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.

Corrosion Control : Metal Coating

Metal coating:

Metals are coated over the surface of a base metal (which is to be protected) by electrode position, flame spraying, cladding or hot dipping.  Metal coating is deposition of a protective metal over the surface of a base metal is called metallic coating.

They are of 2 types: namely anodic coating & cathodic coating.

(i) Anodic Coating: Anodic coatings are produced by coating a base metal with more active metals which are anodic to the base metal. The metal coated on the base metal acts as the anode and undergoes sacrificial oxidative corrosion giving protection to the metal underneath.

Eg: Iron is coated with anodic and active metals like Zn, Mg, and Al.

          One advantage of anodic coating is that, even if the coating is ruptured, the base metal does not undergo corrosion. The exposed surface of the base metal is cathodic with respect to the coating metal and coating metal only undergone corrosion there by protecting the base metal. Therefore, anodic coating is also known as sacrificial coating.

Galvanization is a process of coating a base metal surface with Zinc metal. Galvanization is carried out by hot dipping method.

The galvanization process involves the following steps.

(1)  The metal surface is washed with organic solvents to remove organic matter on the surface

(2)  Rust and other deposits are removed by washing with dilute sulphuric acid.

(3)  Finally the article is well washed with distilled water and air-dried.

(4)  The article is then dipped in a bath of molten zinc, maintained at 425 – 430°C and covered with a flux of ammonium chloride to prevent the oxidation of molten zinc.

(5)  The excess zinc on the surface is removed by passing through a pair of hot rollers, which wipes out excess of zinc coating and produces a thin coating.

(6)  Finally the article is annealed at 650⁰C and then cooled slowly.

Note: Zinc has lower electrode potential (-0.076V) compared to iron (-0.44V).  Iron does not rust as long as some zinc remains on iron.

Applications:

• It is used to protect roofing sheets, water pipes, barbed wire, buckets etc.
• Galvanized articles are not used for preparing and storing foodstuffs, since zinc dissolves in dilute acids producing toxic zinc compounds.

(ii) Cathodic Coating :

Cathodic coatings are produced by coating a base metal with a more noble metal (having higher electrode potential) which is cathodic.

    Eg: Cu, Ni, Sn or Ag is commonly used as cathodic coatings on steel.

These noble metals are less reactive than the base metal and are less susceptible for corrosion. The cathodic coating provides protection only when it is undamaged, continuous and non porous.

Tinning is a process of coating a base metal with tin. Tinning of iron or steel sheet is carried out by hot dipping method and the process involves the following steps.

1.    The sheet is first washed with organic solvents to remove grease or oil deposits.

2.    Then treated with dilute sulphuric acid to remove rust and scale deposits.

3.     Finally it is washed well with distilled water and air-dried.

4.    The cleaned and dried sheet is passed through molten zinc chloride flux. The flux helps the molten metal to adhere to the metal sheet.

5.    It is then passed through a tank that contains molten tin.

6.    Finally passed through a series of rollers immersed in palm oil. The palm oil protects the hot tin-coated surface against oxidation.

7.    The rollers remove any excess of tin and produce a continuous, thin coating of tin on the sheet.

Application: 

·         It is used to produce tin coated steel for making containers used for store food stuffs.

·         Tin coated copper or brass vessels prevent contamination of food with poisonous copper salts.

biles, 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.