Over the years, post-welding technology has evolved, ensuring a cleanliness (o polishing) of the weld bead very quickly and with good restoration of the passive layer of the stainless steel. There are several models and different technologies on the market that promise great results. The idea of having in your hands a pickling device with exaggerated powers could be the right choice, but at what price? Very often people choose devices that, within their company, do not center the goal of minimizing cost and maximizing scope. In other cases, the goal is primarily to minimize the risks involved in using a system that uses hazardous substances and must be used with all possible caution. In a world, such as electrochemical pickling, it is easy to receive disappointments when expectations are high and the machine fails to meet them. Expectations are mainly related to the choice of technical characteristics of the machines and the possible advantages that the device offers.
So the suitable parameters to differentiate the devices can be:

• Electrical power;
• Electric current transmission technology;
• Types of electrolytes;
• Safety devices;
• Pickling process optimization technologies;
• Type of weld, weld length;
• Price.

Among the various parameters, we did not mention the quality of the welding. 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;
• Nonmetallic compounds (silicates, sulfides, etc.);
• Intermetallics (chromium carbides, etc…);
• Cracks and fractures;
• Non-conforming joint shape (“glue” effect).

The promise of quick and easy joint cleaning is always linked to the customer’s desire to produce a welded joint of a quality that facilitates the pickling process. Expectations therefore must rest on the fact that electro-pickling removes metallic 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 parameters of the welding process. There is no fictitious way out to elevate the finished product to a higher quality and beat the competition.

Electrical Power and Electronic Technology

Most pickling machines fall into two categories:

• Inverter technology;
• Transformer Technology.

Inverters vs Transformer

The inverter is an electrical energy converter. It transforms the supply voltage it receives as input (if alternating) into a direct voltage (possibly raising or lowering its value, just as the transformer does, albeit with a completely different principle), and then recreates, in a completely electronic way, an output voltage, which can be fixed or variable, alternating (periodic or aperiodic) or direct.

Inverter efficiencies are typically better than transformer efficiencies, do not depend on equipment weights, and depend less on component cost. Instead, they depend on the type of circuit (there are a variety of circuit solutions to achieve the desired conversion) and the quality and quantity of components used. Obviously, complex circuitry and high number of components used can expose the inverter to higher incidence of failures, and reliability coefficients typical of simple electromechanical equipment, such as transformers, are achievable with inverter electronics only by using individual components with very high reliability (affecting equipment costs).

For the same amount of convertible power, the inverter can weigh half as much as a transformer, combining light weight with versatility of use (the transformer works at fixed input and output voltages, the inverter can be built to accommodate a wide range of input voltages and provide virtually arbitrary output voltages).

A transformer is an electrical machine that, through a double conversion of energy from electric to magnetic and then, again, from magnetic to electric, converts voltage and current at the input into different voltage and current at the output. Depending on construction technologies, it has maximum electrical efficiencies ranging from 55-60% to 85-90%. Transformers with higher efficiencies are built with valuable technologies and materials and have rather high costs.

Transformers, moreover, are made mostly of copper and iron (or, as an alternative to iron, with more energy-efficient ferromagnetic materials), so they are typically heavy. The more electrical energy they have to transfer from their input to their output, the more material must be used in their construction, thus significantly burdening the equipment into which they are integrated.

Transformers are typically employed applications in which the supply voltage is not subject to change (either input or output), although they can be used as the first stage of energy conversion in more complex (mixed electromagnetic/electronic) systems. Transformers remain, however, in a power range between a hundred watts and several kW, the most economical power converter that can be chosen in an electrical installation.
The transformer does not have the ability to adapt electrical parameters. This results in considerable energy expenditure.

Not modulating the electrical parameters results in the provision of maximum power, which could lead to the creation of pitting when the carbon fiber is not fully wetted by the electrolytic liquid, causing continuous sparks and microscopic surface damage. Unfortunately, this defect is visible through an optical microscope, so the operator recognizes the damage now when thestainless steel has been installed and receives the first signs of corrosion (generalized o pitting).

The transformer like all conductors must cope with the Joule effect. The latter depends on the resistance and the square of the current. In order to minimize this effect one must therefore decrease the resistance or current intensity. In order to reduce the resistance, the cross section of the conductors must be increased, but there is an economic and technological limitation in the sizing of power lines, which is also related to the phenomenon of voltage drop in the lines themselves. In order, therefore, to lower the current intensity, a transformation is made by increasing the voltage for the same power. Lowering the current intensity runs the risk of having a slow pickling process compared to that with inverter technology which keeps the current constant and modulates the voltage instead.

Types of electrolytes

The electrolytes are very important to the pickling process because they contribute to the detachment of weld residue by creating an ion exchange between the anode and cathode. The reaction is catalyzed by electric current. Thus, an alternating current allows the hydrogen ions in the solution to accelerate against the metal surface and the dissolution of the weld residue. The amount of hydrogen ions is determined by the pH of the solution. The more acidic the solution the higher the concentration of ions, thus favoring the detachment of the residues. This could be favorable, but if the solution contains acids that are too aggressive, it could lead to ruining the surface finish of the base metal, changing the aesthetic effect and surface finish. Solutions containing strong acids can lead to over-aggressing the surface to the point, for example, of leaving acidic inclusions that could lead to corrosion phenomena if not neutralized properly.

A very important aspect is the restoration of the passive layer. So if the electrolytic solution remains anchored on the surface or attacks the surface too much, stainless steel that might be in a section of the company immersed in iron dust or from other contaminants does not have a chance to form the protective layer that makes it so attractive to many manufacturers.

Another important factor is the conductivity of electrolytic solutions. At the same pH, for example, an electrolytic solution that has a higher conductivity has the ability to clean the surface of stainless steel much faster. Conductivity is strongly related to the amount of acid present in solution. As the concentration of acid in solution increases, the electrical conductivity of the solution increases to a maximum value, above which, the conductivity strongly decreases. By taking advantage of these two parameters, an electrolytic solution can be obtained 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 according to application and hazard:

• Acid concentrations above 50%: Applications for cleaning/polishing demanding welds such as electrode and MIG/MAG;
• Acid concentrations up to 25%: Applications for cleaning TIG welds;
• 0% acid concentrations : Applications in pharmaceuticals, nuclear, food or situations where the company chooses an environmentally friendly and safe approach.

For the same power output from the electrical source and type of weld, reducing the amount of acid within an electrolytic solution results in lower pickling speed. With these conclusions, one might think of using a very high acid concentration to optimize the pickling process even on non-demanding welds such as GTAW, but if the operator managed the pickling process in such a way as to increase the electrical power and reduce the amount of acid,for the same pickling speed, he would have the advantage of:

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