Inorganic Coating

INORGANIC COATING

Inorganic coating are a type of protective layer applied to material, typically on metals, to enhance their durability and resistance to corrosion. Non‒organic compound on to the surface.

These coatings are produced on the metal surface by immersing the metal in the solution of a suitable chemical which reacts with the metal surface producing non‒organic adherent coating. Some important surface conversion coatings are discussed below.

1. Phosphate coatings (or) Phosphating

Phosphate coatings are produced by the immersion of base metal in a bath of aqueous solution containing phosphoric acid and metallic phosphates. Zinc, iron and manganese phosphates are generally used.

Phosphate coatings are usually applied to iron, zinc, steel, cadmium and aluminium. During the phosphate coatings, phosphating solution dissolves and reacts with the surface of the base metal forms an insoluble metal‒phosphate compound. metal‒phosphate compound forms an adherent deposit over the base metal. It is grey in colour.

Uses

1. It is used as a base for the paints lacquers, which increase the resistance of the films to humidity.

2. It also impair welding strength.

2. Chromate coatings

Chromate coatings are produced by the immersion of the base metal in a bath of aqueous solution containing acidic potassium chromate followed by the immersion in a bath of neutral chromate solution.

Chromate coatings are usually applied to zinc, cadmium, aluminium and magnesium. During the chromate coating, chromating solution dissolves and reacts with surface of the base metal forms an insoluble metal‒chromate compound containing mixture of Cr (III) and Cr (VI). This metal‒chromate compound is amorphous, non‒porous and more corrosion resistant than phosphate coatings. It is golden brown or black in colour.

Uses

It is also used as a base for paints, lacquers and enamels.

3. Anodization (or) Anodised Coating

Anodization or anodic oxidation is an electrolytic process in which a thick oxide coating is produced on the base metal.

Anodized coatings are generally produced on non‒ferrous metals like Al, Zn, Mg and their alloys by anodic oxidation process, in which the base metal is made as an anode. It is carried out by passing a moderate direct electric current through a bath in which the metal or alloy is suspended from anode. The bath usually contains sulphuric, chromic, phosphoric, oxalic or boric acid.

Example: Anodizing on aluminium.

Anode (base metal): Aluminium.

Cathode: Innert metal.

Electrolyte: Dil sulphuric acid, oxalic acid, 3% chromic acid.

Temperature: 40‒50°C.

Current density: 0.2‒2mA/cm².

Anodized coating on aluminium is done by making aluminium as an anode in an electrolytic bath containing sulphuric, chromic or phosphoric acid at moderate temperatures (about 35‒40°C) and the cathode is a plate of lead or stainless steel. On passing the current oxidation starts at anode and oxygen combines with the anodic metal to form the oxide. The oxide film, initially will be very thin and grows from the metal surface very thin and grows from the metal outwards and increases in thickness as oxidation continues at aluminium anode. The oxide film is very porous and soft and this can be sealed by immersing it in boiling water. This treatment converts porous alumina (Al₂O₃) into hydrated alumina (Al₂O₃ • H₂O), which occupies more volume thereby the pores are sealed.

Advantages of anodization

Anodization provides

(i) an insulating coat for the electrically conducting base metal (aluminium),

(ii) very good resistance to corrosion,

(iii) thicker oxide coating and hence it is more protective,

(iv) the film may be coloured with organic (or) inorganic dyes,

(v) anodized metal (like anodised aluminium) is used in automobile engine piston.

Table 4.7 Differences between Electroplating and Anodising

Electroplating

1. It involves deposition of a thin layer of coating metal to the base metal by passing current through an electrolytic solution.

2. The base metal to be plated is made cathode of an electrolytic cell.

3. The base metal to be plated may be either a pure metal or an alloy.

Anodizing

1. It involves production of a thick oxide layer over the base metal by passing current through an electrolytic solution.

2. The base metal on which oxide coating to be produced is made anode of an electrolytic cell.

3. The base metal should be non‒ferrous and only those metal which gets oxidized easily can be used here.

4. Vitreous (or) porcelain enamel coatings

Vitreous or porcelain enamels are glass like decorative and protective films applied to steel and iron articles.

1. Vitreous enamel coatings

Vitreous coating can be done either by wet process or dry process.

(a) Wet process

In wet process, first frit (glass) is made by fusing together acidic refractory substances like quartz and feldspar with basic fluxes like borax and and sodium nitrate and the molten mass is mixed with water. For making opaque film, opacifiers such as lead oxide, titanium dioxide, etc. are mixed with it. Then the frit is applied on steel or iron surfaces and fired in a furnace.

(b) Dry processing

In dry process, the dry powdered frit is sprayed on the surface and fired.

Vitreous enamels provide glossy and permanent colour to the base metals. These coatings are corrosion resistant and withstand temperatures upto 1000°C.

Uses

Enamelled articles are not widely used because of brittleness. They are used as sanitary wares. Vitreous enamelled steel is used in kitchen utensils, containers and basins.

2. Ceramic coatings

Ceramic coatings are made of mixing high refractory oxides such as chromium trioxide with silica materials. The paste of silica material and refractory oxide is sprayed and then fired.

The ceramic coatings have high chemical and corrosion resistance.

Uses

Ceramic coatings are used in nozzles, thermo couple tubes, blades in gas turbines and electrical insulations.