The use of silicone resins as corrosion protective coatings for magnesium alloys

The future of magnesium sems to be as a structural material due to its light weight and ether excellent physical properties. In the chemical industry the use of the metal as a structural material is limited by its rapid corrosion, particularly in the presence of acids and salts. The recently developed silicone resins appear to be logical materials for use as a protective coatings for magnesium under these conditions due to their properties of heat resistance, chemical resistance, and water repellency.

The purpose of this investigation was to evaluate the silicone resins (DC 801, 802, 803, and 804) as corrosion protective coatings for magnesium alloys, and to determine the cause of difficulties encountered in the application of the resins to magnesium.

In flexibility and adhesion tests of the resins applied to magnesium alloy FS-1 and cured at 225°C. for periods of 1, 8, and 16 hours, it was found that resins 801 and 802 had excellent properties, but that resins 803 and 804 were unsatisfactory.

It was found that resins 802 and 804 have excellent oil resisting properties in oil resistance tests of the resins in contact with Texas crude oil at 100°C. for four hours. Resin 801 was found to be satisfactory only when baked for a period of 16 hours or more at 225°C. Resin 803 was found to be unsatisfactory due to crazing.

Cobalt naphthenate was shown to be a more efficient curing accelerator than benzoyl peroxide when used in a concentration of 0.30% based on the solid resin content. Benzoyl peroxide was proved to be better than no curing accelerator at all.

Polymerization tests of silicone resins 802 and 803, conducted in atmospheres of oxygen, nitrogen, carbon dioxide, and air for five hours at a temperature of 220°C ± 20°C., showed that oxygen is not necessary for the polymerization of the resins.

Service tests were conducted, on magnesium alloy YS-1 heat exchanger tubes coated with the silicone resins, by means of a double pipe, single pens heat exchanger. A three per cent sodium chloride solution was used as a corrosive medium at a flow rate of seven g.p.m. Steam at 260°F. Was used as a heating medium, and the outlet and inlet temperature difference of the apparatus was 17°F ± 4°F. The duration of the tests were from two to six hours. From the results of these tests it was found that resins 801 and 802 are not satisfactory protective coatings for magnesium, due to penetration of the corrosive medium at pinholes in the film. Silicone resins 803 and 804 were found to be unsatisfactory protective coatings for magnesium due to erasing of the resin film. The failure of the resin films was shown not be due to chemical attack by the sodium chloride solution; but due to mechanical failure caused by the corrosion products of the metal formed under the resin, by the corrosive medium penetrating to the metal substrate through mechanical defects in the film.