1. Causes of flash rust in water-based coatings
There has never been an accurate explanation for the reason why the water-based steel structure anti-corrosion coating produces flash rust during the construction process. Some experts in this field have said before that the flash rust problem of water-based coatings is generally that the water-based coatings of new constructions will have rust spots during its drying process. This phenomenon mainly occurs on the corroded interface or the newly activated steel surface. The occurrence of flash rust may be related to the dissolution and precipitation of iron compounds. The severity of flash rust depends on the coating system. When the coating is at low pH, low temperature and high humidity, the contact time between water and the substrate is prolonged, and it is easy to Flash rust occurs.
There is a process for the film formation of water-based paint. No metal is absolutely pure and absolutely smooth, which will produce small pieces of different potentials one by one, and current will be generated between the small pieces of different potentials. Metal oxidation is caused by This occurs to form rust spots, that is, flash rust.” From these different statements, it can be seen that the generation of flash rust is an electrochemical process.
Since the water-based paint itself is a conductor, a charge migration channel is formed between different regions with potential differences, so that corrosion occurs within a short period of time during the film-forming process of the water-based paint, resulting in flash rust. Obviously, the longer the wet paint film is kept, the more serious the flash rust phenomenon, which can explain the reason why the flash rust phenomenon is more serious under the condition of low temperature and high humidity.
In fact, because flash rust is an electrochemical phenomenon, the reactive metals that are ranked ahead of hydrogen will encounter this problem when applying water-based paints. In the coating test of steel, cast iron, aluminum and aluminum alloy, galvanized sheet and other substrates, we found that the conditions prone to flash rust are roughly: cast iron, aluminum, carbon steel, galvanized sheet (polished), tianshi wax, tinplate (grinding), iron plate or wire, aluminum alloy.
2. Causes of accelerated flash rust
①Slow drying conditions: low temperature and high humidity (especially serious when RH is greater than 85%)
②Iron substrates with unclean and unstable pretreatment: Usually, the newly treated steel surface is prone to flash rust; rusted carbon steel, cast iron and other metal materials are also prone to flash rust after grinding.
③Coating system: The lower the PH value, the easier it is to flash rust; to deal with this problem, generally select adsorptive products for water-based coating systems with lower PH value.
④ The degree of difficulty of flash rust on metal substrates is: metal weld (submerged arc welding) > cast iron > carbon steel > zinc plate (grinding) > tinplate (grinding) > iron plate or wire > aluminum alloy.
3. How to prevent flash rust
1. Use an anti-flash rust agent: A reasonably matched anti-flash rust agent can effectively prevent flash rust from occurring.
2. Accelerated drying: high temperature and low humidity are conducive to the prevention of flash rust.
3. Increase the PH value of the coating: the higher the PH value, the less likely it is to produce flash rust.
4. Control the thickness of the paint film: the thicker the paint film, the easier it is to produce flash rust.
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