With the rapid development of the world economy and industry, the problem of metal corrosion has become increasingly serious, causing huge losses to the national economy. The annual corrosion rate of existing steel and metal equipment in the world is about 10% [1], and it is imminent to take protective measures for metal materials. . Fasteners are a type of mechanical parts used to fasten connections and are widely used. For commonly used ordinary steel fasteners, surface treatment is a common method to improve its protective performance, and it is also a practical and effective method to improve surface corrosion resistance. Way.

Traditional surface protection technologies include electro-galvanizing, hot-dip galvanizing, chemical plating, painting, anodic conversion coating, etc., looking for more effective means of fastener protection, zinc-aluminum coating technology came into being under this background. Zinc-aluminum coating, also known aszinc-chromiumcoating or Dacromet coating, is a kind of zinc-aluminum powder, chromic anhydride, wetting agent, dispersant, deionized water and related additives prepared in a certain proportion. The coating liquid is coated on the base material, and is sintered and solidified to form a film layer.

The coating has no hydrogen embrittlement and high corrosion resistance.

The technology was developed by American Marine in 1963, initially to solve the problem of rusting of automotive chassis fasteners when exposed to salty rain and snow. This technology was introduced into our country in 1993, and has been popularized and used subsequently. But the fly in the ointment is that the Cr6+ in the zinc-aluminum coating has strong toxicity and carcinogenic effect, which will have a great impact on the environment and human body [3-5]. Therefore, chromium-free zinc-aluminum coating has become a hot topic in the field of scientific research. Research [6]. At present, the research on chromium-free zinc-aluminum coatings mainly focuses on the substitutes of chromic anhydride, but related studies have shown that the modification of the metal powder in the coating can improve the corrosion resistance of the coating [7-9], such as In the preparation of the plating solution, the chromium-free zinc-aluminum alloy coating was prepared by using the flaky zinc-aluminum alloy powder instead of the flaky zinc-aluminum alloy powder, and good results were obtained.

Chrome-free zinc-aluminum coating

Zinc-aluminum coating, also known as Dacromet coating, is a kind of anti-corrosion coating with simple process but excellent corrosion resistance. However, due to the strong toxicity of Cr6+ contained in the coating, it will have an impact on the environment and the human body. Both have promulgated some relevant regulations/directives and standards to restrict or prohibit the sale and use of products containing heavy metal chromium. The European Union promulgated the RoHS directive as early as 2003 to reduce the use of toxic substances, and banned the use of Cr6+-containing anti-corrosion coatings after July 1, 2007. Faced with this severe situation, my country's state has also stepped up efforts to limit the pace of chromium, zinc, and aluminum coatings. Chromium-free zinc-aluminum coating is to apply a plating solution made of flake zinc powder, flake aluminum powder, chromium-free passivator, wetting agent, dispersant, deionized water and other additives on the surface of steel and other substrates. After drying, sintering and curing, an excellent corrosion-resistant coating is formed on the surface of the metal substrate. Since chromate mainly acts as a binding agent and a passivating agent in the zinc-aluminum coating, choosing a substance that can replace chromate is the first problem to be solved for the chromium-free zinc-aluminum coating. Domestic research on chromium-free Dacromet coating is mainly in the laboratory research stage, but some progress has been made. For example, domestic BNC water-based chromium-free zinc-aluminum coatings have also been industrialized.

Chromium-free zinc-aluminum alloy coating

The chromium-free zinc-aluminum alloy coating is based on the traditional chromium-free Dacromet coating technology, and the scaly zinc-aluminum alloy powder is used as the metal pigment to replace the traditional scaly zinc-aluminum alloy powder. The coating liquid is prepared and coated on On the protective film layer on the surface of the metal material. Since the traditional chrome-free dacromet coating solution mainly uses flake zinc-aluminum mixed powder to mechanically stir to prepare the coating solution, the density of zinc powder and aluminum powder is different, and the volume varies greatly under the same mass, and the uniform dispersion in the coating solution not tall. , lead to segregation of the plating solution [16], after sintering and solidification into a film, the zinc and aluminum components of the coating will be unevenly distributed, resulting in local zinc deficiency and affecting the coating.

Corrosion resistance is poor and cannot provide effective protection for the substrate. The use of flaky zinc-aluminum alloy powder solves the phenomenon of uneven dispersion of metal powder and pigment. Zinc-aluminum alloy powder is used as a single metal powder, and there will be no delamination during the preparation of the plating solution and the formation of the coating film. At the same time, due to its high degree of alloying and small degree of composition segregation [17], the coating obtained after sintering and solidification into a film is denser and has smaller porosity. According to microstructure and composition analysis, the coating has almost no microscopic cracks, uniform distribution of components, and no obvious holes, which makes the coating have higher corrosion resistance compared with traditional chrome-free zinc-aluminum mixed powder pigment coatings.

The corrosion resistance mechanism of the coating

The corrosion resistance mechanism of chromium-free zinc-aluminum alloy coating can be attributed to the following three aspects: physical shielding effect, passivation protection effect and cathodic protection effect.

(1) Physical shielding effect. The chrome-free zinc-aluminum alloy coating is mainly formed by stacking flake zinc-aluminum alloy powder layer by layer, and the complex produced after passivation by the chromium-free passivator fills in the gaps of the scaly alloy powder, thereby forming a dense film layer by layer , Effectively prevent the infiltration of corrosive media such as water, oxygen, and chloride ions, and avoid corrosion caused by direct contact between the metal substrate and corrosive media.

(2) Passivation protection. In the chromium-free zinc-aluminum alloy coating, the chromium-free passivation agent plays a vital role. In the process of preparing the plating solution and sintering and curing the film, the chromium-free passivation agent will interact with the flaky zinc-aluminum alloy powder and the matrix. A complex chemical reaction occurs, and the alloy powder will be oxidized to form a dense passivation film on its surface. Chen Ling et al. [19] Analyzed by electrochemical means, the film surface formed by passivation has fewer active points. It can effectively prevent the diffusion of corrosive medium to the substrate, greatly reduce the self-corrosion current density of the coating, and slow down the corrosion rate of the coating. Due to the layered structure of the coating, when the surface coating is damaged by corrosion, the layered alloy powder on the lower surface will also be passivated, so that the corrosion resistance of the coating will not decrease significantly, and it can still be used for a period of time. Maintain good corrosion resistance inside, just make the plating thinner [20].

(3) Cathodic protection. Since the self-corrosion potential of the zinc-aluminum alloy powder is lower than that of the steel substrate, when the steel substrate protected by the alloy powder coating is in a corrosive medium, the coating can be used as the positive electrode of the primary battery, loses electrons and undergoes active dissolution, thus protecting the cathode as the cathode. steel base.

Summary and Outlook

As another breakthrough of Dacromet technology, chromium-free zinc-aluminum alloy coating technology is favored by more and more researchers for its excellent corrosion resistance. The coating is based on the traditional chromium-free zinc-aluminum coating technology, and the metal powder pigment is modified, and the zinc-aluminum alloy powder is used instead of the zinc-aluminum mixed powder to prepare the coating. Compared with the traditional chrome-free Dacromet coating, due to the alloying of the metal powder and the pigment, there is no delamination and composition segregation during the preparation of the coating liquid and the sintering and curing process, and the microscopic morphology of the coating is denser and more uniform. No obvious cracks; At the same time, compared with the traditional chromium-free zinc-aluminum mixed powder coating, its self-corrosion current density is smaller and the polarization resistance is larger, which greatly reduces the consumption rate of the coating in the corrosive medium, and the corrosion of the coating Resistance is improved.