Question

a) Explain the causes of the formation of; mill scale on steel plate.

b) Describe the preparation necessary before application; of conventional paints to under water surface of; the hull.

Answer.

(a) Steel plates supplied to ship builders have patches of black oxide known as mill scale, formed during the steel manufacturing process, adhering to the surface. This is insoluble and if maintained over the surface reduces corrosion. It is very brittle and does not expand mechanically or thermally at the same rate as steel plate. Hence this mill scale has to be removed before painting.

The proper procedure for surface preparation is

1) Wipe off all dirt, grease and oil with cotton rags,( rags to be disposed off safely as they can spontaneously ignite or cause pollution.)

2) Use a solvent or degreaser to remove any oily stains

( The first application of ;the solvent will only dissolve the oil and spread it. Two or three applications may be needed.)

3),Wash the solvent off with a mild steel detergent and rinse with fresh water taking care not to let the washing go overboard.

4) Remove all rust and scale.

If the plate is exposed to atmosphere rust forms under the mill scale. On wire brushing majority of mill scale is removed. This is known as weathering.

If the plates are immersed in a weak solution of sulphuric acid-or hydrochloric acid for a few hours the majority of scale is removed. This is pickling. The pickled plate must be hosed down with fresh water on removal from the tank, allowed to dry before painting. During drying a light coat of rust is formed and must be removed before painting.

Flame cleaning, oxy acetylene torch having several jets is used to brush the surface, burns the dirt and grease, loosens the surface rust, loosen the mill scale. The surface is immediately wire brushed and the priming coat applied while still warm.

Mill scale is removed by shot blasting. This removes dirt, rust and grease as well. This removes 95% to 100% mill scale and results in slightly rough surface which allows adequate adhesion of the paint. The plate is spray painted on emerging from the shot lasting machine