Corrosion is essentially the conversion of metal to a hydrated form of oxide. In the presence of oxygen and water, the overall corrosion reaction of steel is:

4Fe + 3O₂ + 2H₂O -> 2Fe₂ O₃ H₂O

The surface of steel is never uniform (steel has a crystal structure which vary in composition particularly at crystal boundaries) and if it is in contact with a thin layer of aqueous electrolyte. then small electrical imbalances from site to site will lead to the formation of an electrolyte cell. At anodic sites oxidation occurs:

4Fe - 8e -> 4Fe²⁺ (Iron loses 8 electrons)

At cathodic sites, sites reduction occurs:

4H₂O + 2O₂ + 8e -> 8 OH- (Water gains 8 electrons)

The cation formed migrateSthrough the electrolyte towards the more negative cathode sites and the anoins produced migrate towards the anodic sites. Within the metal, electrons flow from anodic to cathodic sites and within the electrolyte migrating ions meet to form soluble ferrous hydroxide:

4Fe²⁺ + 8OH- -> 4Fe (OH)₂

When sufficient oxygen is present this is oxidised to insoluble ferric oxide

(rust).

4Fe(OH)2 + O2 -> 2Fe₂O₃H₂O + 2H₂O

Thus, it can be seen that corrosion (dissolving of the metal) is an electro-

chemical process. Rusting does, however require an additional oxidation step. If the final corrosion product can-be formed as a strongly adherent insoluble and impermeable layer on the metal surface corrosion will decline but if it is loose or permeable the corrosion process will continue.

In real life, the above processes are modified by the presence of other chemical species even in trace amounts, and on steel the layer is commonly loose and permeable.

Corrosion can be inhibited by one of the following techniques:

1) Keeping the surface completely dry, so that no conducting electrolyte can form on it.

2) Starving cathodic areas of oxygen.

3) Forming a film impermeable to electrons on cathodic sites.

4) Forming a film impermeable to electrons on anodic sites.

Coating metal with paint might be assumed to be taking preventative action by all four mechanisms but in fact no paint is completely impermeable to water or oxygen so that after an initial delay barrier properties are lost and corrosion eventually begins. It is, therefore, necessary either to include inhibitive ingredients within the paint (anti-corrosive pigments) or to apply an inorganic chemical pretreatment before painting or do both.

The object of using inorganic chemical pretreatments often called conversion coatings, are two-fold:

1) To passivate the surface by forming upon it a relatively stable, strongly adherent, corrosion-inhibiting layer.

l 2) To provide a surface to which paint coatings readily adhere. The conversion coatings are thin and easily damaged subsequent layers of paint protect the film from damage and if they contain inhibiting pigments, repair faults and damage where they occur. The main conversion coatings for steel are iron phosphate or zinc phosphate.

Zinc phosphates are preferred for best corrosion resistance outdoors. Iron phosphates are amorphous coatings are very thin with 0.1 to 1 gram per meter square and contains primary sodium or ammonium phosphate, together with degreasing and oxidising accelerator. The bath runs at 50 to 80°C, steel is immersed in this bath and a chemical reaction takes place producing a final product of about “0% ferric oxide and 60% ferric phosphate (Fe PO”).

Zinc phosphate coatings are crystalline and somewhat thicker 1-5 grams per meter square. The bath or spray contains primary zinc phosphate, phosphoric acid and oxidising accelerators. The pretreatment operates between pH 1.5-3.3 and temperatures of 25-90°C. The process begins with acid attack on the steel, oxidation and precipitation of some ferric phosphate (Fe PO4).

The zinc phosphate is in finely balance equilibrium with the other species. The removal of small amounts of hydrogen ion by acid attack on the steel upsets this equilibrium locally. Producing sparingly soluble secondary zinc phosphate and insoluble tertiary zinc phosphate which deposits at the site where the reaction has taken place forming a protective crystalline layer mainly hopeite Zn₃ (PO₄)₂ 4H₂O

Both iron and zinc phosphates contain small amounts of porosity, probably caused by the escape of hydrogen in the early stages of phosphating. The free pore area can be reduced by rinsing in hot dilute solution of chromic acid (0.01-0.051). Insoluble iron chromates are formed in the pores passivating these areas and increasing the corrosion resistance almost four-fold. Thus, the protective properties of phosphating and chromating are combined.

However, in many cases the phosphate pretreatment is designed to give good results without the chromate rinse, either for reasons for economy, or because of concerns about the toxicity of hexavalent chromium and the disposal of the effluent.

Iron phosphating is widely used on panel radiators. refrigerators and other items not subject to severe exterior exposure. Zinc phosphating is used to protect motor car bodies and other articles which require maximum corrosion protection. Since phosphating works well on zinc. it is used on electro-zinc for washing machines, and hot dipped galvanised steel and coil-coated steel

for building claddings.

The use of zinc-coated steel in automobile bodies is increasing rapidly, so that phosphate pretreatments have to be designed to treat both zinc and steel surfaces.