Practical experience shows that most of the coating defects are from poor surface treatment. No coating can perform its best performance on poorly treated surfaces. Painting on rusty and greasy surfaces is a great waste of time and money. High-grade surface treatment quality will extend the life of the paint.
Surface treatment purpose
1) Structural treatment: Certain treatments must be carried out on the substrate itself, such as sharp edge grinding and chamfer rounding, spatter removal, repair welding and flattening of weld holes.
2) Surface cleaning: remove the substances on the surface that can damage the coating, especially oxide scale, rust, soluble salt, grease, moisture, etc.
3) Surface roughness: The surface roughness increases the contact surface with the coating, and has a mechanical effect to improve the adhesion of the coating to the substrate. The roughness cannot be too large, otherwise the thickness will often be insufficient at the wave crest and cause early pitting corrosion.
Surface treatment object
Usually metal surface will be attached with dust, oil, oxide scale, rust layer, pollutants, salt or loose old paint film. According to different objects, the processing methods adopted are also different.
(1) Oxide scale
Oxide scale is the most common but most easily overlooked part. It seems to protect steel, but it is not trustworthy. Oxide scale is a dense oxide layer produced during high-temperature forging and forming of steel. Its surface layer is chemically stable FeO, the middle layer is FeO, and the metal is FeO. Under the action of water and oxygen, it is easy Generate hydroxides. In addition, its coefficient of expansion is smaller than that of steel, and it is more brittle than steel itself. It is easy to crack after cooling and heating cycles, so it is not firmly attached and will peel off over time, and it is a cathode, which will accelerate metal corrosion. The best way to remove oxide scale is sandblasting.
(2) Rust
The rust on the surface of steel is loose iron oxide, which often traps moisture and oxygen, and the rust products often contain soluble salts. If it is not removed before painting, it will not only affect the adhesion of the coating, but also the rust will continue to develop. In SO-containing industrial environments, the rust layer contains ferrous sulfate; in the rust layer near the sea, the rust layer contains NaCI. These impurities will promote corrosion, so the rust must be removed. The rust can be removed by grinding, sandblasting and other methods.
(3) Soluble salt
Soluble salts such as ferric sulfate and ferrous chloride not only directly damage the coating, causing osmotic pressure, blistering, and poor adhesion, but also a catalyst that will aggravate corrosion. The best way to remove soluble salts is high-pressure fresh water washing. The old paint film with uneven surface and cracks, including invalid or spongy anti-fouling paint film, is easy to contain salt and dirt, so it is necessary to wash it carefully.
(4) Zinc salt
With the extensive use of zinc-rich paint, zinc salt (also known as white rust) also constitutes a hazard to the coating. This loose zinc corrosion product also affects the adhesion of the coating. It can be removed by high-pressure fresh water, sanding or with a hard brush.
(5) Grease
Oil stains are another common pollutant. Most paints and grease have repulsive force. If not removed, it will seriously affect the adhesion of paint. Grease on the substrate will cause shrinkage and poor adhesion. And the more high-performance coatings such as epoxy and polyurethane coatings, the lower the adhesion at this time as conventional oil-based coatings. A large amount of grease must be scraped off first, and then scrubbed with a solvent (aliphatic hydrocarbon or pine perfume), but this tends to expand the surface of the grease, making the situation worse. So it has to be repeated several times, each time wiped with a clean cloth to be effective. Commercial chemical cleaning agents are very effective in removing oil, but must be confirmed to be harmless to the coating. Washing with hot water is also very effective.