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.
Examples of formulation
Aluminum is a very complex metal to be treated for its chemical ductility, a characteristic that makes it also a metal of great value and increasingly used. There are three basic types of treatment for aluminum, acid or alkaline etching and anodizing (or other electrochemical processes).
Being a material very reactive with air, on its surface there is always a thin layer of natural aluminum oxide which preserves it over time. To clean the aluminum, it is therefore necessary to remove this irregular oxide layer, degreasing the surface and recreating a regular oxide layer that uniforms the nuance. This is possible through an alkaline etching (usually based on soda) or through a etching based on fluorinated acids or ammonium bifluoride. The strong reactivity creates insoluble hydroxides and gases that must be contained and aspirated. After alkaline etching, acids, such as nitric acid, can be used to remove the particles and irregularities still present on the metal.
Depending on the performance and the final appearance that you want to achieve, the aluminum surfaces may undergo different finishing processes, for which we refer you to an expert company in the sector, Aluservice srl of Lainate.