Degreasing is the operation that allows the removal of oily, greasy and foreign products from the surface of the stainless steel, which can decrease:
- The adhesion of galvanic coatings
- Corrosion resistance
- Mechanical resistance
Very often these impurities are caused by debris or pastes coming respectively from a mechanical process of brushing or polishing or from sticky resins of protective films.
Degreasing differs from pickling by the nature of the substances we want to remove from the surface. In the degreasing organic pollutants (mineral, animal or vegetable oils) that come from mechanical processing, heat treatments and mechanical cleaning are removed. In the pickling, inorganic substances (rust, welding residues, stains, solid particles) are removed through the use of acidic substances.
The cleaning time depends on:
- Composition of the degreasing mixture
- Composition of the substance that pollutes the steel
The first must contain substances that promote the emulsion, the solubilization, the flocculation and the wetting action. In short, it must act on the surface tension of the polluting particle. The second can consist of polar groups that are absorbed by the surface of the steel or by free fatty acids (stearic acid) which form metal soaps that adhere to the metal surface. This adhesion is further strengthened if the steel undergoes a rise in temperature.
The degreasing process can be separated in 4 parts:
- Organic solvents
- Chemistry through alkaline solutions
- With ultrasound
Degreasing with organic solvents is also called “pre-degreasing”. This technique is widely used when the particular has to undergo subsequent processing not in the aqueous phase (mechanical processing) or in concentrated chemical solutions (chemical baths for electropolishing).
This type of degreasing brings in solution the various greases adhering to the surfaces of the pieces. It can be carried out by rubbing, immersion, or solvent vapors.
In all cases it is necessary to perform a chemical or electrolytic degreasing to eliminate any residue on the surface.
Organic solvents can be of two types:
- Not flammable
The former are no longer used due to fire hazards. The latter are liquids heavier than water, clear, colorless, which can be toxic. These substances are used in appropriate machines that include:
- Heating the solvent (boiling)
- Suction of fumes
- Solvent distillation and recovery
- Spray jets to facilitate the detachment of impurities
The duration can vary from 2 to 5 minutes. Particular attention is given to the positioning of the product, which must be dry, inside the degreasing chamber so that the degreasing liquid involves all parts of the product and no stagnation areas are created inside the cavities.
The chemical degreasing exploits the chemical reaction between an alkaline substance and the impurity to remove said saponification. In this reaction the chemical has the possibility both to bind compatibly with oils and / or fats, and to bind with the water molecule. In this way the substance allows to trap impurities inside micelles and move them away from the metal surface. This substance is called surfactant. The surfactant lowers the surface tension and facilitates wettability. The surfactant concentration in the degreasing mixture is very important because it ensures the formation of solutions consisting of micelles instead of molecular solutions. The surfactant confers a spherical shape to the fat / oil particle. In this way the contact area with the metal surface decreases, favoring its detachment.
Detachment of the impure particle is also favored by adhesion of the surfactant on the metal surface. In this way a repulsion is generated between the metal, coated with surfactant, and the impurity now in micellar form.
The surfactant concentration is usually very low (less than 0.2%) and can be of various types:
Lowering the surface tension involves the following phenomena:
- Emulsification: the fatty substance detaches from the surface in the form of very fine liquid particles
- Dispersion: the fat is dispersed in the aqueous phase, decreasing its concentration on the metal surface.
- Flocculation: once the fat has been removed from the surface and dispersed, the particles are aggregated and precipitated away from the surface.
These three characteristics form the basis for a chemical solution that allows the surface to be degreased in optimal way.
The application of the degreasing mixture can be:
- By immersion
The first application is used to eliminate large quantities of grease and cleaning residues. The application involves the use of nozzles which, at high pressure, strongly spray the degreasing mixture. The mechanical action of the spray greatly favors the detachment of fats. Usually the mixture is composed of alkali with phosphates and silicates but without foaming surfactants.
The second application involves immersing the product in a high temperature solution inside an appropriate tank. The high temperature reduces the immersion time because it reduces the viscosity of the impurity and increases the speed of chemical reactions. Therefore the degreasing power is mainly due to the combined action of:
- Temperature (90 ° C)
- Emulsifying power of the solution
- Mechanical agitation
The immersion time depends very much on the starting state of the product and on the quality of the base metal.