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The protective ( barrier ) effect of the resulting corrosion products depends not only on their quantity but mainly on their composition and properties . Corrosion resistance of the iron-zinc phases of the diffusion zinc coating is higher than that of pure zinc , primarily due to participation of ZnO oxide along with zinc ferrites ( ZnFeO2 , ZnFe2O4 ) and complex zinc-based salts in passivation of the coating surface . Corrosion of diffusion zinc coatings in time has a decaying character , i . e . it decreases .
The increased corrosion resistance of diffusion zinc coatings is also due to formation of barrier layers through accumulation on the surface of the corrosive system of an inert phase present in the coating as a component or formed on the diffusion zinc coating during its interaction with the corrosive medium .
When evaluating the corrosion resistance and protective properties of zinc coatings , type of the coating ( determined by the method of its formation ) is important . Therefore , on the basis of the above comparison , the most typical applications for various types of zinc coatings can be offered . Comparison of the main properties and the results of corrosion tests of various zinc coatings shows that diffusion zinc coatings applied on the products by chemicalthermal treatment in zinc-based powder mixtures are most appropriate for severe operating conditions ( oil and gas production , sea or industrial water flow ).
Zinc-filled coatings are compositions consisting of zinc powder and a binding component . Depending on the binder type , inorganic- and organic-based zinc-filled coatings are distinguished . In inorganicbased zinc-filled coatings , liquid glass ( silicates of sodium , potassium , lithium ) is most often used as a binder . Various synthetic resins ( epoxy , epoxy ether , alkyd , phenolic , polyurethane , etc .) are used as organic binders .
Properties of zinc-filled coatings are largely determined by the binder nature ( its adhesive power and electrical conductivity ), binder / zinc powder ratio , coating thickness , chemical and granulometric composition of the zinc powder , and shape of its particles . Of particular importance is chemical composition of powders for organic-based zinc-filled coatings as this determines their electrochemical properties .
Choice of the binder is mainly determined by the properties to be possessed by the zinc-filled coating in a particular application . Inorganic zinc-silicate coatings have high adhesion power , hardness , heat resistance ( up to 500 ° C ), resistance to solvents and oils . Such coatings feature good manufacturability and for their application , use is made of nonscarce binders , i . e . they have a low cost . Due to their electrical conductivity , inorganic zinc-silicate coatings have a protective action ( for example , electrical conductivity of pure sodium silicate is 2.62∙10-3 ohms ).
Ethyl silicates , organic derivatives of silicic acid , are widely used as binders for zinc-silicate coatings . Zincfilled coatings based on the ethyl silicate binder have a short drying period and a higher protective ability in comparison with the liquid glass based coatings . They can be applied using an airless spraying method . Disadvantage of coatings with ethyl silicate binders consists in their inflammability during application in connection with the use of acetone , ethyl or isopropyl alcohol as solvents .
Combined coating is a combination of a zinc coating of any kind and a paint or polymer coating . In the world practice , such coatings are known as “ duplex systems ” which possess not only electrochemical protective effect of zinc coating but also the waterproofing protective effect of paint , varnish or polymer . New opportunities open to enhance corrosion resistance and operational reliability of metal products with the use of a protective duplex system .
As a whole , the duplex system on a steel surface can be defined as a combination of a metal coating ( zinc , zinc-aluminum or iron-zinc alloy ) on which one or more layers of paint or powder coating are applied . The aggregate of metallic and organic coatings in the duplex system provides optimum anticorrosion
24 TUBE NEWS June 2018