CORROSION PERFORMANCE of ELECTROGALVANIZES STEEL TREATMENT with Cr (III)-BASED PRODUCTS
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CORROSION PERFORMANCE OF ELECTROGALVANIZES STEEL TREATMENT WITH Cr (III)-BASED PRODUCTS C R Tomachuk1, A R Di Sarli2, C I Elsner2 1Instituto de Pesquisas Energéticas e Nucleares, IPEN/CNEN-SP, Brasil 2CIDEPINT (CICPBA ± CONICET), La Plata, Argentina SUMMARY: The corrosion resistance of pure zinc coatings can be improved through the application of suitable chemical passivation treatments. Hexavalent chromium compounds have widely been used to formulate conversion layers providing better anticorrosive protection as well as anchorage properties to painting systems. However, taking into account that they are produced using hazardous chemical LQJUHGLHQWV WKH GHYHORSPHQW RI DOWHUQDWLYH DQG ³JUHHQ´ WHFKQRORJLHV ZLWK HTXLYDOHQW SURWHFWLYH performance is a paramount purpose of many R&D laboratories working around the world. In this work, the corrosion behavior of zinc coatings obtained from free-cyanide alkaline baths and latterly subjected to a Cr(III) based passivation treatment, with and without a sealing treatment, was studied. The experimental work involved electrochemical impedance spectroscopy measurements on 0.5 M Na2SO4 solution, surface microstructural and morphological characterization by electronic microscopy as well as chemical analysis by EDXS. The analysis and interpretation of all the data coming from these batteries of tests allowed inferring that both the Cr(III) based conversion treatment + adequate sealer provided a good corrosion resistance and, therefore, together with an adequate painting system, they could be a less pollution and less toxic alternative to the traditional chromate coatings. Keywords: corrosion, impedance electrochemical, green technologies, conversion treatment, electrogalvanized steel. 1. INTRODUCTION The corrosion of galvanized steel is one of the major problems in industry. This material can become more resistant to corrosion if an appropriate protective conversion coating is applied on top of the zinc layer. The term, conversion coating, as used in the metal-finishing industry, refers to the conversion of a metal surface into a surface that will more easily accept applied coatings and/or provide for a more corrosion resistant surface [1]. Conversion coatings for zinc have been in use since the early 1920s and there are a number of different products on the market [2]. Being one of the most important of them, the chromatation layer plays a number of functions by acting like anodic inhibitor, forming a passive layer, lowering the zinc dissolution rate and it is also an efficient cathodic inhibitor, lowering the rate of the oxygen reduction reaction on the metal surface, avoiding the formation of blisters [3]. Responding to increasingly more rigorous environmental protection activities, recent years have shown progressive advances in order to reduce the use of environmentally-hazardous materials. On line with this purpose, the development of various kinds of chromate-free coated steel sheets to be used in food, automotive, appliances, etc. industries is being extensively explored all over the world. In this sense, the most common transitional alternative to Cr6+ is Cr3+, which is used since the mid 1970 [4-8]. The first-generation of trivalent chrome conversion processes were based on fairly stable Cr3+ complexes, which slowed their reactivity rates even at high temperatures. It produced film thicknesses of 20 to 30 nm with limited corrosion resistance. To produce thicker passivation layers, a second-generation trivalent passivation process was developed. It incorporates accelerators, modified complexes, and is operated at higher concentration and temperature to drive the reaction kinetics at a faster rate. When applied as recommended, film thicknesses of 300 to 380 nm, equivalent to those produced from yellow Cr6+ passivating solutions, were obtained. In film in this case consist of an insoluble barrier layer free of hexavalent chrome [9]. 18th International Corrosion Congress 2011 Paper 321- Page 1 The main advantage of Cr3+ plating baths is that these ions are non-toxic and, therefore, they are environmentally benign alternatives. However, their corrosion resistance is generally less than that of the Cr6+ [10-12]. Consequently, many recent studies have focused on the improvement in the corrosion resistance of the conversion layers based on Cr3+ subjecting them to a sealing treatment. Bellezze et al. [10] reported the corrosion resistance of the Cr3+ layer increased dramatically through the sealing treatment in Si based solution and was equivalent to that of the Cr6+ based conversion layer. According to Fonte et. al. [13], the Cr3+ conversion layer formed in a bath containing transition metal ions such us Co2+, Ni2+ and Fe2+ showed higher corrosion resistance than the formed in the bath without transition metal ions and it was confirmed by Tomachuk et. al. [14, 15]. Concerning the formation of the protective layer on zinc, it was demonstrated that the passive layer contains Zn(II) and Cr(III) oxides/hydroxides [17]. Nevertheless, other authors [3] stated that the chromate layer does not contain significant amount of zinc, and it is permeable to zinc dissolution, being the main film composition Cr(OH3).2H2O. The requirements of Cr(VI)-free coatings for electrogalvanized steel sheets is high because they are to a great extent used for automotive industry, audiovisual, office automation, and home appliances. In this context, and taking into account that the bibliography consulted shows that there are a lot of papers dealing with Cr(VI) based pre-treatments deposited in, and H[SRVH WR DQ H[WHQVLYH YDULHW\ RI H[SHULPHQWDO FRQGLWLRQV WKH DXWKRUV¶ LGHD was testing the behavior of new products which, eventually, could became an alternative to the traditional Cr(VI) as corrosion inhibitor. Therefore, the surface FRPSRVLWLRQ DQG PRUSKRORJ\ RI WKH ³DV-UHFHLYHG´ VDPSOHV DV ZHOO DV WKH FRUURVLRQ EHKDYLRU RI electrogalvanized steel panels subjected to Cr(III) based pre-treatments used as the only coating layer have been characterized by means of surface analysis and electrochemical techniques. The corrosion behavior was evaluated as a function of the exposure time in 0.5 M Na2SO4 solution. 2. EXPERIMENTAL 2.1 Samples preparation AISI 1010 steel sheets (7.5x10x0.1cm) were industrially electrogalvanized using a cyanide-free alkaline bath containing Zn2+ 11 g.L-1, NaOH 130 g.L-1, and commercial addition agents (room temperature, cathodic current density 2 A.dm-2). Immediately after finishing the zinc deposition step, each sample was coated with the make up described in Table 1, according to the operating conditions recommended by the respective supplier, Table 2. At the end of this step, samples were rinsed with deionized water, and then dried. The sealing treatments 1 (S1) applied on TA samples (for obtaining TAC samples), and 2 (S2) on Z66 and Z80 samples (for obtaining Z666 and Z806 samples, respectively) contained synthetic polymers and corrosion inhibitors, while the sealing treatment 3 (S3) applied on Z66 and Z80 samples (for obtaining Z665 and Z805 samples, respectively) was constituted by synthetic polymers, non-ionic surfactants and silicon dioxide. Table 1. Samples and description of Cr+6 free make up coatings Sample Description TA Zn + Cr(III) passivation treatment free of Cr+6 and complexing agents TAC TA + S1 (sealing treatment with corrosion inhibitors and a product based on Si ) Z80 Zn + Cr(III) passivation treatment free of Cr+6 and oxidizing agents Z805 Z80 + S3 (sealing treatment with a Cr+6 free liquid dispersion and Si particles) Z806 Z80 + S2 (sealing treatment free of Cr+6 and oxidant products) Z66 Zn + Cr(III) passivation treatment free of Cr+6, oxidative agents and fluorine ions Z665 Z66 + S3 (sealing treatment with a Cr+6 free liquid dispersion with mineral particles) Z666 Z66 + S2 (sealing treatment free of Cr+6 and oxidant products) Table 2. Coating Films and Operating Conditions Parameter/Sample TA TAC Z66 Z666 Z665 Z80 Z806 Z805 Cr 0.02 0.04 0.05 0.02 0.04 0.14 0.17 0.15 Make up + zinc Zn rest rest rest rest rest rest rest rest coating,( % v/v) Si ---- 1.19 ---- 0.68 0.58 ---- 0.95 0.44 Co ---- ---- ---- ---- ---- 0.02 ---- ---- 18th International Corrosion Congress 2011 Paper 321- Page 2 1.6-2.0 9.0- pH 8.0-9.0 1.7-2.2 7.0-8.5 9.0-9.5 1.6-2.1 7.0-8.5 9.5 Bath Temperature 25 25 25 25 25 60 25 25 (ºC) Immersion Time (s) 30 15 30 10 20 60 10 20 Drying 60 60 70 70 90 70 70 90 Temperature (ºC) Agitation Mechanical Activation 0.5% HNO3 solution for 10s and then rinsed in deionized water Green Film color Blue Blue Blue Blue Blue Green Green iridescent Total coating 11.8 10.7 8.4 7.8 8.2 10.4 10.3 9.1 thickness (µm) ±0.86 ±0.93 ±0.43 ±0.37 ±0.35 ±1.43 ±0.77 ±0.28 2.2 Thickness measurements Coatings thickness was measured with a Helmut Fischer DUALSCOPE MP4 according to ASTM B499-09 standard. 2.3 Quali-quantitative chemical analyses and Morphology Coatings morphology was observed by SEM using a LEO 440i microscope, while the composition was determined using energy dispersive X-ray spectroscopy (EDXS) using a Si detector and 20 KeV energy. 2.4 EIS measurements In this work, a three-electrode cell system was used for all electrochemical experiments. A Pt-Rh mesh and a saturated calomel electrode (SCE = +0,244V vs. NHE) was served as the counter and reference electrodes, respectively, while the treated electrogalvanized steel samples, with a defined area of 15.9 cm2, acted as the working electrode. EIS measurements as a function of the exposure time in quiescent and open to air 0.5 M Na2SO4 solution were conducted using a Solartron 1255 Frequency Response Analyzer coupled to a Solartron 1286 Potentiostat/Galvanostat (both controlled by the ZPlot program®), a small potential perturbation (10 mVa.c. amplitude around the OCP) and the frequency range 10-2 < f(Hz) < 105.