Usually, Zinc phosphating process consist of 8 stages but some stages can be included or excluded depending upon the condition of the substrate. A standard 8 stage phosphating process has been shown in the figure below,
It implies removal of surface contaminates such as Oil, grease, dust and waxes. It is the crucial prerequisite for subsequent Zinc Phosphate Coating. Majority of coating defects are culminated due to poor cleaning or degreasing. Effective degreasing can be achieved through potent solvent or alkaline cleaners. However, mechanical and heat acceleration (approx. 80-85°C) are required as an ancillary support.
It plays an instrumental role in eliminating surface contaminants and preventing drag out of chemicals used in the previous stage which may taint the subsequent stages.
It is a surface treatment which is used to clean rust or mill scales from the substrate. The chemical used in this stage comprise of strong inhibited inorganic acids which converts ferrous oxides into water soluble salts.
Low Temperature Zinc Phosphate Coating
Now this practically clean surface is subjected to zinc phosphate solution (FINBON) which forms insoluble inert poly-crystalline zinc phosphate coating which not only provides corrosion resistance but also greatly improves paint adhesion to the substrate. The aforementioned solution can be applied through dip or spray at 10-40 °C
Now the coated surface should be thoroughly rinsed with demineralized water in order to remove phosphate residue and other soluble salts which can cause blistering and flaking of subsequent organic finishes.
Phosphate coatings are slightly porous which contain intercrystalline spaces or voids. These voids can affect the corrosion resistance of the substrate unless they are completely sealed. Therefore, dilute passivation solution is used to seal off these voids or gaps.
After passivation the substrate must be dried off in order to eradicate moister from the surface which could be detrimental for subsequent organic coatings.