Millscale is a thin layer of iron oxides which forms on the steel surface during hot rolling of plates and sections.
This scale is insoluble and, if maintained over the whole surface, would reduce corrosion.
It is however, very brittle and does not expand either mechanically or thermally at the same rate as the steel plate.
Hence, this millscale has to be removed before painting.
Describe the preparation necessary before the application of conventional paints to the underwater surface of hull.
1. Wipe off all the dirt, grease and oil with cotton rags.
2. Use a solvent de-greaser to remove any oily stains.
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 hydro-chloric acid for a few hours, the majority of the mill scale is removed. This system, known as pickling, has been used.
The pickled plate must be hosed down with fresh water on removal from 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, loosens 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.
Describe a coating scheme for the underwater hull using a conventional paints.
The ships bottom has priming coats of corrosion inhibiting paint applied which are followed by an anti-fouling paint. Paints used are required resist alkaline conditions.
The paint should have a good electrical resistance so that the flow of corrosion currents between the sea-water and steel is limited.
Suitable corrosion inhibiting paints for ships bottom are pitch or bitumen types, chlorinated rubber, coal tar/ epoxy resin or vinyl resin paints.
The anti-fouling paints may be applied after the corrosion inhibiting coatings and should not come into direct contact with the steel hull.
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