EXPERIENCE IN DEVELOPMENT AND COMMERCIAL IMPLEMENTATION OF NEW ZINC-RICH COATINGS FOR PROTECTION OF STEEL AGAINST CORROSION

Authors: Prof.Dr.Irina V.Frishberg; Dr.Lilia P. Yurkina, Olga Yu. Subbotina and Dr.Nicolay V. Kishkoparov,
(Fine Metal Powders Company)

Abstract. Two new zinc-rich materials have been commercialized by Fine Metal Powders Company using the basic method of producing fine zinc powder developed at the Institute of Metallurgy of the Russian Academy of Science,Ural Branch.A two-pack self-curing paint ZFES® consists of fine zinc powder and ethyl silicate binder,by metal zinc content 50-60% in the paint and 90-95% in the coating. Application allowable in the temperature range from -15°C to +40°C , operation temperature - up to 150°C. Service life of the coating - up to 20 years in cold climat and up to 12 years in tropics.The ZFES coating displays high adhesion to steel and performs similar to hot galvanizing preventing occurrence of under -coat corrosion. A single-pack paint ZINOL® based on modified polysterene contains 70-80% metal zinc powder (95-97% in dry coating).It is less sensible to the surface quality and very easy in operation. Both materials provide double corrosion protection for steel: shielding (similar to hot galvanizing) and insulating (similar to varnish and paint materials). FMP coatings surpass hot galvanizing in service life at equivalent zinc content of the coat. They are extensively used in construction of buildings, bridges, electric power lines, oil tanks etc.

Zinc-rich coatings (ZRC) have long received worldwide acceptance for corrosion protection of steel, cast-iron, and other products and structures.

Fine Metal Powders Company (FMP) was the first in Russia to set up, on the basis of a new fundamental technology for making ultrafine powders, the commercial production of ultrafine zinc powder suitable for use in ZRC. In last In last 5 years the company has developed and commercialized two new ZRC based on UF zinc powder of the company's own production.The main aim was to meet home customers demand in high quality cost sparing corrosion preventing products.

The first ZRC developed by FMP Company was a double-package compound, brand name ZFES. It consists of zinc powder and an ethyl silicate binder. Tables 1 and 2 present main characteristics of the ZFES compound and coating.

Table 1. Ethyl silicate compound ZFES

Table 2. ZFES Coating

The road from laboratory samples to commercial products was a no rose-paved one.To get admittance to big state and private consumers such as metal work plants, bridge and electric power lines construction enterprises , shipbuilding etc, the new home -developed ZRCs must have been approved and recommended by authorized experts, certified by the State Standard Committee and introduced in official technical guidance papers and industry branch standards. That's why several years were passed in testing the new materials by independent authorized experts. The aim was to show that new ZRCs are not inferior to known analogues, first of all to hot galvanizing, by performance in various media - air, fresh and sea water, industrial zones, oil and petroleum derivatives.

The summary of the tests results is given below.
The ZFES coat arises from the chemical interplay of hydrolyzed ethyl silicate with zinc and iron ions to form at the coat-substrate interface a strong iron-silicate layer resistant to fresh and sea water, oil, petroleum derivatives, the sea atmosphere, etc. This is the so called interphase layer, which chemically binds the metal to the corrosion-inhibiting coat. The interphase layer precludes the "undercutting" of the protective corrosion-inhibiting coat and the propagation of underfilm corrosion.

The identity of behavior against corrosion of hot galvanizing to that of ZFES coating ("cold galvanizing") is illustrated most dramatically by data of comparative laboratory-scale and full-scale corrosion tests. In the laboratory we employed the clamping-cell volt-ampere metering (VAM) method. Also, a metallographic analysis was invoked, the results of which are schematized in Fig. 1.

Fig.1.Schematic picture of corrosion process as seen by metallographic study:a,hot galvanizing;b,ZFES coat; c,zinc-rich epoxy paint

Results of full-scale tests on ZFES coat endurance in sea air and water effected at five marine climatic stations in the Republic of Cuba, as obtained by the VAM method (Fig.2 ,2a and 2b) have shown that both with hot (Fig. 2a) and cold galvanization (Fig. 2b), corrosion protection is effected as a result of the anodic dissolution of zinc. The curves are symbatic in character, but the rate of dissolution of zinc in the ZFES coat is lower than that in a hot-galvanized coat, a fact which manifests itself in lower current density values in this case (Fig. 2a, b, curve 2). This means that by contrast with hot galvanizing, the ZFES coat ensures a longer service life, which is due to the passivating action of the ethyl silicate binder.

• Cold climate > 20 years;
• Moderate climate > 15 years;
• Tropical climate > 12 years.

The predicted service life of a > 150 m thick ZFES coat in sea water amounts to > 10 years.

The ZFES coat displayed high corrosion resistance in crude oil, petroleum derivatives, weak solutions of salts, alkalis, and acids within the limits of pH 5.0 to 8.5 and withstands temperatures of up to 150°C.
In 10 years, no visible changes have been detected in the ZFES coats subjected to full-scale tests.

Extension of the operationable pH interval has been achieved by use of epoxy, vinyl, acrylic, chlorinated-rubber and other top finishes. A silver-grey aluminum- ethyl silicate paint PAES performes very good as a top finish.ZFES displayes a perfect compatibility with all said top finishes. In some cases, when using a multiple-layer ZFES coat, certain amounts of aluminum powder may be introduced into the last ZFES layer for decorative purposes, without reducing the content of zinc.

Accelerated corrosion tests of ZFES coat have been carried out in comparison with one of commercial European polymer based zinc-rich paint, using a technique similar to ISO standard 4623-84 for determination of thread-like corrosion on a steel surface.. We employed the electrochemical method for measuring the variation of current density as a function of the time of holding specimens in a corrosive medium, for which purpose a simple short-circuited galvanic cell was used with HCl 0.02 M solution electrolyte. A 50 m thick coats were applied to shot blasted steel specimens. The zinc content of a hardened ZFES coat was 95%, while that of the other coat was 97% (Fig.3). It's known that to protect steel against corrosion in most common media, the current density should range between 0.05 and 0.2 mA/mm2 [4]. As Fig.3 showes, all imax in the tests exceed those values. Actually, the ZFES coat provides good protection with much lower imax values when compared with two other coats (curve 2). At the same time it displayes the lowest rate of the short-cut current density decrease. This suggests that ZFES coat will not lose its protective property for a longer time then other coats.

The corrosion stability of ZFES coating greatly depends on the quality of surface. A mandatory condition is that the surface be slightly rough (optimally 30 to 50 m) and cleaned to grade 5 a3 (5 a 2 1/2) according to the Swedish Standard 150 8501-1; 1988/55 05 5900. The required surface condition is ensured by shot - or sand blasting.

The effect of the reactivity of the steel substrate in the formation of a ZFES coat was particularly conspicuous when comparing different chemical preparation methods. We used several standard etching-phosphatizing patterns, as well as treatment by means of rust converters. Results of these investigations have shown that an overwhelming majority of standard chemical methods of surface preparation are inapplicable to ZFES because the wetting of the surface in this case is poor. The ZFES coat applied to a steel surface treated with rust converters containing o-phosphoric acid had low adhesion. Good results were obtained owing to phosphatizing, viz., successive treatment with solutions of o-phosphoric acid of different concentrations under specially chosen conditions promoting the formation on the steel surface of a thin amorphous reactive iron phosphate film with respect to the ZFES compound (Table 3).

Table 3. Influence of surface preparation method on ZFES coat adhesion

The investigations under review permit the ZFES compound to be recommended for use either as paintwork for cold galvanizing or as a zinc-containing protecting primer, primarily according to the degree to which the surface is cleaned prior to applying the compound. Cold galvanizing calls for shot blasting to a degree of no less than S a 2 1/2 (Table 3). Here the corrosion protection mechanism is identical to hot galvanizing and is superior to it in protection efficiency.

The metalwork operated at metallurgical plants is prone to considerable corrosive damage due to presence of aggressive gases and vapours. Protective coats based on common varnish-and- paint materials devoid of protective properties turn out to be inefficient in such medias, primarily because of underfilm corrosion occurence.

We have investigations carried out aimed to develop effective protection coating systems for the load-carrying metal structures in the building of the cold rolling mill shop under construction at the Magnitogorsk Integrated Iron and Steel Works, including coats for the frame of the etching bay. The atmosphere of the etching bay belongs to category of aggressive, in some cases hydrochloric acid may condense on the metalwork.

A study has been made of systems based on the ZFES compound, used either as a primer or as an independent coat . For top coats we selected perchloride-vinyl and epoxy varnishes as well as paints possessing high resistance in the above media. Before application of the coat, the specimens were shotblasted and degreased. ZFES compound was applied by pneumatic spraying. The zinc content of a dry coat amounted to at least 90% (mass).

To decide whether ZFES-based systems can be used to protect the metalwork of the etching bay, additional studies have been made of the protective properties of the coats under conditions simulating the production environment of the etching bay; also, we have studied the probability of solutions and hydrochloric acid vapor attacking the surface of the coat.

The testing technique involved dipping specimens into a solution of hydrochloric acid to half their height. The bottom of the specimens was subjected to static exposure to acid solution, while the top was exposed to concentrated acid vapor.

Table 4. Physicochemical properties of the coating systems

*) A perchloride-vinyl paint
**) An epoxy paint

Table 4. Coating systems tested for use in the etching bay