NACE Resource Center
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| Resource Library - (Materials Selection) |
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Stainless Steel Pickling Pickling is the removal of any high temperature scale and any adjacent low chromium layer of metal from the surface of stainless steel by chemical means. Where the steel has been heated by welding, heat treatments or other means, to the point where a colored oxide layer can be seen, there is a chromium depleted layer on the surface of the steel underneath the oxide layer. The lower chromium content gives lower corrosion resistance. To restore the best corrosion resistant performance, the damaged metal layer must be removed, exposing a fully alloyed stainless steel surface. Stress Reference Mechanical removal may leave abrasive or other particles embedded Passivation or may be impractical, so chemical means are usually employed. Procedures incorporating pickling solutions of nitric Stainless steels and hydrofluoric (HF) acids remove the scale and the underlying chromium depleted layer and restore the corrosion resistance. Pickling solutions also remove contaminants such as ferrous and ferric oxide particles. Pickling solutions other than mixtures of nitric and hydrofluoric acids exist and can be used for specialized applications. Pickling pastes, where the solution is mixed with an inert carrier, are commonly used to treat selected areas such as welds. Pickling also removes red rust ()see rougingfrom corrosion of the steel or from corrosion of contaminant iron or steel particles. Note that passivation is not sufficiently aggressive to remove this corrosion product after the free iron has begun to rust. High temperature dark scale is not only undesirable for aesthetic reasons - it also results in a reduced corrosion resistance of the underlying steel surface layer. reference The type of scale and hence the methods to remove it will depend upon the steel grade and the heating conditions involved. The straight-chromium grades such as S41000, S41600 and S43000 scale more readily and unfortunately the resulting scale is also more tenacious. All pickling operations result in metal removal, and the outcome is therefore to some degree a dulling of the visual brightness and perhaps also a significant reduction in dimensions. If pickling does need to be carried out the treatments given in Table 2 can be used. An initial pickle in sulfuric acid is often beneficial as this softens the scale so that it can more readily be removed by subsequent pickling in hydrofluoric and nitric acids. The pickling procedures are summarized in the following table adapted from ASTM A380. Pickling procedures
Notes: Trial treatments should be carried out first to confirm that dulling is acceptable. Pickling should preferably be carried out on fully annealed stainless steels due to risk of grain boundary attack. This problem is especially relevant to steels sensitized in welding. All pickling treatments must be followed by thorough rinsing. Observe all precautions for handling acids - sulfuric, nitric and especially hydrofluoric acid are highly corrosive and dangerous to exposed skin. Pickling Paste A very convenient method for pickling is use of "Pickling Paste". This is a prepared mix of strong acids in a stiff paste which enables it to be applied to small areas and to vertical or even overhanging surfaces. It is especially useful for pickling to remove heat tint following welding. Again precautions for handling acids must be followed and the residue flushed thoroughly to a suitable waste stream after completion. Most commercial pickling paste is formulated for the austenitic grades, so if these are used to clean lower alloyed grades such as 3CR12 the process must be closely monitored to ensure the paste is quickly removed and very thoroughly rinsed off afterwards. Electropolishing Electropolishing is a useful alternative to pickling. Metal removal is achieved, but usually results in a bright, smooth and more highly corrosion resistant finish.
Numerous pages on this web site discuss passivation related topics: Beer, Biomaterials, Blocking, Calcareous deposits, Electrochemical noise, Electrode passivation, Galvanized, Inhibitors, Iron, aluminum bronze, Oxidizers, layer, Passive curve, Passivity, pH, Pickling, Pitting, Potentiodynamic polarization, Rouging, Nickel , Steel, corrosion cracking, Surface contaminants
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