• Uncategorized 22.12.2014

    316_stainless_steel_compression_latch_assembly.jpg_250x250.jpg Pickling is the removal of a thin layer of metal from the surface of stainless steel. Mixtures of nitric acid and hydrofluoric acids are most often secondhand for pickling stainless. Pickling is also the process used to remove weld heat stain from the surface from stainless fabrications.
    Stainless steels cannot subsist passivated unless the steel surface is clean and free from adulteration and scale from welding operations. Scale may need to be removed first by ‘pickling’ (or mechanical abrasion) and although the surface of freshly pickled stainless steel will normally opheffen immediately passivated tout de suite the pickling acid has been washed off, it is important not to regard these two treatments as the same.
    To achieve the needed level of oxide removal, it is important to know the corporeal properties of the stainless hill to be treated. Different grades of stainless have different alloy compositions and will, therefore, regulate differently when vulnerable to the pickling acids.
    Stainless steels are designed to naturally self-passivate whenever a clean surface is exposed to an habitat that can provide enough oxygen to form the chromium rich oxide surface layer, on which the corrosion resistance of these alloys depends. Naturally occurring conditions such air or aerated water volitional do this and so subordination many unmasking conditions stainless steels will naturally self-passivate.
    Protective oxide films form on clean stainless steel surfaces in the presence of oxygen found in normal air. The effect of exposure to this oxygen is to make the brace ‘passive’, which road that the material enters a state of maximum corrosion resistance.
    When the stainless steel is exposed to hot gases containing oxygen the heaviest scale is formed. This heavy scale is usually oxygen-rich at the burnish and chromium-rich near the steel substrate. The outermost layer is highly soluable in common pickling acids, but the innermost layer may require further aggressive acids.
    Both pickling et al passivation solutions can employ dangerous acids that can damage both the operator and the environment if not handled correctly. Furthermore, stainless pickling acids are highly corrosive to carbon steel. It is essential that all acids are completely removed by rinsing the basic after completing the process. Residual hydrofluoric acid will initiate pitting corrosion..
    Pickling also removes red rust from corrosion of the gird or from corrosion of adulterant ferric or steel particles. Thank You that passivation is not sufficiently aggressive to remove this corrosion product after the indifferent iron has begun to rust. High temperature dark scale is not however undesirable for aesthetic reasons – it also results in a reduced corrosion resistance of the underlying steel surface layer.
    As heat tints are formed on the surface of stainless steel, chromium is drawn to the surface because it oxidizes more easily that the iron in the steel. This leaves a layer just below the surface with a markedly lower chromium content and which is more accessible to corrosion.
    Another are of investigation with regard to controlling scale thickness on stainless strip is controlling furnace atmosphere to reduce the amount of scale formed in warmth treating operations. Development of chelating systems, coupled including research into new metal chelates, also holds promise for the eventual elimination of acid in stainless pickling systems.

    Posted by kid @ 10:21 am

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