Phosphate coating is the treatment of iron steel, galvanized steel, or aluminum with dilute solution of phosphoric acid and other chemicals to produce mildly protective layer of insoluble crystalline phosphate.
The particular system considered in this research work is zinc phosphate coating. Recently, a better understanding of zinc phosphate coating has become essential with the fast development of surface treatment process. The available literature clearly points out that the content of accelerators, bath temperature, total acid point age, free acid point age, acid ratio, iron content, time for treatment are some important variables of phosphate coating. Although, the past investigation were helpful in identifying the variable factors to attain good quality of phosphate coatings remain as basic problem for the surface treated. Further, it was recognized that zinc phosphate coating is generally accepted to give the best performance for corrosion protection application base of lubricant, improve wear resistance and base for paint.
In the present work pretreatment and post treatment were carried out in the department. Actual phosphate was done by varying the process parameter. The data obtained were used to reveal physical property of zinc phosphate coating such as appearance, corrosion resistance, Scanning Electron Microscope (SEM) study, with the factors like bath temperature, time and concentration of accelerator.
EAD OF THE DEPARTMENT
TYPES OF PHOSPHATE COATING
- 1. ZINC PHOSPHATE COATING
- MANGANESE PHOSPHATE COATING
- IRON PHOSPHATE COATING
1 :- ZINC PHOSPHATE COATING: – The zinc phosphate coating cover a wide range of weights and crystal characteristics, ranging from heavy films with coarse crystals to ultra thin microcrystalline deposits.
These coatings vary from light to dark grey to color. Coatings are darker as the carbon content of the underlying steel increases, as the ferrous content of the coating increases, as heavy metal ions are incorporated into the phosphating solution, or as the substrate metal is acid pickled prior to phosphating and microcrystalline coatings are usually darker grey than coatings of the same weight with coarser crystals.
2 :- MANGANESE PHOSPHATE COATING:- Manganese phosphate coatings are generally accepted as giving the best performance for purely corrosion protective applications and are widely used for this purpose particularly in the USA and UK. Un accelerated manganese phosphate processes still find limited application and are specified for certain military applications. In particular these are used where rinsing facilities are limited and for components seams of crevices from which it is difficult to rinse out the traces of the treatment bath and where the trapped active accelerator ions can be a potential source of corrosion.
3 :- IRON PHOSPHATE COATING:- These coatings were the first to be used commercially. The original Coslett process was of the ferrous phosphate type, prepared by dissolving iron filing in the phosphoric acid. The original Coslett bath contained the following:
Iron filing 28 g
Phosphoric acid 110-115g
Water to make 4480 ml
Iron and steel articles acquire protective phosphate coating after 2.0 – 2.5 hours treatment in boiling solution.
Initially the bath was essentially based on phosphoric acid. Metallic accelerators such as nickel may now be added and the iron content of the bath is usually achieved by processing scrap rather than addition of iron filings. Oxidizing accelerators are normally avoided as they tend to increase the tendency of oxidation of ferrous phosphate to the insoluble ferric form.