Pickling devices

When the right model makes the difference (part 1)

Dispositivi di decapaggio (parte 1)
Dispositivi di decapaggio (parte 1)

Different Clinox models for different applications

Over the years, post-welding technology has evolved, guaranteeing a very quick cleaning (or polishing) of the welding seam and with a good restoration of the passive stainless steel layer. In the market there are different models and different technologies that promise great results. The idea of ​​having a pickling device with exaggerated power in your hands could be the right choice but at what price? Very often you choose devices that, within your company, do not meet the target of minimizing costs and maximizing the field of application. In other cases the target is above all to minimize the risks deriving from the use of a system that uses dangerous substances and which must be used with all possible precautions. In a world like electrochemical pickling, it’s easy to get disappointments when expectations are high and the machine can’t satisfy them. Expectations are linked above all to the choice of the technical characteristics of the machines and to the possible advantages that the device offers.
Therefore the suitable parameters to differentiate the devices can be:

Among the various parameters we have not named the quality of the weld. This parameter remains constant as we consider it appropriate to consider that the bead is free from defects and that the weld does not generate:

  • Projections;
  • Non-metallic compounds (silicates, sulphides, etc.);
  • Intermetallic (chromium carbides, etc …);
  • Cracks and fractures;
  • Shape of the non-conforming joint (“glue” effect).
Dispositivi di decapaggio (parte 1)

Left: clean welding with CLINOX system
Right: Welding defects

The promise of quick and easy joint cleaning is always linked to the customer’s desire to make a welded joint with a quality that facilitates the pickling process. Expectations must therefore fall on the fact that electro-pickling removes metal residues that react effectively with the electrolytic solution accelerated by an electromotive force and not to resolve any defects in the weld. The latter must be solved by correctly setting all the parameters of the welding process. There are no fictitious exit routes to elevate the finished product to a higher quality and beat the competition.

Electric power and electronic technology
Most pickling machines fall into two categories:

  • Inverter Technology;
  • Transformer Technology.
Dispositivi di decapaggio (parte 1)

Inverter vs Trasformer

The inverter is an electrical energy converter. It transforms the incoming supply voltage (if alternating) into DC voltage (possibly raising or lowering the value, just like the transformer does, even with a completely different principle), and then recreates, in a completely electronic way, a voltage in exit, which can be fixed or variable, alternating (periodic or aperiodic) or continuous.

The efficiency of the inverter is typically better than that of the transformer, it does not depend on the weight of the equipment and depends less on the cost of the components. Instead, they depend on the type of circuit (there are several circuit solutions that allow the desired conversion to be obtained), on the quality and quantity of the components used. It is obvious that complex circuitry and a high number of components used can expose the inverter to greater incidence of faults, and reliability coefficients typical of simple electromechanical equipment, such as transformers, can be reached with the electronics of the inverters only by using single components with very high reliability (with effect on equipment costs).

With the same convertible power the inverter, compared to the transformer, can weigh half the half of a transformer, combining lightness with versatility of use (the transformer works at fixed input and output voltages, the inverter can be built to adapt at a wide range of input voltages and provide virtually arbitrary output voltages).
The transformer is an electric machine that, through a double conversion of energy from electric to magnetic and then, again, from magnetic to electric, transforms input voltage and current into different voltage and current at the output. Depending on the construction technologies, it has maximum electrical efficiency ranging from 55-60% to 85-90%. Transformers with higher yields are built with high quality technologies and materials and have rather high costs.
Furthermore, transformers are mostly made of copper and iron (or, as an alternative to iron, with higher energy efficiency ferromagnetic materials), so they are typically heavy. The more electricity will have to be transferred from their input to their output, the more material must be used in their construction, thus significantly weighing down the equipment into which they are integrated.

Transformers are typically used in applications in which the supply voltage is not subject to variations (neither in input nor output), although they can be used as the first stage of energy conversion in more complex systems (electromagnetic / electronic mixed) . The transformers remain, however, in a power range between one hundred Watts and several kW, the cheapest energy converter that can be chosen in an electrical installation.

The transformer does not have the capacity to adapt the electrical parameters. This results in a considerable expenditure of energy. The fact of not modulating the electrical parameters involves the supply of the maximum power that could lead to the creation of pitting in the moment in which the carbon fiber is not completely wetted by the electrolytic liquid, causing continuous sparks and microscopic surface damages. Unfortunately this defect is visible through an optical microscope and therefore the operator recognizes the damage now when the stainless steel has been installed and receives the first signs of corrosion (generalized or pitting).

Dispositivi di decapaggio (parte 1)

Pitting corrosion following cleaning using a toroidal transformer

The transformer, like all conductors, has to cope with the Joule effect. The latter depends on the resistance and the square of the current. In order to minimize this effect it is therefore necessary to decrease the current resistance or intensity. To reduce the resistance, the section of the conductors must be increased, but there is an economic and technological limitation in the dimensioning of the electric lines, also linked to the phenomenon of the voltage drop of the lines themselves. In order therefore to lower the current intensity a transformation is carried out increasing the voltage with the same power. By lowering the current intensity, there is the risk of having a slow pickling process compared to the inverter technology which keeps the current constant and modulates the voltage instead.

Types of electrolytes

Electrolytes are very important in the pickling process because they contribute to the detachment of welding residues, creating an ion exchange between the anode and the cathode. The reaction is catalyzed by electric current. Therefore an alternating current allows to accelerate the hydrogen ions of the solution against the surface of the metal and the dissolution of the welding residues. The amount of hydrogen ions is determined by the pH of the solution. The more acid is the solution, the more the ion concentration is high, thus favoring the detachment of the residues. It could be a favorable element but if the solution contains acids that are too aggressive, it could ruin the surface finish of the base metal, changing the aesthetic effect and the surface finishing. Solutions containing strong acids can lead to excessively attack the surface until, for example, leave acid inclusions that could lead to corrosion if not neutralized correctly.

A very important aspect is the restoration of the passive layer. Therefore, if the electrolyte solution remains anchored to the surface or too much attacks the surface, the stainless steel that could be found in a section of the company immersed in an iron powder or other contaminating agents does not have the possibility of forming that protective layer which makes it very interesting to many builders.

Another important factor is the conductivity of electrolytic solutions. At the same pH level, for example, an electrolytic solution that has a higher conductivity has the ability to clean the surface of stainless steel much faster. The conductivity is strongly linked to the amount of acid present in the solution. As the concentration of acid in solution increases, the electrical conductivity of the solution increases to a maximum value which, once exceeded, the conductivity decreases strongly. Taking advantage of these two parameters it is possible to obtain an electrolytic solution that has the right amount of hydrogen ions (pH), maximizing its electrical conductivity. The variability of pH and electrical conductivity allows us to differentiate electrolytic products based on the application and dangerousness:

With the same power given off by the electric source and type of welding, the reduction in the quantity of acid inside an electrolytic solution causes a lowering of the pickling speed. With these conclusions one could think of using a very high concentration of acid to optimize the pickling process even on TIG welding but if the operator managed the pickling process in such a way as to increase the electric power and reduce the quantity of acid, with the same pickling speed, would have the advantage of:

  • Keep the surface finish of stainless steel unaltered;
  • Absence of halos;
  • Increase the safety of the pickling process.

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