Chemical pickling

Different pickling process on stainless steel

Chemical Pickling

Chemical pickling is a treatment to remove welding residues that exploits the chemical reaction between the welding oxide and predominantly an acidic compound. The chemical reaction involves the dissolution of the residue over time, facilitating its detachment from the metal surface. By dissolution we mean the process that leads to the formation of a liquid solution, in this way the welding residues dissolve in a liquid (acid) substance. As in all chemical reactions, an exchange of electrons takes place between the parts in play. The reaction therefore between a metal and an acid leads to the formation of a salt (in aqueous solution) and hydrogen (in gaseous form).

The factors that govern chemical pickling on welding can be:

  • Chemical composition of the solution;
  • Temperature;
  • Type of welding.

The chemical composition plays a fundamental role. The more stainless steel contains alloy substances, the more the substances used into the chemical solution must have a high pickling power for the same treatment duration. For this reason, mixtures based on hydrofluoric acid, sulfuric acid and nitric acid are used because they are able to “transform” the metal (solid) into ions and bring them into an aqueous solution. The reason for the danger of this treatment is related to the chemical composition: unfortunately, to clean the weld the compound is toxic. For this reason, the product application process must be performed with all possible protections and all liquids must be disposed of through further chemical processes. With the same type of steel and chemical composition, the duration of the treatment depends strongly on the temperature during the application. As it increases, the reaction energy increases, accelerating the pickling process. The amount of oxides to be removed depends on the welding technology used. MIG welding, which in almost all cases produces a greater quantity of residues than the TIG welding, requires a longer treatment time (keeping the other parameters fixed).

Since the chemical reaction takes place using an aqueous solution, the latter must be concentrated near the welded area. For this reason, the chemical solution is incorporated into a viscous gel. The gel allows the chemical solution to perform its function over time, slowing the evaporation of its components and, at the same time, creating a physical barrier, increasing the treatment time. After the pickling process, when all the residues have been removed, the neutralization phase with high pressure water jets follows in order to remove the gel from the metal surface. In this way unwanted corrosive processes are avoided around the treated area. Compared to mechanical pickling, the “natural” passivation of stainless steel produces a more compact film free from contamination, if the neutralization treatment is done ad hoc. Otherwise, since the presence of acid produces devastating corrosive phenomena (pitting), this process is much more damaging than the contamination of abrasive particles on the surface. The passivation time depends strongly on the substances used inside the gel. Since they are strongly acidic and very aggressive on the metal surface, the “natural” passivation time increases and, to re-enter the production times, it is necessary to adopt chemical solutions that accelerate the formation of the protective layer.

From the aesthetic point of view, the action of chemical pickling is difficult to control. Duration depends on many factors and stopping the process with neutralization at the right time is not always easy. Then in the application area, a halo is formed which outlines the decontamination and pickling process. The halo is caused by the too aggressive action of the chemical compound.

Stainless steel cleaned after pickling gel application

The last very important aspects among those mentioned are operator safety and waste disposal. The pickling gel is very dangerous because it is a Toxic and Corrosive mixture. Its effects on the person are not immediate but can be seen in the long term. Therefore the operator must adapt to the technical safety standards with:

  • Gloves;
  • Antacid suit;
  • Anti-acid protective glasses;
  • Mask against acid vapors;
  • Fume extractor (if necessary).

The chemical pickling must be carried out in wide and open spaces and the neutralized solution (water + gel) must then be stored in special tanks, ready to be disposed of by a company external to the own company or by an internal liquid disposal plant.

Neutralization process

Disposal of neutralized pickling solution

Over the years, the chemical pickling of welding has taken a step forward through the market introduction of Pick & Clean. The Pick is a strip of fabric filled with a chemical solution that is no toxic but only Corrosive which acts locally on the welded area. Neutralization takes place with the second product, Clean, which blocks the action of the Pick and removes all the welding residues, leaving the surface of the stainless steel clean and dry.

Pick & Clean

With this product:

  • The barrier effect of the gel is avoided: the fabric becomes only a vehicle to bring the chemical solution near the welded joint, reducing the treatment time. The wipe adheres completely on the surface of stainless steel creating an area with a smooth and well-defined surface;
  • Corrosive and non-toxic: only certain personal protective equipment must be used (only gloves and glasses);
  • Reduction of disposal costs: the wipes are solid waste;
    Storage space becomes smaller than neutralized liquid tanks. They become an integral part of the waste that is normally generated during the company’s production activity (for example, the cloth filled with oil and metal particles). From the economic point of view it becomes a great savings for companies;
  • Neutralization: the neutralization process involves only the area welded, neutralized with the Clean product . In this way the enormous quantity of high pressure water needed to neutralize the pickling gel is zeroed.

In summary, a chemical pickling leads to:

  • Aesthetic alteration of the product (undesired halos);
  • Use of maximum safety personal protective equipment;
  • Disposal of extremely hazardous liquid waste;
  • Passivation: when neutralization is carried out correctly, a compact and homogeneous layer is created due to natural passivation. Therefore to facilitate passivation it is advisable to have a place free from corrosive agents (eg sulphides, chlorides, etc.) and metal dust;
  • Process suitable for cleaning complex areas (inner part of pipes, cavities, etc.) for which a liquid medium is needed to clean the affected area.

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