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Iron pickling

27/03/2019

Pickling is a specific process to remove oxidations present on metal products. The use of these products is usually intended for heavy carpentry industries or where traditional amorphous phosphating fails.

How does it work

Acid pickling is a very aggressive pre-treatment and is carried out only when there is no possibility of sandblasting. Rust, oxides and calamine are difficult to remove chemically and it is often preferred to remove them mechanically. However, the market offers chemical alternatives, differentiated according to the metals to be treated. For steel it is known to use acids such as nitric, for iron it is preferable to use uncatalyzed phosphoric acid, so as to attack oxidations without excessively converting the clean surface. We will focus on aluminum for a moment, which represents a world apart, very complex for its chemical ductility, a characteristic that also makes it a metal of great value and increasingly used. There are three basic types of treatment for aluminum, acid pickling, basic pickling and anodizing (and other electrochemical processes). The satin finish and the refined nuance of the aluminum surfaces is therefore subject to countless different processes depending on the performance and the final appearance that you want to achieve.

Returning instead to the iron, we can divide the picklers into some categories; those based on inorganic acid, those based on organic acid and neutral ones. Inorganic acids are certainly among the strongest but represent a danger for operators and for the wear and tear of the plants, as well as creating fast reoxidation. The strongest inorganic acid is probably hydrofluoric, then followed by hydrochloric, sulfuric and nitric, or mixtures of these. Organic acids, on the other hand, are milder to attack oxidations but, acting for example on the percentages of use, we are able to have excellent results. Among the most renowned organic we have glycolic acid, sulphamic, lactic and citric. There are also "special" acids that are positioned halfway between an organic and an inorganic, this is the case with methanesulfonic acid. The other category is that of neutral picklers. It is a valid technology based on iron sequestering agents, however it is complex to use due to the difficulty of managing concentration and effectiveness over time. It can therefore be used for occasional work or in tanks with frequent product changes. It also requires a much higher percentage of product compared to a normal acid pickling.

Formulation examples

We have already talked about the acid components, these are often mixed together to create a balance between pickling force and inhibition to reoxidation. Sometimes inorganic technologies are hybridized with organic ones to help in this sense, since acids such as glycolic help slow down corrosion on contact with air. In synergy with acids, we obviously also find important additives, such as sequestering agents, corrosion inhibitors and the appropriate surfactants. The latter are often of an anionic, cationic or derivative nature of quaternary ammonium salts. Many non-ionic surfactants are often not very compatible with the tendency to surface especially by heating the solution. To avoid problems of rapid reoxidation, too rapid acids such as hydrochloric or nitric must be avoided, but phosphoric (which already has an auto-anticorrosive effect) and sulfuric are welcome. Temperature also plays an important role in this sense, if you can keep it below 50 ° C you limit these problems. However, a slightly alkaline final rinse is always recommended.

A classic example of iron paint remover is a concentrated formulation composed of phosphoric acid (even more than 50%), butylglycol, benzalkonium chloride and surfactants chosen according to the application (spray or immersion). Benzalkonium chloride is excellent for inhibiting reoxidation but causes a lot of foam and for this reason it must be well thought out.
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