Rust is the “disease” of metal. We treat metal from rust before painting.
Before you start painting metal products, you must properly prepare the surface. Preparation of metal for painting is carried out according to different technologies. But regardless of the processing scheme, in the first place, the item is freed from rust.
According to the degree of steel damage, corrosion is classified as follows:
- Corrosive stains: characterized by a small penetration depth. Such corrosion spreads in breadth and not deep into iron.
- Pitting corrosion – small dots penetrating deep into the body of the material. With the further development of pitting corrosion, through holes appear on steel.
- Through corrosion – through damage to the material.
- Subfilm corrosion: foci of rust are formed below the surface of the coating. Layers of paint in places of formation of rust swell. But sometimes the undercoat corrosion remains imperceptible until the end-to-end destruction of steel.
Processing of material from rust before painting can be carried out by the following methods:
The mechanical method of removing corrosion has proven to be the most effective. Metal processing from rust is done manually or using a mechanized tool. There are several ways to remove rust mechanically.
Surface cleaning with wire brushes.
It is used to remove small foci of corrosion and cleaning of welds and for the primary treatment of surfaces covered with a thick layer of rust. The quality of cleaning is not very good, do not remove the scale of the brush at all. In addition, a lot of dust is generated during processing.
Abrasive machining of metal using grinding wheels.
It is used for small foci of corrosion. If high-quality disks are used to perform the operation, the result is good. Metal processing abrasive tool has two drawbacks:
- consumption of quality materials;
- requirements for specific job skills.
Anticorrosion treatment of metal using sand blasting installation:
the bombardment of foci of corrosion by a stream of sand supplied under pressure, the so-called sandblast cleaning of metal. The main elements of the sandblasting unit are a tank with sand and a sandblasting gun. For operation of sandblasting installation a small compressor is enough.
The sand is taken ordinary river or construction. Dry well before use. Sand can be used again, pre-sifting, but the cleaning efficiency in this case decreases several times. And the amount of dust increases by the same amount.
Sandblasting removes not only all rust, but also scale, carbon deposits, layers of old paint. When processing places that are inaccessible to the grinding machine and sandpaper (for example, the junction of two parts), this method is the only possible one.
Hydroabrasive metal processing (water sandblasting).
Rust removal occurs under the influence of a jet of a mixture of water and abrasive. By intensity distinguish hydroabrasive processing:
- under ultrahigh pressure: corrosion and all coatings previously applied to the surface to be treated are completely removed;
- under high pressure: removes much of the old coating and corrosion. But there can be patches of especially strong coating and black oxides (magnetites);
- under low pressure: an economical method in terms of abrasive consumption, but after drying, traces of secondary rust remain on the treated surface.
Hydroabrasive surface treatment is an industrial method. Unlike sandblasting, such an installation in the garage can not be erected.
Chemical treatment of steel
This method is based on the removal of rust under the influence of chemically active substances. One of the related types of treatment is electrochemical. The compositions are applied to the surface with a brush or by spraying. Rust removers fall into two categories:
The disadvantage of such compositions is that, when the surface is contacted with water, new sources of corrosion may arise. Therefore, after washing, the material must be quickly and thoroughly dried and treated with anti-corrosion agents;
They are also called soil converters. A product of a chemical reaction cannot be called a full-fledged primer, but nevertheless, such treatment excludes subsequent rinsing, that is, mandatory contact with water is completely excluded.
- Well removes rust aqueous 5% solution of sulfuric or hydrochloric acid. But it is necessary to add a corrosion inhibitor – a substance that slows down a chemical reaction. Most often, urotropin is used as an inhibitor. It should be added in the amount of 0.5 g per liter of solution. Without an inhibitor, acid solutions cannot be used: as a result of this treatment, not only corrosion will dissolve, but also the material.
- An interesting result is the treatment of metal with orthophosphoric acid. If a 15-30% solution of orthophosphoric acid is applied to the surface to be treated, then under its influence rust will turn into a durable coating. This is because during the chemical reaction, iron orthophosphate is formed, which creates a brown protective film on the surface. For better effect, butyl alcohol (4 ml per liter of solution) or tartaric acid (15 ml per liter of solution) should be added to the solution.
- Surfaces strongly affected by rust are treated with the mixture:
lactic acid – 50 g;
vaseline oil (100 ml).
Under the influence of acid, rust turns into a salt soluble in petrolatum oil – iron lactate. The cleaned surface should be wiped with a cloth moistened with vaseline oil.
Surface heat treatment
For heat treatment of steel used a very powerful diesel thermal sand blasting unit, which does not leave a drop of rust.
Specialists of the company “NPP HIMPRODUKT” (Kiev) will help to carry out a complete cleaning of the metal from corrosion by chemical, thermal or sand blasting both on the basis of our company, and on the Customer’s site, with subsequent 3-layer corrosion protection for many years.