Stainless steel degreasing (part 2)
In this article we will introduce:
- Electrolytic degreasing;
- Ultrasonic cleaning.
The electrolytic degreasing is the most used method to prepare the metals before a galvanic deposition because it is the fastest and most efficient method among those classified. In this process the sample serves as a cathode or anode. In both cases the treatment helps the detachment of greases due to a strong gaseous development on the piece which guarantees a strong mechanical stirring, renewing the solution on the piece. The renewal of the solution is not guaranteed in the immersion chemical degreasing because the process is very static. In cathodic degreasing there is a development of hydrogen which generates a strong mechanical action, removing the impurities on the surface. The detachment takes place because the impurities and the surface of the piece have the same polarity (negative), and then they repel.
The disadvantages are:
- Dissolution of metals in solution;
- Hydrogen embrittlement.
In anodic degreasing there is a development of oxygen which allows the oxidation of organic substances on the surface of the stainless steel and to dissolve metal films that are created during the cathodic process. The presence of anionic surfactants greatly favors degreasing. The only disadvantage is: the prolonged use of the treatment could lead to a superficial oxidation of the stainless steel that could be eliminated through an electrochemical pickling acid. Very often, it tries to couple the advantages of the cathodic and anodic degreasing through a degreasing process in alternating current. The chemical solutions involved in the process are almost always alkaline in nature and very similar to those of chemical degreasing. The duration of the treatment extends to a maximum of 3 minutes. Degreasing solutions can be differentiated into:
- High temperature;
- Room temperature.
High temperature is more efficient because :
- The reactions take place rapidly due to the increase in the electrical conductivity of the electrolyte;
- It enhances the phenomena of saponification, emulsion and wettability.
The degreasing at room temperature is widely used only for purely economic reasons. In this situation all the advantages of the high temperature are reduced and the degreasing action is caused by the mechanical action of the gas that gurglings on the surface of the stainless steel. Obviously, following the degreasing, the washing treatment is followed to definitively remove the impurities and the foam that has developed during the treatment. The electrolytic degreasing implies a greater attention than those previously mentioned due to the use of electronic equipment but ensures a faster and more economical treatment time. The most common mistakes you can make are:
- Prolonged degreasing time;
- Voltage too low;
- Percentage too low of the mixture in solution.
If the treatment time is too long, the hydrogen atoms penetrate the crystal lattice of the stainless steel. It follows that a possible galvanic coating is flawed in many parts. We must pay close attention to the design of conductive substrates: if they have too small a section, due to the Joule effect, the electric energy is dissipated in thermal energy which leads to the net breaking of the support. Electrolytic degreasing can be carried out in two ways:
- By immersion;
- Using a pad.
Sample electrolytic cleaned by immersion
The first involves the use of tanks that internally contain the electrolyte liquid. They can be of various sizes and must have smoke abatement systems and a liquid heating system. Together with the liquid you can find the electrodes that can work as an anode or cathode depending on the degreasing treatment. The electrodes are connected to a current generator that provides the appropriate power to support the cleaning process. Electrical insulation is a very important parameter for this technology. Obviously the immersion process involves the use of more tanks to realize more steps, precisely:
If the surface to be degreased is already installed and it is irremovable or the work area is too small to contain the installation of tanks or the investment is too high, you can think of a Nitty-Gritty system called SURFACE PLUS.
Clinox Surface Plus
Clinox Surface in action: degreasing a stainless steel for food factory
The system consists of a current generator, the entire hydraulic and electrical system to concentrate current and electrolyte liquid directly on the precise area, avoiding to spread the liquid everywhere. The system is easy and fast. It allows you to instantly check the success of the process. You can switch from a degreasing process to pickling process quickly, changing the electrolyte liquid contained in the tanks, avoiding the enormous amounts of liquid contained in the tanks, using the immersion process. Unlike the immersion process, the steps to follow are:
In this way the costs, related to the treatment time, decrease. The process can be performed externally, or internally without special suction systems because the danger values are much lower than the limit.
Ultrasonic degreasing can be used for products with complicated shapes such as cavities and undercuts or for types of dirt that can not react chemically with chemical solutions. The main cause of the detachment of impurities is the ultrasonic vibration. These vibrations are not felt by the human ear. The fundamental parameter for this technology is the propagation speed. This depends on the frequency and the wavelength of the matter. If the samples were immersed in a liquid and there was a generator inside the ultrasound, the particles of impurities on the surface would be subjected to ultrasonic vibrations. These particles will then undergo contractions and dilatations, which helps the detachment from the surface of the stainless steel, bringing them into suspension in the liquid.
Ultrasound instruments differ in:
- High frequency;
- Low frequency.
Typical ultrasonic system
The high frequencies are suitable for removing dirt when these are difficult to detach. The difference is also in the packaging of the waves. In the case of high frequencies, the distance between the ultrasonic source and the steel does not affect the intensity of the wave. This situation can be beneficial for cleaning small surfaces. While for large surfaces we need non-homogeneous wave packing in such a way that the whole surface is involved in the process. The effect that must be obtained to help the detachment of impurities is called cavitation. This phenomenon is negative in the design of hydraulic pumps but it is a positive factor in the cleaning of surfaces. During cavitation, vacuum areas are formed inside the liquid which subsequently implode on the surface of the stainless steel. They are generated by strong vibrations with a long interval of dilatation of the liquid. In most cases, cavitation phenomena are generated with a succession of 20 thousand times per second.
The degreasing liquid can be an alkaline solution or a perchlorethylene based solution. The liquid must be permanently filtered to free it from the suspended impurities. Because of the packing of the waves, if you choose the high frequencies you have to place the piece in a certain position because the area is very narrow. Applications can be:
- Precision mechanics;
- Complex parts;
- Automotive industry.