Effects of zinc powder on the organic and inorganic zinc-rich primer

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Inorganic zinc-rich primer has corrosion prevention effect. In the presence of water and oxygen, metal zinc will have anodic reaction, in which the corrosion products vary with different medium, including zinc oxide, zinc hydroxide, basic zinc carbonate, basic zinc chloride, zinc oxychloride and so on. These products have compact structure and are the kinds of stable compounds that are soluble in water, which can deposit on the zinc layer surface and form passivated shielding layer to prevent water, oxygen and salt penetration as well as reduce the electrochemical reaction rate of sublayer zinc and decline the corrosion rate. The paint film of inorganic zinc-rich primer will be cracked if the coating is too thick. Therefore, the film thickness of majority inorganic zinc-rich primer is recommended to be 50-100μm. When the film thickness exceeds 120μm, it will be cracked. Experiments show that by adding a little phosphorus iron powder, the coating crack condition will be greatly reduced. Zinc powder is a kind of important electrochemical antirust pigment, which has below characteristics compared with other metals: it is lighter than iron and has malleability, in addition, it can be made into alloy with iron. The most important characteristic is its electrochemical activity, in which the standard electrode potential is -0.76V, more active than iron that has the standard electrode potential of -0.44V. Besides, it has “self sacrifice spirit”. Fine particles of high purity zinc powder can be made by fusing, processing and purifying zinc, which can be used as the important antirust pigment in the anticorrosive paint. When the film is eroded, zinc powder will be rusted as the anode and the steel base material will be protected as the cathode. As the oxidation products formed in the process of zinc powder used as the anode has closure effect on the coatings, the protection effects of coatings on substrate will be strengthened. Zinc powder will be gradually consumed in the process of protecting the substrate from corrosion with very low speed. Due to the formation of its corrosion products, the potential difference between coating and substrate will be reduced. When the paint film is broken, new metal zinc will be exploded and the potential difference will be immediately increased, thus will generate a new round of strong cathodic protection, therefore, the zinc rich paint corrosion will not spread around from its paint film lesions. Provisions have been made in the world to stipulate the percent of zinc powder content contained in the zinc-rich paint in total mass of dry film so as to ensure the electrical conductivity and sacrificial anode effects caused by the close integration between zinc powder and steel in the zinc rich primer, as well as ensure the zinc-rich primer operating mass. For example, “SSPC-paint20” issued by the United States Steel Structure Paint Council specifically stipulates that the zinc powder content of inorganic zinc-rich paint should not be less than 74 percents and the organic zinc-rich paint should not be less than 77 percents. As organic zinc-rich paint has poorer electrical conductivity than inorganic zinc-rich paint, its zinc content is a little higher than the inorganic zinc-rich paint. Zinc content of many other single-channel coating and long-acting anti-corrosion inorganic zinc-rich coating products reaches 75 percents in water soluble paint and more than 82 percents in the solvent based coating. Besides, the requirements on zinc powder will vary in different service environment.

Epoxy zinc coating is most commonly used among the organic zinc-rich coatings. Chlorinated rubber zinc-rich paint and other coatings are also commonly used. However, as it is a kind of thermoplastic resin and will become soft when heated, its applications are not very much. Epoxy zinc coating is produced by using zinc metal powder as antirust pigment, epoxy resin as bond makings, polyamide resin or amine adduct as curing agent and configurating them with appropriate mixed solvent. It has excellent cathodic protection, mechanical wear resistance, heat resistance, small damage area in the paint film when welded and so on. Various data shows that by coating epoxy zinc coatings on the steel, the tensile, cold-bending and welding center impact value of the splice as well as the yield point, tensile strength, ductility, percentage reduction of area and other technical data of weld metal after being welded are same with the data that not coated with primer under same conditions, which shows that this product has no harmful effects on the welding quality of steel. In addition, it can dry quickly as well as has short recoat interval time, good adhesion, good crashworthiness and heat resistant temperature up to 120 degrees Celsius, besides, it can assort with most of the finishing coat. Therefore, it is a kind of good primer in multichannel coating system. Epoxy zinc coating is preferred no matter in the new construction or in the field service and other constructions due to its outstanding corrosion resistance performance, especially for the anti-corrosive construction on bridges, offshore facilities, tunnels and other steel structures. The excellent performance of epoxy zinc coating mainly depends on the stable carbon-carbon bond, ether bond and benzene ring contained in the molecular chain before the film formation of epoxy resin except for the important effects of zinc powder in its formula, which will lead to the compact film structure, good shielding and excellent penetration resistance after solidification as well as good water resistance, salt fog resistance and excellent chemical resistance. Moreover, the large polarity of hydroxide radical in the epoxy coating molecule and the gravitation caused on the abutting interface will react with the free bonds on the metal surface, thus will form strong chemical bonds and greatly enhance the coating adhesion and expand its adaptability to the substrate. Meanwhile, as the hydroxide radical in the benzene ring has been etherified after the film formation, it will have stable quality, rigid-flexible coating, good wear-resisting performance, high solid content of coating and will not generate large amount of small molecular by-products in the solidification process. Besides, it also has small volume shrinkage, low coefficient of thermal expansion and other characteristics. Compared with inorganic zinc-rich coating, the organic zinc-rich coating has poorer corrosion resistance. This is because it has low electrical conductivity, which will lead to the weak cathodic protection; in addition, it has low acceptable chloride content and large amount of fine zinc oxide smoke in the welding and cutting process, which is not conducive to the environmental protection. However, it also has wide application as it has lower requirements on the abrasive blasting pretreatment of coated steel substrate and better mechanical impact resistance than inorganic zinc-rich coating, and easy to assort its upper shield finishing coat.