First, introduction to hot-dip galvanizing technology:
Hot-dip galvanizing is a way to prevent steel from rusting. Galvanizing has been widely used for the protection of steel since its first application more than 170 years ago. Hot-dip galvanizing is divided into different forms such as continuous galvanizing of wire rods, continuous galvanizing of plates, semi-continuous galvanizing of steel pipes, batch structural steel galvanizing, and mechanical galvanizing.
Second, the hot-dip galvanizing process is as follows:
Degreasing-washing-pickling-washing-flushing-drying-hot-dip galvanizing-separation (rapping)--cooling (passivation) A good hot-dip plating operation should be under strict control of each process. Fully utilize the function of this process. And if the previous process is not good, it will cause a chain reaction of adverse reactions in the subsequent process, which will greatly increase the operating cost or cause poor hot-dip galvanized products. If the pretreatment is not good, the molten zinc cannot react completely with the steel to form the most perfect galvanized film structure. If the post-treatment is not good, the appearance of the galvanized film will be damaged, and the commercial value will be reduced. Each process of hot-dip galvanizing has its own technical characteristics, and some have related control requirements for the upper and lower processes. In recent years, new degreasing agents have been used in the degreasing process, acid mist and over-pickling inhibitors have been used in the pickling process, and compound galvanizing solvents have been used in the solvent treatment process. Add anti-oxidation to the zinc bath, increase the fluidity of the zinc solution, inhibit the reaction rate of iron-zinc alloying, generate spangles, and improve the appearance of the coating. In the cooling process, there are passivation and non-passivation, and passivation has chromium and no chromium.
Third, advantages and disadvantages of hot-dip galvanizing technology:
Advantages: Long anti-corrosion time, up to more than 20 years. Disadvantages: 1. The technical process is complicated. From the above introduction, it can be seen that any improper handling of the process will cause the decline of the anti-corrosion effect of the coating. 2. High energy consumption, the temperature of hot-dip galvanizing needs to be more than four or five hundred degrees, and it needs a lot of power consumption, which greatly increases the cost of the product. For China, which is short of energy, this is undoubtedly a waste of precious energy. 3. The pollution is serious, a large amount of three wastes will be produced in the galvanizing process, which will cause serious pollution to the local atmosphere, water and soil environment, affecting people's life and health, and at the same time, it will also affect the health of front-line workers. 4. The comprehensive cost is high. If you want to avoid the above phenomenon, it is necessary to invest a lot of money to solve the process problems and carry out the three wastes treatment. 5. Zinc salts will be produced within two to three months of use, making the rod body black and unable to be cleaned and brightened.
Fourth, corrosion mechanism of steel:
The corrosion of steel is due to its chemical and electrochemical reactions, from the outside to the inside, the steel is destroyed and the result of losing its original properties. Steel has an inherent spontaneous tendency, that is, from the metal to its more stable natural state. Transformation is the natural corrosion tendency of steel. In nature, due to different corrosive media, there are various types of steel corrosion, which can be divided into: atmospheric corrosion, water and seawater corrosion, soil corrosion and chemical medium corrosion. Steel is composed of ferrite and traces of cementite, and in water with electrolyte, the following electrochemical reactions take place: Anode reaction: Fe → Fe2+ + 2e Cathode reaction: 2H+ + 2e → H2↑ or in solution: Fe2+ + 2OH- → Fe(OH)2 Under the action of water and oxygen, Fe(OH)2 generates hydrated iron oxide, that is, rust.
2 Fe(OH)2+ H2O +1 O2 → Fe2O3 x H2O 2H2O + O2 + 4e → 4OH- In the electrochemical reaction, the anode is corroded.
1. The composition of rust: From the perspective of chemical composition, rust is composed of: FeO﹒ from the inside to the outside nH2O;Fe2O3﹒ nH2O;Fe3O4﹒ It is composed of three layers of nH2O, and rust can also be regarded as the process of iron redox into original ore. Therefore, like natural iron ore, it is composed of FeOOH, Fe3O4 and some amorphous substances of different crystal forms (α, β, γ, δ). , From the structural point of view, the rust layer is a loose and porous water-containing solid. From the composition point of view, it is a relatively stable iron compound such as Fe3O4, Fe2O3, α-FeOOH and more active iron compounds such as γ-FeOOH, Fe(OH)
2. Fe(OH)
The mixture of 3. The new rust is mainly γ-FeOOH, and the old rust is mainly Fe3O4. The corrosion process of steel is: O2 →(rearrangement) α-FeOOH Fe3O4, Fe → FeO→ Fe(OH)2 → Fe(OH)3→γ-FeOOH→(Fe(OH)2) Fe2O3→(dehydration).
Because rust is loose and porous, the rust layer often contains a lot of water, which causes the metal to continue to corrode, and the primary rust on the steel surface and the steel substrate adhere firmly. As the rust progresses, the original primary rust gradually ages and is squeezed To the surface of the rust layer, the rust on the surface of the steel can be divided into two layers, the outer layer is loose rust (floating rust), which is not firmly attached to the steel; the inner layer is fast rust, which is firmly attached to the steel substrate. 2. Difficulty in removing rust from steel: It is very difficult to completely remove the inner rust of steel rust. As long as there is residual rust on the surface of the steel, the rust nucleus can make the steel substrate continue to rust. Even if the rust is very clean, the surface area of the new metal will increase. If it is large, the amount of water absorbed will increase, and it will be greatly affected by the environmental corrosive medium, and the possibility of rusting will be high. According to the above situation, use conventional anti-rust paint for metal protection,
The effect will not be ideal. At present, ordinary paint is used to prevent rust, and the paint film cracks and falls off in half a year to a year, and the paint film is rusted and stained, which can not achieve rust prevention at all. The key to using paint to prevent corrosion and rust is to make the metal surface no longer rust. The use of rust-bearing paint should be vigorously promoted in anti-rust paint. It is a new type of paint that can be directly painted on the surface of steel with residual rust. coating. 3. Permeable rust paint: Permeable rust paint originated from European and American countries. It uses the wetting and penetrating effect of paint and additives on rust to separate and surround the rust in the paint. At the same time, with the help of special additives It plays a role in preventing rust and prevents the further development of rust.
Fifth, form of anticorrosion:
The more mature forms of anti-corrosion mainly include: ordinary paint, Dacromet paint, ZY anti-rust paint, hot-dip galvanizing, electrostatic spraying, multi-element gas co-infiltration, etc. In addition to ZY anti-rust paint, other anti-corrosion forms are used to remove rust on steel. The grade requirements are high, the anti-rust effect of ordinary coatings is poor, and the anti-corrosion treatment costs of Dacromet coatings, hot-dip galvanizing, electrostatic spraying, and multi-component gas infiltration are high, the process is complicated, and it is not suitable for on-site construction.
Sixth, introduction of ZY products
This product is composed of rubber and plastic resin, flake zinc-aluminum powder (solid content of 80%) chemical corrosion inhibitor, auxiliary agent, and environmentally friendly diluent. Does not contain benzene solvents, lead, mercury, chromium and other toxic ingredients.
A. Anti-corrosion mechanism of ZY anti-rust paint
ZY-D anti-rust paint forms countless extremely fine zinc flakes on the surface of metal parts. According to the theory of electrochemical corrosion, in the corrosion battery, the anode with negative electrode potential is corroded. Fe → Fe2+ + 2e Zn → Zn2+ + 2e The standard electrode potential is the standard electrode potential is 0.44 V—0.50 V -0.76V The primary battery composed of Fe and Zn, Zn is the anode, Fe is the cathode, because the zinc powder adheres to the steel surface , to form a conductive protective layer. Zinc is slowly consumed during the protection process, but the speed is extremely low. In addition, due to a small amount of aluminum powder in ZY-D anti-rust paint, it can effectively prevent the corrosion of salt water and increase the weather resistance and rust resistance. And resistance to alternating dry and wet, but also inhibit the precipitation of zinc.
It also contains special additives and solvents in the composition, which can chemically react with rust to form a protective complex, which has sufficient penetration ability to the rust layer, so that the rust layer can be bonded into a continuous closed coating. To seal the entire rust layer and the steel surface, in addition, it has enough reactivity to passivate or convert the harmful iron compounds in the rust layer into harmless fillers, forming a passivation film on the steel surface, its special polymer structure, It has the ability to resist ultraviolet rays, can delay the aging of the paint film, and has a long anti-corrosion life, generally more than 10 years.
B. Product advantages:
1. Excellent water resistance and salt water resistance.
2. Excellent rust resistance, durability, impact resistance and good adhesion.
3. No zinc salt is produced, and the rod body will not turn black and fade after long-term use.
4. It can be washed with water, and the surface is not easy to deposit dust.
5. No substances harmful to human body and the environment will be produced during the construction process.
C. Construction technology
1. The coating is simple, using spray brush or roller.
2. Shot peening (sand) removes the oxide scale, rust, oil, dust, water, etc. on the surface.
3. The surface temperature of the steel during coating construction is 3°C higher than the dew point temperature.
4. When the steel surface is affected by rain or ice and snow, it cannot be painted.
5. It can be coated on hot-dip galvanized and partially rusted rods at the same time.
D. Precautions for construction
1. When opening the barrel, the dust and debris outside the barrel should be removed first to avoid mixing into the barrel.
2. When painting, use special thinner according to the methods of spraying, brushing and rolling.
3. Due to the large proportion of zinc powder, the coating process should be continuously stirred to prevent the zinc powder from precipitation.
4. When painting with rust, it is necessary to ensure that the thickness of the paint film is met, and the thickness of the paint film is required to be more than three times that of the rust layer.
E. Coating supporting process
1. Selection of paint: Primer: ZY-S high-permeability rust-preventive paint Finish: ZY-D rubber-plastic finish (color paint)
2. Interval time between coating: the time between primer and primer is ≥2 hours, the time between primer and topcoat is ≥12 hours, and the time between topcoat and topcoat is ≥2 hours
(two-component) (one-component)
Note: After the workpiece to be painted is ready, adjust the two-component paint according to the approximate usage, and add the curing agent to the oil
Stir evenly in the paint, use it up within 3 hours when the temperature is higher than 25℃, and use it up within 6 hours when the temperature is low. Depends. 6. Quality inspection: If any defects such as missing coating, sagging, wrinkles, pinholes and cracks are found during coating, they should be dealt with in time. Before each layer is painted, the previous layer should be checked, and the interval between painting should meet the requirements.
F. Main features: It has excellent acid and alkali resistance, salt spray resistance, seawater resistance, aging resistance, wear resistance, improves the adhesion between paint films, and no longer peels off or peels like traditional paints. The total dry film thickness is not less than 170um.