Phosphating Process Tight Control for High Quality Phosphate Coatings

To obtain high quality phosphate coatings, strict control and regular replenishment of the baths in the phosphating process is a first requirement.

The phosphating bath is acidic, so its control includes measurement of total and free acid by 0.1 N NaOH titration with phenolphthalein and bromophenol blue (or methyl orange) as indicators.

The total acid can vary from 20 to more than 100 points (ml of titrant consumed), depending mainly on the desired phosphate layer thickness, while free acid is determined by the total acid to free acid ratio, e.g. 6-7.

The free acid determines the rate of substrate metal dissolution and thus the formation of the phosphate coating in the alkalized thin layer of the electrolyte near the metal surface.

Phosphate baths often contain an oxidizing agent that accelerates the deposition of the phosphate layer. It is controlled by redox titration with 0.1 or 0.042 N KMnO4 and the pointage depends on the type of accelerator and titrant concentration.

An alkaline activator bath preceding the phosphating bath is often controlled by pH measurement and visual observation of phosphate crystal size. The purpose of the activator bath is to deposit small particles on the surface of the object to be phosphated. The particles serve as nucleation centers for the growth of the phosphate crystals and enable the formation of a fine-grained structure and a compact coating.

Surface cleanliness is critical to the successful phosphating process. Therefore, the first step is immersion in an alkaline cleaner bath controlled by total alkalinity and/or free alkalinity titration with 0.1 N H2SO4 using phenolphthalein and methyl orange as indicators.

In case you have any further questions or require additional information, please do not hesitate to reach out.