Mechanical pickling

Different process to clean the stainless steel welds

Decapaggio meccanico

Types of welding pickling: mechanical pickling (right), chemical pickling (center), electrochemical pickling (left)

Over the years, the use of stainless steel has grown exponentially due to its mechanical properties and corrosion resistance qualities that make it attractive for the pharmaceutical, oil and food industries. Those who produce stainless steel often face customers who need to clean and protect the weld and the entire surface of the product.

To meet these needs, various production processes have been developed that can be used sequentially or taken individually. The use in sequence has an important weight because it could compromise the corrosion resistance of stainless steel if the sequence of treatments have a wrong order.

The pickling processes can be split in 3 sections:

  • Mechanical pickling;
  • Chemical pickling;
  • Electrochemical pickling.

Mechanical pickling

Mechanical cleaning involves abrasive brushes that remove the welding oxides and a part of the material below it. As a rule, the abrasives that must be used for cleaning must not be harder than the surface of the metal to be treated. Abrasives are usually in powdery form and incorporated with substances that allow adhesion to the roller and a lubricating action. The requirements for an abrasive suitable for cleaning are:

  • Energetic cutting action;
  • Brightening action;
  • Lubricating action;
  • Fat melting point between 50 and 60 ° C.

To get the right surface finish, mechanical cleaning has different grains. The grain indicates the size of the abrasive particles. By increasing the grain value, the size of the abrasive particles decreases. By decreasing the particle size, it is possible to switch from a degreasing to a finishing process. For stainless steel:

  • Degreasing : Grain 80-100;
  • Semi-finishing: Grain 120-180;
  • Finishing: Grain 240-320.

After a mechanical treatment, the worked surface could be like this:

Welded joint that has undergone mechanical pickling treatment

As can be seen, to reach a satisfactory condition, the surface must go through three stages. This determines an increasing of the treatment time, surface deformations, abrasive particles contamination and substantial aesthetic changes that could lead to a difference between the worked area and the unchanged one. These factors cause a reduction in the anti-corrosive properties of stainless steel near a point, such as welding, which is very delicate.

Evolution of the abrasive particle during the mechanical pickling treatment

In this figure the path made by the abrasive particles during the treatment is regulated by 2 steps:

  • At first time,  the particles hit the metal surface, generating the desired roughness;
  • During the process, the abrasive particles shatter on the jagged surface.

In this last step, The abrasive particles, together with the lubricant used during processing, create the perfect conditions to stimulate a corrosive process.

In this layer, the passive layer, which could form on the surface, is not compact and thick. Therefore, the stainless steel loses its property of resisting corrosion when it is exposed to more aggressive corrosive agents, such as the marine environment.

The satin finishing of stainless steel welds is a mechanical process that is required in kitchen furnishing to obtain a uniform surface with the rest of the surface. During the production cycle, after the satin-finishing the product is packed in protective films (sometimes acidic). To optimize the processing times, the time required for the steel to form the passive layer is not allowed. Sometimes this passivation called “natural” is not sufficient to withstand more severe conditions and attention must be focused on chemical or electrochemical processes.

In summary, a mechanical pickling leads to:

  • Aesthetic alteration of the product;
  • Contaminations of abrasive particles and lubricants on the surface;
  • Passivation: inconsistent and contaminated.

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