled by the addition of strontium chromate in the paint primers; however, there is substantial interest in chromate replacements due to environmental reasons. This work describes an experimental study of inhibition with specific relevance to the cut-edge situation; essentially equivalent to a small galvanic cell between zinc and steel. Although chromate initially acts as an anodic inhibitor for zinc corrosion at the cut edge, over a few hours of immersion, it was found to also strongly inhibit the steel cathode, hence reducing the cathodic protection current requirement on the zinc and thus acted as a mixed inhibitor in the cut-edge galvanic cell. Although individually, zinc phosphate and calcium ion-exchanged silica pigments had relatively poor inhibition, they showed a strong synergistic effect. Thus, a mixture of the two compounds had comparable inhibitive efficiency to chromate. This was found to be due to a similar mixed inhibition mechanism as chromate. Thus, anodic inhibition of zinc was evident as well as strong cathodic inhibition on the steel due to the formation of a compact, thin film containing zinc, calcium, and phosphate species.Combined metal/polymer coatings are widely used in industrially developed countries for protection of different objects from atmospheric corrosion. Their application has a great economic benefit compared with organic and metallic coatings applied separately 1-3 because the durability is dramatically increased. These combined coatings consist of a metallic layer ͑e.g., zinc, aluminum, or their alloys͒ applied by different methods with hot dipping mostly used for this purpose. Further, an organic coating, which includes a primer and top coat, is applied onto the metallic layer after pretreatment. These are then usually profiled before being used for cladding ͑roofing and siding͒ of buildings. Although coil-coated products are used for other applications including domestic appliance casings ͑''white goods''͒ and in the automotive industry, this work focuses on their use in the building industry.As the application of the anticorrosion system is carried out under factory conditions the quality of the protection achieved is very high. However, mechanical damage to the coating may take place either during transportation or construction. In addition, the edge of the steels where they have been cut are clearly uncoated. Thus, the unprotected cut edges corrode under the additional influence of a galvanic couple between the exposed rim and steel. 4,5 Under the circumstances the zinc layer serves as a sacrificial anode and the steel as a cathode. The zinc layer has a typical thickness of 15 m, while the typical thickness of steel is from 0.4 to 1.4 mm. Thus, the current density on the anode is relatively high and accelerated zinc dissolution will take place. One effective means for the control of zinc dissoluti...
