Amorphous phosphating is one of the most common metal pretreatment processes in the world. This is a technology that has been proven for many decades, mostly used on ferrous materials.
How does it work
The process is able to guarantee both the cleaning of the metal and the formation of an amorphous phosphate layer on the surface, making it bluish. The amorphous phosphate is constituted by the chemical reaction between the phosphoric acid and the iron, this reaction is usually very rapid to take place, it is catalyzed both by the temperature, both by the strength of the spray, and by the chemistry of the formulation. Given all these variables, the reaction is variable over time, it is incomplete and imperfect, generating different phosphating layers over time. Furthermore, there is a large formation of waste reaction products, called phosphating sludge. Such sludge must be filtered continuously to avoid saturation of the bath, clogging of the nozzles etc. A phosphating bath usually lasts 6 months, after which it is advisable to change it in order to avoid defects in the pretreatment. A typical phosphating industrial plant also consists of only 3 stages: phosphating bath - rinsing - rinsing. The suitable temperature for correct phosphating is 45-55 ° C and the duration of the spray is generally between 2 and 3 minutes, depending on the percentage of the product. It must be said that the phosphating agent is not used alone, but in synergy with surfactants, which also clean the pieces, this double action is called phosphodegreasing. Often the detergent additive is combined in the plant parallel to the phosphating agent, other times a single-component product is instead supplied. The advantage of the two-component is that additive can be added based on the amount of dirt and oil to be removed. The presence of surfactants can cause, as in many other processes, problems of excessive foam, for this reason it is not necessary to abuse them with the quantity and act correctly on the dripping times to avoid finding foam even in the rinses. This problem is however generally less worrying in acid processes than in alkaline processes.
Amorphous phosphating is not able to offer remarkable corrosion resistance performances, however it represents a good quality / price compromise. Phosphate also allows the pieces to be stored or transported before they are painted, preventing them from re-oxidising. By using specific passivating agents after rinsing it is possible to slightly increase the corrosion resistance
Examples of formulation
A phosphating agent is always composed of buffered phosphoric acid, this is to create a solution with a pH that is not too strong and stable over time. The conjugated salt is sodium monophosphate, but small percentages of soda can also be used to further raise the pH, without obviously compromising acidity. They are indispensable in phosphating agents of reaction catalysts, such as sodium molybdate, without which phosphating would be greatly impaired. To complete the formula there may be secondary acids to refine the phosphate crystal, and sequestering agents to reduce the formation of sludge. In the case of single-component phosphates, specific surfactants are indispensable. Given the acidity and the presence of salts the compatibility of many common surfactants can be difficult and it is necessary to convey them with quaternary ammonium salts or cationic surfactants. A small percentage of partially propoxylated surfactant is always recommended for foam control.