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1 Hot-Dipped Al-Mg-Si.Hwp CORROSION SCIENCE AND TECHNOLOGY, Vol.9, No.6(2010), pp.233~238 Hot-dipped Al-Mg-Si Coating Steel - Its Structure, Electrochemical and Mechanical Properties - Tooru TSURU† Department of Metallurgy and Ceramics Science Graduate School of Science and Engineering Tokyo Institute of Technology 2-12-S8-7, O-okayama, Meguro-ku, Tokyo 152-8552 Japan (Received October 13, 2009; Revised November 5, 2010; Accepted November 8, 2010) Hot-dipped Al-Mg-Si coatings to alternate Zn and Zn alloy coatings for steel were examined on metallographic structure, corrosion resistance, sacrificial ability, formation and growth of inter-metallic compounds, and mechanical properties. Near the eutectic composition of quasi-binary system of Al-Mg2Si, very fine eutectic structure of α-Al and Mg2Si was obtained and it showed excellent corrosion resistivity and sacrificial ability for a steel in sodium chloride solutions. Formation and growth of Al-Fe inter-metallic compounds at the interface of substrate steel and coated layer was suppressed by addition of Si. The inter-metallic compounds layer was usually brittle, however, the coating layer did not peel off as long as the thickness of the inter-metallic compounds layer was small enough. During sacrificial protection of a steel, amount of hydrogen into the steel was more than ten times smaller than that of Zn coated steel, suggesting to prevent hydrogen embrittlement. Al-Mg-Si coating is expected to apply for several kinds of high strength steels. Keywords : Hot-dipped coating, Al-Mg-Si alloy, corrosion resistance, sacrificial protection, inter-metallic compounds, high strength steel, hydrogen embrittlement 1. Introduction 2. Search for optimum alloy composition Zinc and zinc alloy coatings for steel has been widely It is well known that aluminum and Al-Si coated steel used, because of its high corrosion resistance and excellent have excellent corrosion resistivity and high temperature sacrificial ability for underlying steel. In recent years con- oxidation, however, their ability for sacrificial action is sumption of zinc, not only for galvanized steel but also not enough to protect underlying steel. Magnesium was for other usage, rapidly increased especially in China. considered as an alloying element to add sacrificial ability, Clarke Number of zinc is known as 0.004% and it is small- and from metallurgical reason will be described latter, sili- er than that of nickel and copper as 0.04%. It is also re- cone was also added. By simultaneous vacuum evapo- ported that the reserve-production ratio of zinc is about ration of magnesium and aluminum or magnesium and 22years.1) So we can not say for zinc as an abundant re- Al-Si mother alloy, evaporated coatings having wide com- source material. position range were obtained and electrochemical proper- To alternate zinc for steel coating, my research group ties, such as polarization curves and galvanic couple cur- started to use aluminum alloys and this is supported by rents were measured. the ministry of education and science for five years pro- From preliminary experiments,3) it was found that in- ject.2) The final goal of this project is that to develop a creasing the magnesium content lowered the corrosion po- new aluminum based hot-dipped alloy coating which has tential and increased sacrificial ability. Fig. 1 shows effect almost same level to zinc and zinc alloy coatings of corro- of silicone content on the polarization curves of Al- sion resistivity and sacrificial ability, to apply the coating 10%Mg-x%Si (in mass%) alloys in 0.5 M Na2SO4 + 0.1 for high strength steels and not to change present hot dip M NaCl solution. It is seen from the polarization curves processes drastically. This paper reports the results of the that the corrosion potential ranged in -1.1 to -1.3 V (vs project for recent two and half years for fundamental Ag/AgCl electrode, SSE) and passive currents ranged in researches. 0.1 to 1 μA/cm2. The corrosion potential was shifted more negative and passive current was increased by increasing †Corresponding author: [email protected] silicone content. The passive current increased above -0.6 V. 233 TOORU TSURU -3 10 0.3 -2 Al-26.2%Mg 10-4 Al-25.7%Mg-2.6%Si /Acm i 0.2 10-5 Al-50.1%Mg Al-10Mg-5.1Si 10-6 Al-50.1%Mg heated* 0.1 Al-10Mg-0Si Intensity, a.u. 10-7 Al Mg Al-10Mg-1.1Si 2 Pure Al Current density, Al-10Mg-2Si 10-8 0 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 020406080100 Potential, E / V vs SSE Diffraction angle, 2θ/deg Fig. 1. Anodic polarization curves of Al-10Mg-×Si coated steels Fig. 2. XRD results of as deposited and heat treated Al-Mg and Al-Mg-Si alloys. in 0.5M Na2SO4+0.1M NaCl. Fig. 3. Al-Mg-Si ternary phase diagram and typical micro-structures. At the eutectic composition of quasi-eutectic system of Al-Mg2Si, almost all the phases composed of fine eutectic structure. Pitting corrosion or localized dissolution of evaporated ture like amorphous alloys so pitting corrosion or localized coating was hardly observed after the polarization meas- corrosion hardly observed. urements. Based on the results of polarization measurements and Results of X-ray diffraction (XRD) analysis for evapo- corrosion tests, primary precipitated massive Al3Mg2 and rated coatings are shown in Fig. 2. It is interesting that Mg2Si preferentially dissolved and result localized corro- all the evaporated coatings, even high magnesium content sion, and eutectic fine structure of α-Al and Mg2Si dis- alloys, show only the peaks belonging to α-Al, and Al-Mg solved homogeneously. Phase diagram of Al-Mg-Si sys- compounds peaks appear after heat treatment. This sug- tem and typical micro-structures are shown in Fig. 3. At gests that the evaporated coating has homogeneous struc- the quasi-eutectic composition of Al-Mg2Si system, almost 234 CORROSION SCIENCE AND TECHNOLOGY Vol.9, No.6, 2010 HOT-DIPPED AL-MG-SI COATING STEEL - ITS STRUCTURE, ELECTROCHEMICAL AND MECHANICAL PROPERTIES - all the phase composed of fine eutectic structure. Anodic polarization curves of casted and hot-dip coated steel al- most similar shown in Fig. 1, however, they showed rapid current increase above -0.6 to -0.5 V indicating start of localized corrosion.4) After X-cut corrosion tests of evaporate coated steel and hot-dip coated steel in 0.5 M Na2SO4 + 0.1 M NaCl sol- ution for 20 h and 1 week, no red rust was observed on the steel surface. Galvanic couple corrosion test for bare steel and evaporate coated steel, area ratio was 1:10, in the same solution for 1 week resulted no red rust on the steel surface indicating complete sacrificial action. Stable sacrificial current was observed and the coating dissolved Fig. 4. Amount of hydrogen entered into substrate steel during almost uniformly. wet and dry corrosion cycle. Both coated steels scratched 1 mm width and 0.2 μL of 0.1 N NaCl solution was placed on the 3. Effect of Micro-structure on corrosion resis- scratch. tance and sacrificial action In a casted alloy and hot-dip coated layer, primary pre- trode potential, so that for Al-Mg-Si alloy coated steel cipitated massive Al3Mg2 and Mg2Si preferentially dis- can be expected several orders of magnitude smaller than solved and they caused localized corrosion.4) Furthermore, zinc coated steel. Using a zinc coated and Al-Mg-Si coated hot-dipping bath contains considerable amount of iron steels with a scratch in wet and dry cyclic corrosion con- which dissolved into the melt bath during dipping. Iron dition, amount of hydrogen permeation current against the precipitates in the coating layer as Al3Fe or Al-Fe alloy corrosion potential is shown in Fig. 4. The alloy composi- which acts as cathode site for corrosion reaction. Around tion of evaporated coating used is Al-12%Mg-3%Si, a lit- these precipitates, aluminum depletion leads to precipitate tle bit different from the quasi-eutectic composition and massive Al3Mg2 and Mg2Si. After the corrosion test, Al3Fe the corrosion potential is much negative than usual, even or Al-Fe alloy remained in the coating and preferential so the amount of hydrogen entered is five times smaller dissolution was observed around these precipitates. It was than that of zinc coated steel.5) This suggests that Al-Mg-Si confirmed that all the phases such that Al3Mg2, Mg2Si, coating can be used for high strength steels without risk Al3Fe and also quasi-eutectic structure, should be as fine or more safely use for hydrogen embrittlement. as possible to prevent localized corrosion of coating layer. This indicates importance for controlling the micro-struc- 4. Control of reaction layers at the substrate ture of coating layer. and coating layer As a test for sacrificial action of quasi-eutectic alloy coating, Al-8.2%Mg-4.8%Si was used for X-cut test and It is well known that Al-Fe inter-metallic compounds galvanic couple test in a solution of 0.5 M Na2SO4 + 0.1 or Al-Fe alloy layer formed and developed rapidly and M NaCl for 1 week. No red rust observed on the bare these layers are usually loose the mechanical ductility of steel surface and the galvanic couple current continued coated layer and causes flaking of the coated layer. It is between 60 to 90 μA/cm2 for the steel. Zinc has excellent also known that addition of silicone decreases the alloy ability to supply large anodic current to protect a wide layer growth by forming Al-Si-Fe layer prior to the alloy surface area of steel such like a sacrificial anode for catho- layer formation.
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