Amorphous phosphating is one of the most popular metal pretreatment processes in the world. It is a proven technology for many decades, used mostly on ferrous materials, although it can also be used with some precautions on other materials.
How does it work
The traditional process (phosphodegreasing) is able to guarantee both the cleaning of the metal and to form a layer of bluish amorphous phosphate on the surface (0.2-1.0 g / m2), through a chemical reaction between the phosphoric acid and the iron. This reaction is catalyzed by the temperature, by the activators, by the pressure of the spray and by the degree of acidity. Given all these parameters, the reaction is variable over time and, if not well managed, it can become unbalanced, generating different phosphating layers over time. During the process there is a sustained formation of waste reaction products, called phosphating sludge. This sludge must be continuously filtered 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 to avoid defects in the pretreatment.
A typical industrial phosphating plant can consist of only 3 stages: phosphating bath - rinsing - rinsing. The temperature suitable for correct phosphating is 45-55 ° C and the duration of the spray is generally between 2 and 3 minutes, depending on the acid concentration. An indicative percentage of phosphating is between 0.8% and 1.5% with a pH variable from 4.5 to 5.8.
The cleaning action of a phosphodegreaser is possible thanks to the use of surfactant mixtures that can be supplied as a separate product or together with the phosphating agent (single-component). The advantage of the two-component is that the surfactant additive can be added according to the amount of dirt and oil to be removed, on the other hand a single-component is more convenient and economical to use. As for rust and oxidation on the pieces, phosphoric acid is able to attack and remove them (up to a certain amount and until the conversion into phosphates occurs).
The presence of surfactants can cause, as in many other processes, problems of excessive foam, so we must not abuse the quantity and act correctly on the dripping times to avoid foam even in the rinses. However, this problem is generally less worrying in acid processes than in alkaline processes.
Phosphodegreasers can also be used for manual lance applications, this is sometimes useful if you do not have a washing tunnel or if you have to work on complex, large pieces or those that do not require special care. The percentage of acid to be used will be lower since the high pressure will already catalyze the phosphating reaction in itself, otherwise rapid re-oxidation would be obtained on the pieces.
Amorphous phosphating is not able to offer remarkable corrosion resistance performance (about 200 hours in salt spray), however it represents a good quality/price compromise. The phosphate also allows the pieces to be stored or transported before they are painted, preventing them from re-oxidation. Through the use of specific passivating agents after rinsing it is possible to slightly increase the performance.
It has nothing to do, in terms of corrosion resistance, with tricationic phosphating and similars.
A phosphating product is always composed of buffered phosphoric acid, this to create a solution with a pH that is not too high and stable over time. The conjugated salt is the sodium monophosphate, but small percentages of soda can also be used to further raise the pH, without obviously compromising acidity. Reaction catalysts, such as sodium molybdate, are essentials in phosphating agents, without which phosphating would be greatly impaired. To complete the formula there can be secondary acids to refine the phosphate crystal, and sequestrants to reduce the formation of sludge. In the case of single-component phosphating agents, specific surfactants are indispensable. Given the acidity and the presence of salts, the compatibility of many common surfactants can be compromised and it is necessary to convey them with quaternary ammonium salts or cationic surfactants. A small percentage of partially propoxylated surfactant is always advisable for foam control.
The addition of corrosion inhibitors is not
recommended as they can negatively interact with phosphating.
Phosphating (without surfactants)
- Demineralized water: 45-50%
- Phosphoric acid (75%): 20-30%
- Sodium Mono Phosphate: 6-9%
- Sodium Molybdate: 0.5-0.8%
- Sequestering agent: 1-2%
- Soda (30%): 5-10%