Picking up rust when exposed to atmosphere bare.
Presence of oil, grease, rust and black scale on the surface.
Mild Steel material is known for it’s tendency of going under rapid oxidation when exposed to atmosphere bare. The result of oxidation is the formation of oxide layer on the surface which we commonly call as ‘RUST’. To avoid this oxidation or rusting, the mild steel material is not allowed to get directly exposed to atmosphere. For this, we generally find rust preventive oil on the surface of mild steel material. The oil does not allow the material to come in direct contact with air and thus delays oxidation. However, this doesn’t entirely eliminates the problem of rusting and it only delays the process of rust formation. Some mild steel materials are hardened for strengthening and similarly some are welded during fabrication. Both these processes leave black scale on the surface. In short, mild steel parts generally have rust and/or grease-oil and/or black scale on the surface. All these cause severe problems of bonding if not removed before application of paint.
The bare metal being extremely susceptible to atmospheric corrosion should be applied with conversion coating
Before painting, once the rust, oil and scale are cleaned, the bare metal is again exposed for oxidation. To avoid this, the material has to be coated with some type of conversion coating which prevents further oxidation before applying paint and also gives adhesion for paint.