What is Red Oxide?
Over the years there has been a lot of misinformation provided concerning red oxide, primarily due to its widespread use in red oxide primers where its principal application is to camouflage rust creeping through from the steel underneath. Red oxide is actually the common name for iron III oxide (Fe203) which in colour and chemically is very similar to the hydrated iron oxide (rust) which hides any early signs of rust. Red iron oxide when purified and sold as a pigment for use in paint and ceramics has the advantage of being stable having already oxidised and so resistant to further oxidation and heat changes at most temperature found in common applications. It is also hard and very resistant to mechanical damage. All of these characteristics have led to its widespread use in primer paints or direct to metal paints for iron and steel for many decades. However, unless combined with an active pigment, usually a phosphate to protect the steel there is little actual protection provided by the red iron oxide other than as barrier and here it is often confused with a more effective form of barrier coating based in micaceous iron oxide which is another form entirely and uses its lamellar plate like structure to provide some physical barrier to the penetration of water and oxygen, both of these are needed for corrosion to take place.
Corrosion is an electrochemical process, very much like a battery in which the iron in steel is oxidised by losing electrons to become positively charged iron ions:
Fe » Fe2+ + 2e-
The electrons produced by this reaction combine with hydrogen ions in the water as well as with dissolved oxygen to produce water:
The formation of rust requires iron, water and oxygen. Although it’s a complex process, the chemical equation is simply:
4Fe + 3O2 + 6H2O → 4Fe(OH)3.
The First Step: Oxidation of Solid Iron
It’s common knowledge that rust occurs when you leave water on a metal implement or you leave it exposed to moist air. That’s because the first step in the rusting process involves the dissolution of solid iron into solution. The formula for this is:
Fe(s) → Fe2+(aq) + 2e-
The electrons produced by this reaction combine with hydrogen ions in the water as well as with dissolved oxygen to produce water:
4e- + 4H+(aq) + O2(aq) → 2H2O(l)
The consumption of hydrogen ions that occurs as iron dissolves leaves a preponderance of hydroxide (OH-) ions in the water. The iron(II) ions react with them to form green rust:
Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)
BUT that isn’t the end of the story. The iron(II) ions also combine with hydrogen and oxygen in the water to produce iron(III) ions:
4Fe2+(aq) + 4H+(aq) + O2(aq) → 4Fe3+(aq) + 2H2O(l)
These iron ions are the source of the reddish deposit that gradually eats holes in all corroding steel and iron from car bodywork to metal roofing to farm ironwork. They combine with the extra hydroxide ions to form iron(III) hydroxide:
Fe3+(aq) + 3OH–(aq) → Fe(OH)3
This compound dehydrates to become Fe2O3.H2O, which is the chemical formula for rust. Note how similar it is t the chemical formula for Red iron oxide pigment Fe2O3 and this is why red iron oxide paints help to camouflage but do not prevent corrosion when used as red iron oxide primers or paints – for that you need a pigment that actively prevents corrosion such as Nanoguard ACP