Corrosion Control : Inorganic Coating

b) Inorganic coating:

Inorganic coatings are generally chemical conversion coatings. A surface layer of the metal is converted into a compound, by chemical or electrochemical reactions, which forms a barrier between the underlying metal surface and the corrosion environment. There are two methods.

i) Anodizing: It is a process in which a protective passive metal oxide film is produced chemically or electrochemically on certain metals. Anodized coatings are generally produced on non-ferrous metals like Al, Zn, Mg and their alloys by anodic oxidation process using suitable electrolyte and electric current. The process anodization of aluminium consists of following steps.

1. The Al article to be anodized is first de-greased with organic solvents and polished to get clean surface. Then it is used as anode.
2. It is then dipped in 5-10% chromic acid solution (even sulphuric acid and boric acid can be used).
3. Pb is generally used as cathode.
4. A current density of about 150 mA/cm² at 40V and temperature of about 35-40⁰C is used.
5. The aluminium oxide films of about 5-20nm thickness formed on aluminium. The oxide coating is slightly porous. The pores are sealed by dipping in boiling water.
6. Al₂O₃ gets hydrated to Al₂O₃.H₂0 which occupies more volume and hence protects the metal from corrosion.

Application:

• Anodizing is used to produce articles such as window frames, household articles.
• Anodized Al is used in aircrafts.
• Anodization is used for aluminium pressure cookers and cooking pots.

ii) Phosphate coating:

The coating of phosphates by the chemical reaction of base metal when it is immersed in an electrolyte solution of a phosphate and phosphoric acid is called phosphating.

The mechanism involves following steps.

1. First dissolution of metals as metal ions.
2. Metal ions reacting with phosphate ions to form insoluble metal phosphate.
3. Deposition of the metal phosphate on the surface of the base metal.

The process involves following steps.

1. The surface of the metal is first de-greased with organic solvents and polished to get clean surface.
2. The article is immersed in a phosphating bath containing a mixture of phosphoric acid & Zn or Mn phosphate solution along with an accelerator like metal nitrites, hydrogen peroxide, etc to enhance the rate of deposition. Deposition is carried out at 35⁰C by maintaining the pH of 1.8-3.2.

Application:

• Phosphate coating is given before painting bodies of automobiles, refrigerators, washing machines.
• Phosphating is widely used for protection of metal parts which cannot be easily painted or plated. Eg: Nuts, bolts, moving parts of machinery.
• Galvanized iron (which is difficult to paint satisfactorily) is phosphated before painting.