Armour Iron Oxide Paint

Thermite is the explosive compound of iron powder and aluminium powder, ignited by magnesium strips. It can be used to create a coating on ferrous surfaces that prevents corrosion. The coating acts to inhibit oxidation by creating dense and insoluble zinc soaps. It also has a high heat resistance. The concept of armour paint is almost forgotten today, but it could be useful for objects with unique heritage value that demand historically authentic materials. Adapting the concept with VOC-free renewable binders and adding complex phosphate primers and zinc oxide addition could lead to a new generation of anticorrosive paints that are sustainable, with good performance, and suitable for use on heritage steel surfaces.

The aim of this article is to study the corrosion behaviour of a historical anticorrosive linseed oil paint with aluminium pigment (MIO) in combination with hematite and purified linseed oil. This paint was called ‘armour paint’, or ’aluminium paint’, and it was used on outdoors located ferrous substrates from the early to mid-20th century. The corrosion research programme at IVA in the 1930s and 1940s led to the conclusion that power pylons exhibited excellent weathering resistance when coated with two coats of aluminium-pigmented AP, a 2 + 2 system (Laurell 1937).

It was found that MIO reacted very quickly with oxygen on the surface of hematite and created an aluminium oxide film. The oxide layer was impermeable to water, and it prevented the hydration of iron oxide that causes rusting of iron. The opacity of the MIO film was high, and it did not allow light radiation to pass through. This was important in order to avoid thermal oxidation of the linseed oil binder, which would shorten the lifetime of the paint.