Scale (high temperature oxides) can be removed by immersion in suitable acid solutions. Paste style de-scaling solutions are also available for localised treatment on large structures or components.
Passivation / Removing Ferrous Contamination
Stainless steel can pick up carbon from machine beds, tools, racks, etc., which can result in unsightly rust staining. This can be removed by using dilute nitric acid and this is sometimes called “calcination” because of the ability of oxidation acids to accelerate the formation of acid film. Etching operations are conducted on stainless steel by dry or wet methods.
The wet methods use strong acidic solutions, typically nitric/hydrochloric and nitro/hydrofluoric acid solutions.
The extent and depth of the etching can be controlled by the type of solution and by limiting the contact times of the etching solution. Etching is, to a considerable extent, an art and a skill which is acquired through experience as well as extensive experimentation.
The term “polished” defines a range of finishes which generally are of two types, either: (a) satin or grained or (b) brightened and mirror polished. Polishing improves appearance and consistency, make cleaning easier and aids practicality to fabricate and repair/blend after welding and to mask minor damage. Satin Polished stainless steel is practical in use, widely available, relatively low cost and the most commonly used.
Ground – 1G Hot Rolled / 2G Cold Rolled
A coarse but uniform finish, generally obtained using abrasive belts and grits. Usually applied to hot rolled material and generally not to material under 2.5 mm thick. Often used to recover a rough surface prior to further polishing or texturing. This coarse finish can lead to tears and laps in the surface layer, resulting in potential corrosion problems due to localised oxygen starvation, which prevents the formation of the natural protective oxide film that endows stainless steel with its corrosion resistance.
Typical uses include food processing equipment, scientific apparatus and brewing equipment.
Dull Buffed – EN No. 2J for Cold Rolled
A semi-dull finish with a uniform grain generally applied to the supplied surface of the mill finish stainless steel. No pre-grinding is involved, so only the smoother mill finishes such as 2A or 2B can be used. This finish is obtained using a bristle type brush or fine abrasives. It is used where a dull, but even, finish is required and where appearance is not critical.
1J for Hot Rolled / 2J for Cold Rolled
Obtained using fine abrasives or a series of successively finer abrasives if surface defects have to be removed. A smooth, soft polish is produced which has a low level of reflectivity and uni-directional texture. The surface is normally applied to No. 2 D and No. 1 mill finish or to 2A or 2B when removal of all defects is essential. This finish is used for a variety of engineering applications where an even matt ground surface is the main criterion. This finish suffers from problems of consistency and is also harder to keep clean due to the dry-ground nature of the polishing process.
Typical uses include dairy equipment, bus shelters and lorry components.