Zinc-Magnesium and Zinc-Aluminium- Magnesium Coatings Produced by Magnetron Sputtering and Melt Dipping PDF Download

Author: Caizhen Yao
Publisher:
ISBN:
Category : Corrosion and anti-corrosives
Languages : en
Pages : 306

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Book Description
Zn coating is the most economic and widely used sacrificial coating to protect steels against corrosion. Since the corrosion resistance of pure Zn coating is not satisfactory under alkaline or high humidity environments, over the past few decades, demands for better corrosion resistance and mechanical properties arising from various industries and sectors have driven the research and study on alternative coatings of pure zinc coating. The most successful substitutions are Zn-Al alloy coatings, e.g. zincalume (55% Al-44% Zn-1% Si) and galfan (5% Al) coatings. These coatings combine the sacrificial protection of zinc and a long lasting physical barrier of alumina together, thus corrode 5-10 times slower than pure Zn coating. Recently, Zn-Mg and Zn-Al-Mg coatings have been developed from the traditional Zn and Zn-Al coatings by the addition of small amount of Mg. A great attention has been attracted due to their excellent properties. Evidence suggests that up to a 10-fold drop in weight loss has been found in Zn-Mg coating in comparison with pure Zn coating. The performance of Zn-Al-Mg coating in salt spray test is better than that of Zn coating by 10-20 times and Zn- Al coating by 2-5 times. Furthermore, Zn-Al-Mg coating is found to have self-healing capability. Various coating methods, including hot dipping and physical vapour deposition (PVD), have been employed in past studies, and each method results in its unique microstructure and properties in the coating. Those coatings can be used in perforated plates in civil construction, automobile bodies and parts, green house structures in agriculture and switch cabinets in electric power and telecommunication applications. The improvements of corrosion resistance properties from addition of Mg in Zn-(Al)-Mg coatings have been evidenced by researchers. However, detailed information on the corrosion mechanism of Zn-(Al)-Mg coatings is still lack in open literatures, and a number of unclear factors need to be investigated. For example, the effect of Mg content on the microstructure of Zn-(Al)-Mg coating; how does the microstructure interrupt the corrosion process of Zn-(Al)-Mg coating; is there an economic way for the mass production of Zn-(Al)- Mg coating to substitute the traditional Zn coating, and the environment that Zn-(Al)-Mg coating can be used. What's more, as a novel technology, electrochemical method is seldom used in the study on corrosion properties of Zn-(Al)-Mg coating. This research aims to study the processing methods, microstructure and properties of Zn- (Al)-Mg alloys and coatings with varying Mg contents, to investigate the mechanisms of microstructure formation and corrosion behaviour. The ultimate aim is to apply this new type of coating into industrial practice. The processing methods adopted in this research are hot dipping, electroplating and magnetron sputtering. Alloys and coatings are characterized and tested by optical microscopy (OM), environmental scanning electron microscopy (ESEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), microhardness test and electrochemical tests e.g. Open circuit potential (OCP)-time curve, potentio-dynamic polarization curve, electrochemical impedance spectroscopy (EIS) have been used to investigate the morphologies, chemical compositions, mechanical and corrosion properties of coatings. Salt water immersion test (SWI) and neutral salt spray test (NSS) was performed to further investigate the corrosion properties and mechanisms of coating specimens. The main findings from this study include: (1) The microhardness of Zn-(Al)-Mg alloys increased with increasing Mg content, probably due to the grain refinement strengthening effect and the formation of more intermetallic phase at grain boundary areas. Potentio-dynamic polarization curves indicated that the Ecorr of Zn-3 wt.% Mg alloy is more positive than that of Zn, with a low icorr of ~34% of Zn, probably due to its nanostructure. The nano-structure of Zn + Mg2Zn11 eutectic in Zn-3 wt.% Mg alloy may contribute to a general precipitation of Mg-modified simonkolleite and retard localized corrosion, contributing to the excellent corrosion resistance of Zn-3 wt.% Mg alloy. Zn-5 wt.% Al-2 wt.% Mg alloy contains a large amount of Mg2Zn11, and it has the highest impedance 5.11×103 ohm according to EIS results and the lowest icorr 1.03×10-3 A/cm2 according to the polarization curves among tested Zn-Al-Mg alloys. We assume that the improved corrosion property may relate to the formation of Mg2Zn11 intermetallic. (2) The Polarization curves showed that corrosion resistance of Zn coating was enhanced significantly by the magnetron sputtered Zn-Mg layer. Salt water immersion test in 3.5 wt.% NaCl solution also showed corrosion property improvement. The corrosion products on magnetron sputtered Zn-Mg coating mainly contain Mg modified simonkolleite and magnesium hydroxyl carbonate. The much improved corrosion resistance of Zn-Mg coating can be attributed to the formation of the uniform layer of Mg modified Zn5(OH)8Cl2H2O compound. During the corrosion process, less noble Mg reacts preferentially and a layer of magnesium hydroxyl carbonate forms on the coating surface, which is electrochemically inert. The magnesium hydroxyl carbonate has the effect to neutralize the OH- associated with Zn5(OH)8Cl2H2O formation, resulting in the precipitation of Zn5(OH)8Cl2H2O. This uniform Zn5(OH)8Cl2H2O layer formed on coating surface further reduces the corrosion rate of Zn-Mg coating. (3) For hot dipped Zn-Mg coating, with the increasing of Mg content, a significant improvement in the microhardness was observed as a result of grain refinement strengthening effect and the formation of intermetallic. Based on the results of XRD and EDS analyses, it was concluded that laminar eutectic MgZn2 and Zn formed at Zn grain boundary areas during the solidification of the coating. Electrochemical test indicated that the current density of Zn-3 wt.% Mg coating was appreciably lower than that of Zn coating, suggesting that Zn-3 wt.% Mg coating possessed improved corrosion resistance. This result was further proved in salt water immersion test. The formation of flocculent type of simonkolleite may be a reason for its improved corrosion resistance. (4) For hot dipped Zn-Al-Mg coating, optical microscope images showed that with the increasing Mg content, Zn grain size decreased and eutectic areas at Zn grain boundaries increased. Zn-5 wt.% Al-1.5 wt.% Mg coating has two continuous layers. Mg is prone to exist in the surface layer while Al is prone to exist in the inner layer. The inner layer is composed of Al5Fe2Zn0.4 intermetallic and the outer layer is composed of Zn grains surrounded by Zn and Mg2Zn11 eutectic. This is a well combination of Zn, Al and Mg structure: the inner intermetallic layer containing Al increased the microhardness and adhesive properties of the coating and the outer layer containing Mg contributed to the corrosion resistance of the coating. Zn-5 wt.% Al-1.5 wt.% Mg coating showd the best corrosion resistance among tested Zn-Al-Mg coatings. The outstanding corrosion resistance property of Zn-5 wt.% Al-1.5 wt.% Mg coating is due to the formation of flocculent type of simonkolleite. The structure of simonkolleite prolongs the micro-path and impedes the movement of O2 and H2O, ultimately retards the overall corrosion process of Zn-5 wt.% Al-1.5 wt.% Mg coating. (5) For Zn-Al-Mg-Cu coating, three different compositions (in wt.%) of dipping bath were prepared: Zn-0.1Cu (G), Zn-5Al-0.1Cu (ZA) and Zn-5Al-1Mg-0.1Cu (ZAM). Results showed that ZAM coating consists of five different phases: hcp Zn phase, base centered Al5Fe2Zn0.4 phase, laves phase MgZn2, cubic lattice Mg2Zn11 and Zn-Fe intermetallic compound. The microhardness of ZAM coating was improved to 178 HV comparing with 43 HV of G coating and 89 HV of ZA coating. The improved microhardness of ZAM coating is due to the strengthening effect of grain boundary at which intermetallic compounds of Al5Fe2Zn0.4, MgZn2 and Mg2Zn11 precipitated. ZAM coating has the best corrosion resistance among three types of coatings as evidenced by electrochemical test and salt spry test. The protective nature of ZAM coating may be attributed to the initial corrosion of Mg-rich phases. The corrosion products of Zn, Al and Mg agglomerate on the cathodic area, which act as inhibitors, blocking the corrosion paths (the micro paths for the diffusion of O2 and H2O) along the grain boundaries of Zn crystals, and increasing the impedance of coating surface, Thus, the overall corrosion process of ZAM coating is retarded. Future works for this research are also suggested in the end of thesis.