Acid pickling

In the metal finishing, the use of acids for the removal of corrosion products prior to coating is widely practiced. The acids used depend on the metal or alloy being treated as it is essential that once the corrosion products have been removed, there is no attack on the exposed metal surface. For use in conservation the aim is to selectively dissolve the rust and leave the parent metal unaffected. Thus 5 M HC1 plus hexamine as corrosion inhibitor has been used for 

The main drawback of this method is that it removes surface layers and thus alters the shape of the artefact. Indeed, if the artefact is completely mineralised, the complete artefact will dissolve and there will be nothing left This contravenes one of the important aims of conservation in that the artefact s shape must be unaltered. Another serious shortcoming is that traces of acid may be trapped in crevices and cracks in the artefact, which will continue to rapidly corrode the metal after conservation during storage or on display. Overall, apart from lead-based artefacts, this method is not recommended in the majority of cases. 

As an alternative to the use of acids, sequestering agents have been employed to dissolve the corrosion products without attacking the parent metal. The most effective formulations are based on the derivative of ethylene diamine-tetra acetic acid (EDTA). Lead artefacts from the Mary Rose were cleaned in a 10% solution of this compound. The use of EDTA is not recommended for cast iron as the graphite flakes embedded within the corrosion products are also dissolved. As with the use of acids, the shape of the artefact is altered if the corrosion layers are very thick and it is also difficult to wash out all the solutions from cracks, crevices and pores in the artefact after cleaning. 

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Acid phosphatase Acid phosphatases Acid pickling Acid rain... 

Alkaline permanganate pretreatment of steel for the removal of heat scale and smut prior to acid pickling results in faster descaling and reduced metal attack (see Metal surface treatments Metal treatments). Stainless steel alloys can also be cleaned by alkaline permanganate followed by pickling in nonoxidi2ing acids (260). 

The inherent solderabiUties of selected alloys are Hsted in Table 9. Class IV and V ratings with this particular flux indicate the presence of oxides or other surface contaminants that may be removable with more aggressive flux or acid pickling. 

Acid Dips and Acid Pickles. Acids are used to remove inorganic soils. A distinction is made between acid dips and acid pickels. Where there is no mst or scales, a dilute acid dip is used for activation and as a rinse aid. Caustic residues from cleaners are notoriously difficult to remove with water rinsing alone. By contrast, strong acid pickles are used to remove mst and scale. 

After being cleaned, ware that is to be pickled is immersed successively in one or more tanks of water at 80—95°C and then transferred to the acid pickling solution. The pickling solution of 6—8% sulfuric acid is contained in a stainless steel tank or, alternatively, a lead-lined wooden tank at 60—65°C. 

Good Good 7.5 Widely used for sulfuric acid pickling equipment also for propeller shafts in motor boats precautions needed to avoid sulfur attack during fabrication... 

This is a process to remove heavy black scale and rust from the surface. Hot-rolled sheets that may have such scale formation need only be acid pickled. Cold-rolled sheets, which may carry no such scales, need not be acid pickled. Depending upon the type of surface, one of the following methods may be adopted. 

Acid pickling This can be done under the following operating conditions, either with sulphuric acid (H2SO4), or hydrochloric acid (HCl). H2SO4 releases a lot of fumes and is ineffective under cold conditions. It forms iron sulphate, which forms a hard deposit at the bottom of the tank and is difficult to remove (see table on next page). 

To reduce metal attack and fumes, use an acid inhibitor during acid pickling, A 0.01 % concentration is recommended,... 

Mildly scaled and mildly rusted surfaces Now the process of acid pickling may be eliminated if desired. Instead, only the de-rusting process can be used, as indicated in column 2 of Table A13.3. Alternatively acid pickling may be carried out as before, but at a lower concentration and temperature, as noted in column I. Since one cannot always be certain of the quality of sheet surfaces it is advisable to follow the process of acid pickling. 

Acid Mineral A Hydrochloric acid Pickling of metals... 

British Standard Aerospace Series, BS EN 2334. Chromic-sulphuric acid pickle of aluminum and aluminum alloys. BSi, 1977. 

Gelbbleierz, n. yellow lead ore, wulfenite. gelb-braun, brauolich, a. yellowish-brown. Gelbbreone, /. pickle (for brass), gelbbrennen, v.t. (Metal.) dip. pickle. Gelbbreonsaure, /. pickling acid, pickle. 

Acid Pickling This process is widely used for removing rust and millscale from steel, or for removing internal scales from boilers. The objective of the process is the dissolution of iron oxides or hydroxides or hardness scales, but at the same time the iron will also tend to corrode in the acid, with hydrogen... 

In situations such as the acid pickling of steel or the use of steel pipes to handle sour oil streams, the use of suitable inhibitors can give a significant reduction in hydrogen entry. In this context it is important to emphasise that the efficiency of an inhibitor in reducing hydrogen entry is not the same as its efficiency in reducing corrosion. Thus arsenic and antimony compounds... 

Before steel strip or rod can be cold rolled, tinned, galvanised, or enamelled, etc. any scale formed on it by previous heat treatment must be removed. This can be done by mechanical and other special methods, but if a perfectly clean surface is to be produced, acid pickling is preferred, either alone or in conjunction with other pretreatment processes. 

The furnace scales which form on alloy steels are thin, adherent, complex in composition, and more difficult to remove than scale from non-alloy steels. Several mixed acid pickles have been recommended for stainless steel, the type of pickle depending on the composition and thickness of the scale For lightly-scaled stainless steel, a nitric/hydrofluoric acid mixture is suitable, the ratio of the acids being varied to suit the type of scale. An increase in the ratio of hydrofluoric acid to nitric acid increases the whitening effect, but also increases the metal loss. Strict chemical control of this mixture is necessary, since it tends to pit the steel when the acid is nearing exhaustion. For heavy scale, two separate pickles are often used. The first conditions the scale and the second removes it. For example, a sulphuric/hydrochloric mixture is recommended as a scale conditioner on heavily scaled chromium steels, and a nitric/hydrochloric mixture for scale removal. A ferric sulphate/ hydrofluoric acid mixture has advantages over a nitric/hydrofluoric acid mixture in that the loss of metal is reduced and the pickling time is shorter, but strict chemical control of the bath is necessary. 

On ferrous metals immersion deposition in the copper sulphate bath produces non-adherent deposits, and a cyanide copper undercoat is therefore normally used. Where the use of a cyanide strike cannot be tolerated, an electroplated or immersion nickel deposit has been used . Additions of surface-active agents, often preceded by a sulphuric acid pickle containing the same compound, form the basis of recent methods for plating from a copper sulphate bath directly on to steel ". 

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