INFLUENCE OF COATINGS CHEMICAL COMPOSITION ON CORROSION RESISTANCE OF GALVANIZED STEEL

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1 INFLUENCE OF COATINGS CHEMICAL COMPOSITION ON CORROSION RESISTANCE OF GALVANIZED STEEL Juraj GRABAN, Mária KOLLÁROVÁ, Peter ZIMOVČÁK, Ivor KUČERA, Viera KOHÚTEKOVÁ, František HOLLÝ U. S. Steel Kosice, s.r.o., Kosice, Slovak Republic, EU Abstract Long-term corrosion protection of galvanized material is ensured predominantly by the coating thickness, but a significant improvement in corrosion properties of the coated steel material can be achieved by adjusting the chemical composition of the zinc layer. Properly chosen alloys reduce zinc electrochemical activity and thus inhibit the formation of corrosion products, which leads to increased corrosion resistance of coated material. An important group of such hot-dip galvanized coatings consists of coatings enriched by magnesium and aluminum. This paper describes the effect of magnesium and aluminum on the corrosion resistance of steel sheets in various thicknesses, i.e. different coating weights compared to conventional zinc coatings. Corrosion test of coatings was carried out in neutral salt spray according to EN ISO Reviewed was the surface attack by red rust, i.e. perforation-corrosion of the coating. Keywords: corrosion resistance, magnesium and aluminum, zinc coating, red rust, perforation-corrosion 1. INTRODUCTION Currently, the zinc coatings are the most widely used method of steel corrosion protection. One of the most important indicators of the lifetime of galvanized steel sheets is their corrosion resistance in particular corrosive environment. Zinc coatings protect in two ways: Barrier action (prevent action of oxygen and moisture on the steel surface) Cathodic protection at the place of defects (scratches, peeling, cutting edges) Zinc (Zn) itself is unnoble metal with a high susceptibility to corrosion. Nevertheless, its corrosion rate (kinetics) is low in most environments. It's due to the fact that the zinc surface is rapidly covered by corrosion products, which protect against further corrosion the surface is naturally passivated. Important opportunity in improving the corrosion resistance is alloying the zinc by different metals, which reduce the electrochemical activity of zinc or its corrosion products and thus increase its corrosion resistance. A very important group of coatings with increased corrosion resistance consists of zinc coatings enriched by magnesium. In literature, the effect of magnesium on the properties of zinc coatings has been of great attention for several years. The first experiments with improving the corrosion resistance of zinc coatings enriched with magnesium can be dated back to 60ies of last century [1]. The first commercial coatings with significantly improved corrosion resistance, containing, in addition to zinc, a significant amount of magnesium and aluminum were introduced in 1998 in Japan. One of such coatings is e.g. SUPER DYMA coating (Zn-11%- 3%-0,2%Si) made by Nippon Steel, and ZAM coating (Zn-6%-3%), made by Nisshin Steel Company. These coatings with excellent corrosion resistance were intended for the construction industry. Their use in the automotive industry was cumbered by the fact that the addition of in the Zn coatings on one

2 hand increased their corrosion resistance, but at the same time a high content of in the coatings led to difficulties in spot welding. While these coatings have been extensively used in Asia, European manufacturers continued in development of Zn-- coatings that would both significantly increase the resistance of steel to corrosion, and also would be suitable for use in the automotive industry. In 2007, the efforts of European producers led to successful production of Zn-- coatings containing, beside zinc, also 1-2% and 1-2%, by Corus. It was found that the coating allows significant improvement of corrosion resistance, excellent paint adhesion; is suitable for deep drawing, improves weldability and wear resistance. Compared to traditional zinc, i.e. GI (galvanized) coatings, this coating does not stick to the forming tools, leading to galling, and compared to GA (Galvannealed) coatings, this coating does not suffer from powdering and flaking during forming. Additional advantages of zinc coating enriched by and include better surface appearance (suppression of sags, wrinkles) and improved yield (conversion rate) (at a given substrate thickness it covers by 4% more square meters of material per tonne of steel, with half coating thickness) [2]. Currently, similar zinc coatings are also offered by other steel companies, such as Thyssen Krupp, Voestalpine, ArcelorMittal and others [1]. In 2010 the development of alloyed zinc coating began in U. S. Steel Košice. In early 2011, this type of zinc coating was patented and introduced to market under the name ZINKOMAG. The coating is produced in a zinc melt containing 0.8 to 1.0% and 0.8 to 1.0%. 2. EXPERIMENTAL MATERIALS AND METHODS The aim of this report was to assess the corrosion resistance of Zn-- coatings in comparison with conventional zinc coatings at different coating weights in corrosive environment of neutral salt spray according to STN EN ISO Reviewed was the surface attack by red rust, i.e. perforation-corrosion of Zn coating. Tested were hot-dip galvanized sheets with two types of zinc coatings. The first type was the conventional GI (galvanized) coating, and second type was zinc coating enriched by and. Specific quantities of and listed in the report relate to the content of these elements in the coatings. The surface of tested samples was in a pure state, i.e. did not have additional corrosion protection (not chemically passivated). Samples were selected so that they would represent the coating weight groups of g/m 2 and g/m 2 per one side. Content of elements (, ) was measured in the zinc coating (in wt.%) by AAS method (atomic absorption spectrometry). Samples were subject to corrosion test in the atmosphere of neutral salt spray according to STN ISO 9227 [3]. The evaluation was performed according to standard STN EN ISO [4]. Monitored was the time till first corrosion of Zn coating (red rust formation) depending on the content of alloys ( and ) in the coating in two different weights The corrosion test was carried out with the equipment, Cycling and Combining Chamber. 3. RESULTS AND DISCUSSION Due to the presence of alloys ( and ), the structure is changing in the coating, for example, eutectics (Zn--) or structural components with alternating phase precipitation (Fig. 1) are formed. For these coatings, the process of oxidation (rust), is slowed by the presence of 3+ ions and 2+ ions, which form protective stabilizing layer (natural passivation), less conductive corrosion products on the surface, or act as additional electrochemical protection. The chemical composition of the coating has a significant influence on the corrosion behavior in different environments. Furthermore, changes in temperature and aggressiveness of corrosive environment (the presence of salts and other products) can have a major impact on corrosion loss or coating perforation-corrosion.

3 a) GI coating surface c) Zn coating - surface b) GI coating - cross-section d) Zn coating - cross-section Fig. 1 Composition of traditional GI coating and the coating alloyed by and Selection of samples with particular specific weight of zinc coating and alloy content for corrosion test is shown in Table 1. Table 1 Selection of samples for corrosion test Each group contains samples with approximately the same Zn coating weight and different representation of alloys (, ). content in Zn coatings ranges from 0.1 to 1.11% and content ranges from 0.4 to 1.27%. For comparison, each group also includes a sample with conventionally produced GI coating

4 Time to the start of red rust (hours) Time to the start of red rust (hours) Time to the start of red rust (hours) , Brno, Czech Republic, EU containing about 0.3%. l samples were tested simultaneously and continuously evaluated under the same conditions Zn = g/m2 Zn = g/m2 300 Zn = g/m2 Zn = g/m ,2 0,4 0,6 0,8 1 1,2 1,4 (%) 0 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 (%) Fig.2 Impact of content (%) on coating corrosion Fig.3 Impact of content (%) on coating corrosion Figs. 2 and 3 show the impact of alloy content ( and ) on the corrosion resistance of zinc coating. It is obvious that with increasing content of and, the lifetime of the coating in corrosive environment of neutral salt spray significantly increases. At the same time, as expected, further increase of corrosion resistance is achieved by using higher coating weight. 400 Zn = g/m2 300 Zn = g/m Conventional GI coat 0-0,1 0,4-0,45 0,5-0,55 0,79 0,81 1,08-1,11 0,3-0,31 0,4-0,6 0,6-0,7 0,8 0,93-1,04 0,93-1,02 1,21-1,27 % % Fig.4 Comparison of corrosion resistance of two coating weight groups wih different alloy content ( and ) Fig. 4 shows the time till perforation-corrosion of Zn coating in corrosive environment of neutral salt spray. Compared was the coating alloyed by different content of and in two weight groups. GI type coating was used as a reference sample.

5 Fig. 4 shows that the same corrosion resistance typical for GI coating with higher coating weight can be achieved by using Zn coating with lower weight with the content of 0.1% and % in the coating. With increasing content of and elements, the coating corrosion resistance is several times multiplied. It was found that Zn-- coating exhibits, depending on the alloy content, on average, 1.5 to 8 times better corrosion properties than GI-type coating. loys in zinc coating [%] Coating exposition time in corrosion environment of neutral salt spray (hours) Test start Conventional GI coating Fig.5 Kinetics of corrosion attack to zinc surface over the time. Compared are coatings type GI and alloyed coatings with various and contents. Specific weight of coatings is g/m 2, corrosion environment neutral salt spray

6 Fig. 5 shows the growth kinetics of white and red rust on coatings with specific weight of (45-55) g/m 2. The zinc coating alloyed by 1.1% and 1.3% begins to corrode down to steel substrate after about 500 hours of test, while conventional GI coating was at the same time corroded on full-area. To explain the different behavior of the Zn coatings compared with the GI coatings in corrosive environments, the structure of corrosion products forming on the surface of these coatings was studied. Cross-section analysis showed that the corrosion products on Zn coating are extremely thin and dense in comparison to corrosion product formed on traditional GI coating. So infusion of water and Cl -, which are the causes of corrosion, is inhibited [5]. 4. CONCLUSION The most important task of hot-dip galvanized sheets is to protect the steel substrate from the rust attack, so to extend the life of zinc coating. The article describes the impact of and content on corrosion resistance in severe corrosive environment of neutral salt spray. Significant improvement of corrosion properties can be achieved by appropriate combination of Zn coating weight and content of alloys, comparing to conventional GI-type coatings. Samples for the test were used from two weight groups of Zn coating (45-55 g/m 2 and g/m 2 per one side). Each group represented a variety of alloy contents ( 0.1 to 1.11% and 0.3 to 1.27%). Test results were compared with zinc coating of GI type, which contained 0.3%. Tests were conducted in severe corrosive environment of neutral salt spray according to STN EN ISO 9227 standard. Evaluated was the perforation-corrosion of zinc coating over the time. It is obvious that Zn coating, depending on the content of alloys, exhibits on average 1.5 to 8 times better corrosion properties than traditional GI coating. ready low content (approx. 0.1%) in the zinc coating improves its lifetime in the selected corrosive environment. Zn coating of U. S. Steel Košice s.r.o. produced under the name ZINKOMAG increases the resistance of the coating against perforation-corrosion compared to traditional GI coating. Further significant increase in product lifetime can be expected after its subsequent surface treatment, e.g. by passivation, or by organic coatings. LITERATURE [1] VLOT, M., BLEEKER, R., E. van PERLSTEIN, Galvanized Steel Sheet Forum Functional Properties, Dusseldorf, Germany, 2006 [2] VLOT, M., ZUIJDERWIJK, M., TOOSE, M., ELLIOTT, L., BLEEKER, R.,MAALMAN, T.: In: Proceedings of Galvatech 2007, pp [3] STN EN ISO 9227, Corrosion tests in artificial atmospheres, salt spray tests [4] STN EN ISO 10289, Evaluation of test samples and products subjected to corrosion tests [5] SHINDO, H., OKADA, K., ASAI, K., NISHIMURA, K., NISHIMURA, N.: Nippon Steel technical report No.79 January 1999