by Irina FRISCHBERG, Dr. Sc. (Technol.), Director General, ZAO NPP Highly Dispersed Metallic Powders, Yekaterinburg; Olga SUBBOTINA, Cand. Sc. (Chem.), Paint-and-Varnish Materials manager; Andrei SOLOVYOV, Cand. Sc. (Chem.), Head of the Rust Inhibitor Paint-and-Varnish Materials Laboratory
Choosing a protective material capable of prolonging the service life of metalworks and thus of reducing their operational costs is one of the industrial challenges of our time. It is all important to reliably protect paintwork coatings in situations where outfit is exposed to hostile (aggressive) media, say, in pollution-laden industrial areas, marine atmosphere, northern regions, oil-water contact, and so forth. High health and safety standards must also be satisfied.
State-of-the-art-method allows to have the cited problem solved using the paint-and-varnish materials coating systems. These materials feature zinc dust as their pigment piece. Because of its extremely high proportion in the zinc-rich coatings (about 85 to 96 percent by weight), the dust provides for a dependable cathodic protection of steel. The mechanism of this protection is as follows: once a steel surface is exposed to a corrosive medium attack, the zinc, noted for its more negative electrochemical potential, tends to get oxidized, which prevents corrosion and heals corrosion-induced damages. This technology is known as cold galvanizing.
Along with the cathodic (protective) defense mechanism such coatings exhibit the properties characteristic of regular paint-and-varnish coatings. Their microspores gradually accumulate the water-insoluble by-products of zinc corrosion. This is how a coupled defense mechanism is actuated, which adds dramatically to the effectiveness and safety of zinc-rich materials.
Considering the current quality trends in protective paint-and-varnish coatings, in these 20 years our Company has been involved in the development and manufacture of cold galvanizing-oriented materials. In so doing, we employ high-grade film-forming substances. Apart from the well-known TsVES product, our Company turns out the TsINOTAN polyurethane composition, polymer-based epoxy primers TsINEP and TsINOL as well as special-purpose materials known as TsINOTERM, TsINOL-SV, and TsINMASTIK putty compound.
Our products also include materials for interoperational and temporary (masking) protective systems. These are called TsVES-MO and TsINMASTIK (autoconserving agent).
In the production of these materials current use is made of highly dispersed zinc powder of PTsVD grade, whose special quality is provided by the available original technologies and equipment. Modular units of continuous operation enable us to fabricate powders of the tailored chemical and granulometric composition, as well as to modify the particles surface with a variety of substances directly in the course of powder fabrication.
The findings of the electrochemical investigations and proximate tests confirm that, in terms of their protective properties, the PTsVD powders can give an optimal chemical and granulometric composition. This is where a durable cathodic protection effect is ensured. In particular, our powders are notable for the lowest amount of zinc oxide and metals liable to get the electrochemical potential shifted into the range of positive values. Metallic zinc present in PTsVD powders makes up 98 - 99 percent, iron 0.005 percent, lead 0.01 percent, and copper 0.001 percent. These figures are much superior in the purity of zinc powder intended for paint-and-varnish compounds to what is specified by relevant foreign and domestic standards. The particles of this powder are sphere-shaped, averaging 4 to 12 μm in diameter.
On the whole, it is undeniable that the zinc powder-zinc-rich compounds flow pattern affords many benefits compared with the other fabrication routes. First, we do not have to transport, and arrange for prolonged storage of the powder and thus can preserve its properties intact. Secondly, we enforce tight control over the process technology variables and the quality of the fabricated powder. Thirdly, it is feasible to optimize the powder properties depending on the type of a zinc-rich composition and its intended application.
Wherever metalworks are exposed to lower corrosive media, such as mild atmosphere, light oil
products, etc., the zinc-rich coatings may be recommended for use all alone, with the exception of the old venerable practice of using the TsVES ethyl silicate composition for water tanks protection.
However, composite coating systems of zinc-rich protective preapplied coating and coat enamels do better in the moderate or higher corrosive media. As for the coat enamels, our business makes use of polyurethane and acrylic materials based on laminated pigment pieces. Thereby the coating layers reinforce the barrier properties of the coat by screening the preapplied coating against corrosive factors. As a result, zinc oxidation slows down, and its protective action is prolonged. Such layers also improve the coating's hardness and resistance to abrasion, and add to decorative characteristics.
Our experts have developed zinc-rich compositions for long-term protection of a variety of industrial projects and installations. For instance, bridges across the rivers Moskva, Ob, Volga, Syr Darya, Chubuk (Turkey); power transmission line supports in the Ural and Siberia; reservoirs and industrial metal structures of domestic oil and gas enterprises, of metallurgical and chemical works, and of hydroelectric plants; storage tanks for electric heating plants located in the territory of Russia. In the past 10 years the total area of the coating protected metal surfaces has topped 10 mn sq m.
Widespread use of home state-of-the art zinc-rich compositions allows to noticeably increase the service life of metal structures, and approach the world standards in the quality of corrosion protection.
Journal Paintwork Materials, Their Application, Nos. 7 - 8, 2004
Digest prepared by Olga SKOROKHODOVA
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