Friends often ask: Do you want to paint the surface of stainless steel, how to treat it, what paint to use, and how to paint it?
Let’s talk about the corrosion of stainless steel in seawater. What kind of paint should we use? What should be paid attention to in painting? What kind of abrasive is used for surface treatment? Or is it better to deal with it? To what extent is it appropriate? What processing tools should be used for local processing? Everyone is actively discussing!
1. Non-metallic abrasives should be used for surface treatment, and the abrasives should not contain chlorine. Use sand sweeping treatment to increase roughness.
2. Zinc-containing primer should not be used. The chloride ion content in the primer should be controlled. It should not contain corrosion inhibitors commonly used in carbon steel anticorrosive primers that promote the corrosion of stainless steel;
3. The paint is mostly epoxy-based.
Stainless steel is not insured when used in seawater. For example, martensitic stainless steel is guaranteed to be finished in 3 months. Ferritic stainless steel is better, and it can be finished in less than a year. Austenite is the best, 2mm thick perforates in one year. In addition, stainless steel has the problem of stress corrosion cracking, namely SCC. Therefore, try not to use stainless steel in seawater. In the atmosphere, you can use halogen-free coatings for painting, and ensure that metal abrasives are not used in the surface treatment process. The abrasives must not contain chlorine, and the coating must not contain chlorine.
Stainless steel is indeed very dangerous in seawater and has a great hidden danger of perforation. Therefore, when making stainless steel, you must forget the word “stainless”. Stainless steel is embroidered steel in seawater. Therefore, it is necessary to use stainless steel in places, such as Some chemical tankers use stainless steel for cargo oil tanks, so part of its ballast tanks is stainless steel. You must be cautious when doing this part! Don’t be lucky! Do not use chlorine-containing abrasives because stainless steel is prone to perforation in the presence of chloride ions.
The excellent corrosion resistance of stainless steel is due to the formation of an invisible oxide film on the surface of the steel, making it passive. The formation of the passivation film is due to the reaction of steel with oxygen when exposed to the atmosphere, or the result of contact with other oxygen-containing environments. If the passivation film is destroyed, the stainless steel will continue to corrode. In many cases, the passivation film is only damaged on the metal surface and local areas. The effect of corrosion is to form tiny holes or pits, which produce irregularly distributed small pit-like corrosion on the surface of the material.
Pitting corrosion is likely to be the presence of chloride ions combined with depolarizers. Pitting corrosion of passive metals such as stainless steel is often caused by the partial damage of some corrosive anions to the passive film, which protects the passive state with high corrosion resistance. An oxidizing environment is usually required, but this is also the condition for pitting corrosion. The medium that produces pitting corrosion is the presence of FE3+, Cu2+, Hg2+, and other heavy metal ions in C1-, Br-, I-, and Cl04- solutions, or the chloride solution of Na+, Ca2+ alkali and alkaline earth metal ions containing H2O2, O2, etc.
Therefore, abrasives or coating materials in contact with stainless steel should not contain halogen ions, including halogen-containing substances that may be converted to halogen ions.
As for why some standards stipulate that zinc-containing primers cannot be used on stainless steel surfaces, I have not been able to understand. To be answered by experts.
Zinc-rich paint cannot be used on stainless steel, and so far I have not found a reliable explanation. Strictly speaking, the actual zinc powder electrode potential is more negative, which acts as an anode to protect the stainless steel. In theory, there is no danger of bimetallic corrosion.
However, in addition to the theoretical description of the potential difference, there is an exception, which indicates that zinc cannot be in contact with stainless steel, that is, during the welding process, zinc will melt, or stainless steel may be exposed to zinc melt in other situations. The intergranular diffusion of stainless steel causes selective corrosion to occur along the grain boundaries. This is the so-called zincembrittel.
The zinc-containing primer referred to here mainly refers to the metallic zinc (zinc powder). The metallic zinc generates active hydrogen during corrosion, which can activate and reduce the substrate’s passive film and accelerate the corrosion; on the contrary, the hexavalent chromium can be dissolved out by zinc chrome yellow. Passivate the surface of the substrate and slow down corrosion, but due to the presence of chloride ions in seawater, pitting corrosion tends to occur.
A friend asked what kind of paint should be used on the surface of the stainless steel heat exchanger. He wants to be dry at room temperature, and the surface does not need to be too complicated.
The best anti-corrosion effect plan:
1. The surface is blasted with non-metallic abrasives and the roughness is Rz25-50 microns
2. Spray epoxy phosphate sealer, no more than 40 microns;
3. Spray epoxy high-build topcoat with 1 coat of 80 microns or 2 coats of 120-160 microns.
Simple solution:
1. The surface should be polished by hand or power tools, at least with a certain roughness
2. Spray epoxy mica primer with 1 coat of 40 microns;
3. Spray a coat of 80-micron epoxy thick paint.
These are all cured at room temperature and used indoors. If it is outdoor, epoxy high-build paint can be replaced with an acrylic polyurethane topcoat.
1. It is not necessary to use paint for the exterior, and paint cannot be used for the interior. The rust is mainly in the interior after the operation
2. Use an aluminum sacrificial anode for cathodic protection
3. We generally recommend a special cleaning agent for household stainless steel heat exchangers
Stainless steel is an alloy of iron, chromium, and nickel. It is mostly used in tube heat exchangers, condensers, and jacketed reactors in contact with various industrial waters.
The corrosion form of stainless steel can be divided into general corrosion and local corrosion. In various industrial waters, stainless steel has a very low overall corrosion rate, which can only corrode 1 cm every 1 million years under ideal conditions.
Therefore, the harm of general corrosion is minimal. Therefore, there is no need to consider corrosion protection.
Although stainless steel has a very low overall corrosion rate in various industrial glasses of water, under actual industrial production conditions, stainless steel equipment, especially the interior of various industrial water coolers, has frequent corrosion damage accidents. After more than a dozen large-scale fertilizer plants (with an annual output of 300,000 tons of synthetic ammonia and 480,000 tons of urea) in China, the stainless steel water coolers of each plant have been corroded and damaged after one to two years of production and operation.
Cathodic protection is generally used: when the potential is -0.8V (for Ag/AgCl electrodes), the crevice corrosion of CrNi steel in contact with carbon steel can be inhibited, and the cathodic protection effect of an aluminum sacrificial anode is obvious.
Painting with paint is unscientific, because pitting corrosion occurs inside the heat exchanger, under the attachments or deposits, and it is impossible to paint with paint. There is also a kind of strong penetration, peeling, cleaning, and corrosion inhibition. Capable of cleaning agent to clean its stainless steel heat exchanger
Because the condenser tubes in the stainless steel heat exchanger are closely arranged, manual cleaning is not possible, only chemical cycle cleaning can be performed.