The main chemical reaction of polyurethane powder coating is the reaction between the hydroxyl group (-OH) of polyester resin and closed isocyanate group (-NCO), and the degradation speed of polyurethane powder coating mainly depends on the decomposition control of the main chain of hydroxyl polyester. The selection of suitable hydroxyl polyesters has a crucial influence on the chemical resistance of polyurethane powder coatings.
Through the test comparison, the resin with high gloss and good aging resistance was selected as the final film-forming resin for polyurethane powder coatings.
The anode-l polarization curve of the metal in the corrosive medium is curve 1. When 9: corrosion inhibitor is not added, the cathodic polarization curve is P2 curve 2. The two polarization curves intersect at point S, and the corresponding corrosion potential is p . Corrosion current density is ic. At this time, the metal is in an activated state, and the corrosion rate is 9: very large.
When there is enough nitrite in the corrosive medium, it has no effect on the anodic polarization curve, while the cathodic polarization curve moves to the direction of high current. Is due to the depolarization of the corrosion inhibitor, the cathode in Figure 22-4 The polarization curve of the polarization type slow cathodic polarization is shown from curve 2 to curve 3. At this time, the negative and positive curves intersect at S’in the passivation zone, the metal is in a passivated state, the corrosion potential is shifting to ps, and the corrosion current is greatly reduced. The corrosion inhibitor depolarizes the metal to passivate the metal. It is pointed out that when the amount of cathodic depolarization corrosion inhibitor is insufficient, the ptfe powder mixing cathodic polarization is insufficient, and the cathodic polarization curve moves from curve 2 to curve 4. The cathodic anodization curve intersects the S” in the active area and cannot enter the passivation. As a result, the corrosion current density increases from i. to that of corrosion. Therefore, insufficient amount of this kind of corrosion inhibitor is very dangerous. This kind of corrosion inhibitor does not affect the speed of the anode reaction, but because the cathode electrode The increase of chemistry causes the potential of the whole system to move higher than the passivation potential.
The cathode type corrosion inhibitor mainly achieves corrosion inhibition through the following effects.
Increase the overpotential of the cathode reaction Some anionic corrosion inhibitors inhibit the oxygen ion discharge reaction by increasing the overpotential of the oxygen ion discharge. For example, in an acidic medium, arsenic, bismuth, antimony, mercury and other metal salts precipitate on the cathode area of the metal surface during the corrosion process, which can increase the overpotential of hydrogen evolution and hinder the reduction reaction of hydrogen ions on the metal surface to play a corrosion inhibition effect.
The formation of a compound film on the metal surface prevents the depolarizer from reaching the metal surface by producing adsorption or phase film on the surface of the cathode.
Corrosion inhibitors that absorb dissolved oxygen in water can absorb dissolved oxygen in water, reduce the concentration of oxygen, which is the cathode reactant in the corrosion reaction, and slow down the corrosion of metals.
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