Adhesion of film and its relation to metal protection

Adhesion of film and its relation to metal protection

Coating adhesion (binding strength) refers to the size of the binding force between the coating and the base material, that is, the unit surface area of the coating from the base material (or intermediate coating) peeling force required.The coating can protect well only if it adheres well to the substrate surface.Especially for metal substrate, the adhesion of coating is very important, which is mainly due to the close relationship between the metal corrosion process and adhesion under the coating, because the corrosion of the metal under the coating is caused by the electrochemical corrosion of the phase interface.Firstly, good adhesion can effectively prevent the penetration of external electrolyte into the matrix and delay the formation of interfacial corrosion cells.Second, good adhesion only in local cathode and anode electrolyte after connected to likely, strong interfacial adhesion can significantly prevent the corrosion products is an area of the side of the metal cation and through to the cathode diffusion, the spread of these cationic is to balance the cathodic reaction of hydroxyl ions of negatively charged.Diffusion of cations through the coating to the cathode is a very slow process, and without a reduction in viscosity, the electrolyte cannot communicate.

The effect of coating adhesion on metal corrosion under the film is illustrated by contrast experiments of two kinds of coated steel samples with or without adhesion.Two kinds of coated steel samples with and without adhesion (the coating was only attached to the base metal, surrounded by sealed samples) were immersed in 3% NaCl solution and kept at a constant temperature of 24C to visually observe the corrosion changes of the two samples.The potential change with time was measured at a constant time interval within 800h.The onset of corrosion was observed within 10h on the non-adherent specimens, while it took about 25h on the adherent specimens.No adhesion on the sample of the macro separate anode zone and cathode zone, also see under membrane aperture type corrosion situation, this is because the cracks appeared on the paint film, under the condition of – set into anode area, and the adhesion of the coating samples, adhesion to hopefully stop the spread of the corrosive substance along the metal/coating interface, we see on the sample is smaller anode area,No film stripping was observed.For the coated steel coating with adhesion, the total failure occurs in 550~ 750h, while for the sample without adhesion, it occurs in 300~ 450h.Once the coating has reached the destruction 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, cathode stripping often occurs quickly at the beginning, accompanied by blistering of adjacent parts, and then complete destruction.Therefore, the adhesion of the coating is the premise and guarantee of the protective function of the paint film.

Adhesion of film and its relation to metal protection

The essence of adhesive force of coating is a kind of interfacial force, and the adhesive force of organic coating should include two aspects.The first is the adhesion between the organic coating and the base metal surface, and the second is the cohesion of the organic coating itself.Both are indispensable in the whole system of coating protection.The coating must be firmly attached to the substrate during its lifetime.The greater the strength of the attachment force, the better: at the same time, the cohesion of the coating is also very important, the coating must form a tight and continuous film layer, in order to play a good blocking role.Two aspects are indispensable, adhesion is not good, and then the intact coating can not play a role, and the coating itself cohesion is poor, the paint film is easy to crack.These two together determine the adhesion of the coating and constitute a key factor determining the protective effect of the coating.

Coating to be evenly dispersed on the surface by coating, combine with base material well, its premise is sufficient coating has certain viscosity of liquid, powder coating is in high temperature melting, melt flow flat first curing), and obtain the contact area as large as possible, that is to say, on the surface of the coating and the coating doesn’t embellish is the premise to obtain high adhesion performance.