Electrochemical Pickling

The electrochemical pickling has the purpose of removing surface impurities and oxides that are produced during the mechanical process. The process uses chemical solutions, typically acid ones, and the flow of electric current. The metal piece can be cleaned by immersing it in a chemical liquid with appropriate electrochemical baths or by operating directly on the affected area with suitable pads saoked in chemical solution.

Electrochemical process with pad

Electrochemical process with bath

Compared to the bath, the electrochemical process with pad – implemeted in our Clinox Line –  has many advantages:

  • It focuses only on specific areas (welds, corrosion points, localized impurities, etc …);
  • In case of special surface finishes (satin finishes, mirrored surfaces, designs, etc …), it does not alter the whole surface of the piece;
  • It is cheaper because it avoids the construction cost of the bathing plant (equipment, hydro, electrical systems, security systems, etc …);
  • It is more immediate because it doesn’t involve the many stages of the pickling bath (pre-pickling, rinsing, pickling, double rinse, passivation, rinsing).

However, the pickling bath has its advantages:

  • it creates a regular, smooth surface on the whole piece,
  • it reaches all parts of the piece and is able to clean complex geometries not accessible with a pad.

Electrochemical pickling is widely used because it is faster than chemical pickling and garantees high quality cleaning. In the case of stainless steel welding, electrochemical pickling is used not only for aesthetic reasons but also to increase corrosion resistance. After welding, chromium carbides form on the surface and contaminations occur.  These two factors deplete the anti-corrosive properties of the steel because they create favorable conditions for a corrosion process to start which, in time, will lead to dangerous failures. Pickling is used to remove any residue of iron powder that may have deposited on the steel, especially when mechanical drilling, turning, welding are carried out in the same environment in which the mechanical piece is found. The area where iron powder deposits is the most subjected to generalized or localized corrosion.

Crack over the welded parts

Generalized Corrosion

The electrochemical process is widely popular because:

  • Compared to chemical pickling, it does not leave halos and stains;
  • It does not use chemical solutions based on nitric acid and hydrochloric, which are extremely aggressive;
  • Compared to mechanical cleaning, it does not produce contamination caused by abrasive particles that get stuck on the steel’s surface.

Different cleaning methods

The electrical component of the process is very important because it allows to optimize and increase the speed of the process. Indeed, when the electric current flows on the piece, it can act as cathode or anode. If the piece undergoes a cathodic polarization, molecular hydrogen develops on the surface due to the presence of hydrogen ions.  The acidity of the solution generates these hydrogen ions.
The cathodic polarization:

  • Produces a “mechanical effect” of detachment and rupture of the residual welds due to the presence of gas (hydrogen);
  • Acts as an inhibitor against metal solubilization. This method is called cathodic protection.

Cathodic process on the welds

If the piece undergoes anodic polarization, the following reactions take place:

  • Metal dissolution reaction: the metal turns into ions, which flow into the chemical solution;
  • Production of oxygen. When the metal is hard to oxidize (for example gold) or is slightly soluble (lead in sulfuric acid), the process is faster.

With this type of polarization, in case the metal piece is composed of carbon steel or iron, the surface becomes very reactive and tends to oxidize again when exposed to air. In this polarization, pickling is greatly accelerated.  The process must stop when the residual has been removed, so as to avoid the removal of excess metallic material. This process is used by the machines of our Clinox line to obtain a shiny, clean weld or electro-polish metal pieces by immersion.

Polished welding

Polishing process with Clinox Rec

The most widespread current mode, also used in Clinox machines, is an alternating polarization of the part. In this case, the piece is polarized cathode and anode alternately, combining the effects of the two processes and obtaining:

  • Development of gaseous hydrogen (cathode);
  • Solubilization of the weld (anode).

These two processes work together to detach residues and protect the metal underneath.  This makes alteranting polarization faster.
The current transmission technology used is very important. The difference is made by the use of either a transformer or an inverter.

Transformer VS Inverter Technology

As you can see in the figure, Inverter technology has many advantages, compared to Transformer technology :

  • The device is lighter;
  • The current is constantly corrected during the pickling process (Power Factor Correction). In this way, the Clinox machine controls the current in order to avoid sparks and short-circuits that may damage the surface of the steel and create microscopic pits;
  • The power supply cost is lower: efficiency is doubled.

The electrolytic pickling process ends with neutralization. After pickling, the surface is covered by a layer of acid that, if not neutralized, can compromise the entire cleaning process and produce irreparable damage.

White halo after pickling

Not neutralized surface

Neutralising the surface means treating it with a solution that will restore its pH values, leading it to neutrality. The neutralising solution is basic. When it reacts with the acid solution used in the electrochemical pickling, it raises pH values to 6-7.

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