Film Properties of Anionic Electro-Coatings (II):Effect of Electro-Coatings Processes on Scab Corrosion
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概要
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By using two kinds of zinc phosphates (Bonderite #3050, #3112) and three kinds of anionic electro-coat paints (Samples A, B and C), the effects of solvent wiping prior to an automatic surface treatment process, electro-coat application methods in terms of increasing the voltage, and baking conditions for the electro-coat paints on scab corrosion were examined.<BR>The scab corrosion of a given coatings system was produced first by damaging the film with a fall of a sharp arrowhead of 5 g from 1 m in height and a knife scratching, then exposing the damaged film for 6 months on the sea water of Toba Beach in Mie Prefecture.<BR>In addition to the marine exposure tests, a combination of the salt spray and humidity test with a given time cycle was investigated as an accelerated test method of the scab corrosion.<BR>The scab corrosion resistance was evaluated by measuring corrosion areas in mm2 for the arrowhead method and in mm for the knife scratch method.<BR>The test results thus obtained showed followings ;<BR>1) With Bonderite 3050 (shower + half dipping type), Samples A, B and C showed a very good scab corrosion resistance, irrespective of the changes in the electro-coat process.<BR>2) With Bonderite No. 3112 (shower type), however, remarkable differences were observed on the scab corrosion resistance of the three samples. With Samples B and C, solvent wiping, increasing the voltage in a two-step way (100 V, 5, vm + 200 V, 20um→ 25, vm) and the standard baking condition (170°C x 20 min) gave much better results than no solvent wiping, one-step way (200 V, 25, vm), and low or over-baking conditions (140°C × 20min, 190°C × 20min). Regarding Sample A, however, no significant effects of these factors on the scab corrosion were observed and it showed the best scab corrosion resistance in the three samples.<BR>3) This indicates that one of the most important thing to improve the scab corrosion resistance is to find an appropriate combination of a zinc phosphate and an electro-coating material.
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