Corrosion of Painted Steel Plate in Sea Water

Abstract

Vinyl paint is superseding oil paint as ship's bottom paint. The characteristics of vinyl paint film are better than those of oil paint film. However, there is a fear that, when some part of paint film happens to be defective, heavy local corrosion develops intensively at the defective part, as is often experienced on ships in service. In order to prevent the hull from corroding, cathodic protection is widely adopted along with the use of anti-corrosive paint. This method has proved effective, but there still remains a problem of deterioration of paint film as blistering and peeling. We investigated this problem by making immersion and rotation tests for 6 to 7.5 months in the sea and a continuous cathodic protection test for 5 months. This article deals with the effect of oil and vinyl paints applied on steel plates upon corrosion of the plates and the effect of cathodic protection upon paint film damage and deterioration. The durability of paint film of vinyl paint in sea water was good, but corrosion was apt to concentrate on the defective part of paint film. The pitting of vinyl painted plate was about two times as deep as that of oil painted plate. However, the application of cathodic protection with the use of vinyl paint even when the protected supply potential is as less noble as -1.0 V (vs. S.C.E.), had little possibility of blistering and peeling of paint film. Another advantage was that the required current density was only about 3 mA/m2.