Coating adhesion (bonding strength) refers to the size of the bonding force between the coating and the substrate, that is, the force required for the coating per unit surface area to be peeled off from the substrate (or intermediate coating). The coating can only play a good protective role if it adheres well to the surface of the substrate. Especially for metal substrates, the adhesion of the coating is very important, which is mainly due to the close relationship between the corrosion process of the metal under the coating and the adhesion, because the corrosion of the metal under the coating is caused by the electric current of the phase interface. caused by chemical corrosion; first, good adhesion can effectively prevent the penetration of the external electrolyte to the substrate and delay the formation of interfacial corrosion cells; second, good adhesion is only possible after the local cathode and local anolyte are connected Occurrence, strong interfacial adhesion can greatly prevent the diffusion of corrosion products—metal cations through the interphase side to the cathode region. The diffusion of these cations is to balance the negatively charged hydroxide ions generated by the cathode reaction. Diffusion of cations through the coating to the cathode is a very slow process, and the electrolyte cannot communicate without a decrease in viscosity.
The corrosion experiments of two coated steel samples with and without adhesion illustrate the effect of coating adhesion on metal corrosion under the film. Soak two coated steel samples with and without adhesion (the coating is only attached to the base metal and sealed around the sample) in 3% NaCl solution, keep a constant temperature of 24C, and visually observe the corrosion of the two samples change of circumstances. And the change of potential with time was measured at a certain time interval within 800h. The onset of corrosion was observed within 10 h on the non-adhesive specimens, whereas on the adherent specimens corrosion did not occur until about 25 h later. Macroscopically separated anode and cathode regions occurred on the sample without adhesion, and crevice corrosion under the film was also seen. This is due to cracks appearing on the paint film, which became anodic regions under certain conditions. For the coating sample with adhesion, the adhesion can prevent the diffusion of corrosive substances along the metal/coating interface, so what is seen on this sample is a small anodic area, and no peeling off of the paint film is observed. For the coated steel with adhesion, the total failure of the coating occurs at 550~750h, while that of the non-adhesive samples is between 300~450h. Once the coating reaches the failure stage, it will peel off by electrochemical and physicochemical mechanisms. When there are defects (such as small holes or cracks) and ions (such as NaCl ions) in the paint film, cathodic disbonding often occurs relatively quickly at the beginning, accompanied by blistering in adjacent parts, and then completely destroyed. Therefore, the quality of coating adhesion is the premise and guarantee for the paint film to exert its protective function.
The essence of the adhesion of the coating is an interface force, and the adhesion of the organic coating should include two aspects. The first is the adhesion of the organic coating to the base metal surface, and the second is the cohesion of the organic coating itself. These two are indispensable in the whole system of coating protection. The coating needs to be firmly attached to the substrate surface for the lifetime of the coating. The greater the strength of the adhesion force, the better: at the same time, the cohesion of the coating is also very important. The coating needs to form a dense, firm and continuous film layer in order to play a good blocking effect. The two aspects are indispensable. If the adhesion is not good, no matter how good the coating is, it will not work. If the cohesion of the coating itself is poor, the paint film will easily crack. These two together determine the adhesion of the coating and constitute a key factor in determining the protective effect of the coating.
The coating should be evenly dispersed on the surface of the film to be coated and well combined with the substrate. The premise is that the coating is a liquid with a certain viscosity (the powder coating is also melted at high temperature first, and the melt is leveled and solidified), and it has been obtained as much as possible. A possible large contact surface, that is to say, the complete impregnation of the surface of the coating and the object to be coated is a prerequisite for obtaining high adhesion properties.
