3. PRETREATMENT FOR MILD STEEL

– TENDENECY OF MATERIAL

picking up rust when exposed to atmosphere bare.

– NEED OF CLEANING

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.

– NEED OF CONVERSION COATING

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.

PRECLEANING OF MILD STEEL

DEGREASING – Removal of oil and grease from the surface.

This can be done by

Solvent Based Degreasing:

Petroleum based solvents have excellent degreasing abilities but are not commonly used because they are highly inflammable.

Alkaline degreasing powders:

This is generally a blend of alkalies and surfactants. This is advised to be used under hot conditions. However, alkaline powders are also used under cold conditions as knock out degreasing stage.

Acidic Degreasing:

These are solvent based liquids which are acidic in nature. They remove rust as well as grease under cold conditions. The bath of these chemicals is however, much expensive as their concentration is much higher than alkaline degreasing.

DERUSTING – Removal of rust and light scale from the surface.

These are necessarily acidic chemicals unlike degreasing, which may be alkaline in nature. The chemical is generally a blend of mineral acids like phosphoric acid, sulfuric acid and hydrochloric acid with added inhibitors. Higher acidity will result in faster removal of rust. However, this will affect the life of the coating applied later.

Phosphoric acid based derusting chemicals are most suggested for pretreatment of mild steel before applying paint.

DESCALING – Removal of heavy scale and heavy rust from the surface.

These are strong mineral acid based chemicals with added inhibitors.

CONVERSION COATING FOR MILD STEEL

The most common types of conversion coating include :

• Zinc phosphate coating
• Iron phosphate coating

Zinc phosphate coating is crystalline heavy coating gray in colour. While iron phosphate coating is amorphous conversion coating that ranges in color from iridescent blue to gray.

Iron phosphate coating gives minimum sludge formation and hence smoother finish than zinc phosphate. Apart from this the bonding characteristics are also good. Iron phosphate processes are much easier to operate than zinc phosphate processes and require fewer process stages (zinc phosphate coating requires a predip in activation chemical prior to phosphating to have micro crystalline grain structure), but iron phosphates do not provide the degree of corrosion protection imparted by zinc phosphates.

Iron phosphate systems are therefore used for a range of products requiring a durable finish that are not exposed to severely corrosive environments. Zinc phosphating processes have been developed to provide exceptional painted part durability in corrosive requirements. Typical industries using zinc phosphate processes include automotive, appliance and truck and bus.

POST-TREATMENT

After a metal surface receives a conversion coating, the surface is water rinsed to remove unreacted conversion coating chemicals and a post-treatment is applied. The post-treatment can provide a two- to ten-fold increase in corrosion resistance and humidity resistance when compared to conversion coatings without final rinses. Post-treatments are generally based on chromic acid.

SEQUENTIAL PROCESS FOR PRETREATMENT OF M.S.

Seven (or eight) stage treatment for zinc/iron phosphate include following stages:

1. Degreasing
2. Water rinse
3. Derusting
4. Water rinse
5. Activation: This gives more compact and uniform coating of zinc phosphate. This step can be omitted for iron phosphate process.
6. Phosphating: This forms coating of zinc phosphate (4 to 6 microns) OR iron phosphate (1 to 1.5 microns) on the clean surface of M.S.
7. Water rinse
8. Passivation: This will seal the pores of phosphating.
9. Drying.

CLEANER-COATER SYSTEM – 3 IN 1 CHEMICAL.

Cleaner-coater system for mild steel is a much simplified treatment than 8 tank process. This is done in a single tank with 3 in 1 che1mical. In this system, all three processes of degreasing, derusting and iron phosphating are done with single chemical, thus making the process time effective. However, its corrosion resistance and bonding properties are not as good as that of 8 tank process.)