These include sheet, strip, tube, wire, wrought shapes and castings. When the product is thin and of a high surface finish, scratches, gouging, tears, etc., may ruin the product to the extent of the damaged area. Seawater and fresh water produce stains and seawater may give rise to some corrosion. So long as the stains are superficial, the product can be used. If rough and pitted, the material may be rejected. Slight corrosion by general atmospheric conditions may also occur, forming verdigris. Manufacturing difficulties such as scabbiness, roughness or folds ("pinches"), can be readily identified, as these cannot be produced by rough handing or by corrosion in transit.
Surface defects, apart from corrosion pits, scratches and dents, can usually be attributed to faulty manufacture. Scabbiness is caused by the metal splashing when it is poured into the mold and cooling in droplets on the side. An oxide film is formed around these droplets which prevents them mixing with the rest of the metal in the ingot. Roughness is caused by the ingot sticking to the mold, causing a tearing of the surface on removal. Folds ("pinches") are caused in casting with too low a pouring rate, allowing a series of oxide films to be formed. All three defects originate in a faulty ingot and the faults remain because of the inability of oxidized surfaces to weld together. Superficial staining or slight corrosion may be removed by the use of mild abrasives or ammonia solution, or both, and provided the product is not made to accurate dimensions, scratches, etc., may be polished out with an abrasive. Ammonia solutions, with or without mild abrasives, may be used for cleaning by wiping provided the operation is carefully controlled and the surface is washed thoroughly afterwards.
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