Zn Al Coatings Research Articles

Material Characteristics and Corrosion Performance of Heat-Treated Al-Zn Coatings Electrodeposited on AZ91D Magnesium Alloy from an Ionic Liquid

This study examined co-deposition of Al and Zn on active AZ91D magnesium alloy using ionic liquid electrodeposition. A room temperature Lewis acidic ionic liquid made of aluminum chloride (AlCl3) and 1-ethyl-3-methylimidazolium chloride (EMIC) with a mole ratio of 60:40 was used as the bath for pure Al electrodeposition, while the addition of 1 wt% ZnCl2 to this electrolyte was used for co-deposition of Al-Zn on AZ91D magnesium alloy. The material characteristics and corrosion performance of the as-deposited films formed under various deposition charges and those annealed at 200 and 300°C were investigated. The experimental results showed that both pure Al and Al-Zn coatings provided adequate passivity for the AZ91D magnesium alloy. The re-distribution of the chemical composition after heat-treatment, including the formation of intermetallic compounds, gave rise to an increased microhardness and improved passivation behavior of the Al-Zn coated AZ91D magnesium alloy.

The effect of pH on the selective dissolution of Zn and Al from Zn–Al coatings on steel

Selective dissolution of Al and Zn from 5wt.% Al–Zn (Galfan) and 55wt.% Al–Zn (Galvalume) galvanized steel coatings was investigated by comparing Zn and Al dissolution rates in 30mM NaCl at pH from 2 to 12 using in situ kinetic analysis (atomic emission spectroelectrochemistry, AESEC) and in a 5day immersion test. The selective dissolution of Zn occurred at low pH and selective dissolution of Al at high pH. Results from AESEC and from the immersion test were compared and interpreted in terms of the inhibiting and passivating effect of corrosion product films.

Improvement of cement-based mortars by application of photocatalytic active Ti–Zn–Al nanocomposites

Photocatalytic active TiO2 has been extensively applied in construction material science due to its ability to remove pollutants from material surfaces. New inorganic–inorganic nanocomposite photocatalyst based on layered double hydroxides (LDHs) associated to TiO2 were introduced in order to increase the compatibility of photocatalyst with cement-based mortars. Different materials were prepared and characterized: cement paste with Ti–Zn–Al powder, mortar with cement paste finishing layer containing Ti–Zn–Al LDH and mortar with Ti–Zn–Al coatings. Degradation of methylene blue under UV light was selected as photocatalytic test reaction. The correlation between surface properties and photocatalytic activity was analyzed. The synergetic effect between TiO2 and Zn–Al-LDH contributes to pronounced photocatalytic performances, improving at the same time mortar performances as an important indicator of the compatibility of photocatalyst with mortar. The photocatalyst introduction procedure influences the active sites surface concentration, giving preference to surface coating method. However, introduction of photocatalyst into the bulk, having lower photocatalytic activity, results in an overall more stable system for prolonged application.

The anodic dissolution of zinc and zinc alloys in alkaline solution. II. Al and Zn partial dissolution from 5% Al–Zn coatings

The polarization behavior of a 5wt% Al–Zn steel coating (Galfan™) has been investigated in alkaline solution using atomic emission spectroelectrochemistry (AESEC). The instantaneous Zn and Al dissolution rates were measured as a function of time during a linear scan and potential step transients. The formation rate of insoluble oxides was determined from the difference between the convoluted total current and the sum of the elemental dissolution currents. It was found that, over a wide potential range, the zinc and aluminum partial currents behaved in a similar way to pure zinc and pure aluminum independently. However, during the period in which zinc was active, aluminum dissolution was inhibited. This is attributed to the inhibitive effect of the first and/or the second states of zinc oxide that are formed during the active potential domain. The third form of zinc oxide, observed at higher potential and responsible for the passivation of zinc dissolution, does not have a measurable effect on the Al dissolution rate.

Study on Corrosion Mechanism of Arc Sprayed Zn-Al-Mg Coatings by XRD and EIS

Cored wires and arc spraying were used to produce high Mg content Zn-Al-Mg alloy coatings on low carbon steel substrates. And the corrosion mechanism of the Zn-Al-Mg coatings was investigated comparing with Zn-Al alloy coatings by X-ray diffractometer (XRD) and Electrochemical impedance spectroscopy (EIS). The results show that, The Zn-Al-Mg coatings show higher electrochemical corrosion resistance in salt solution than Zn-Al coatings. With addition of Mg, the corrosion products can block off the pores in the Zn-Al-Mg coating, which is so-called self sealing, and thus prevent attack on the underlying steel substrate.

Effect of Mg on the Microstructure and Electrochemical Corrosion Behavior of Arced Sprayed Zn-Al Coating

Cored wires and high velocity arc spraying technique (HVAS) were used to produce high Mg content Zn-Al-Mg alloy coatings on low carbon steel substrates. The microstructures, mechanical properties and electrochemical corrosion behaviors of the Zn-Al-Mg coatings were investigated comparing with Zn and Zn-Al alloy coatings. And the electrochemical corrosion mechanisms of the coatings were discussed. The coatings show a typical aspect of layered thermal sprayed material structure. Chemical analysis of the coating indicated the composition to be Zn-14.9Al-5.9Mg-3.0O (wt.%). The main phases in the coatings are Zn, Mg2Zn11, Al12Mg17 and MgAl2O4, together with a little Al2O3 and ZnO. The corrosion potential of Zn-Al and Zn-Al-Mg coatings decreased a little and then increased towards the noble potential. With addition of Mg, the corrosion products accumulated to form stable passive film can block off the pores in the Zn-Al-Mg coating, and thus may prevent attack on the underlying steel substrate. The Zn-Al-Mg coatings show higher electrochemical corrosion resistance in salt solution than Zn-Al coatings.

Study on H<sub>2</sub>S Corrosion Resistance of Arc-Sprayed Pseuado Zn-Al Alloy Coatings

Pseuado Zn-Al alloy coatings were made by using of dissimilar metal wire arc spraying technology on the surface of the steel Q235 and the H2S corrosion experiment was carried out on the coatings. The corrosion rate was calculated with weight-loss method. The configurations of Zn-Al coatings before and later suffered corrosion were observed with scanning electron microscope, and the coating surfaces were analyzed with energy spectrum. The corrosion potential of the coatings was tested by using the electrochemistry testing system. The corrosion results showed that Zn-Al coatings can protect the covered steel surface and the steel substrate exposing in the corrosion environment. Being processed with pore sealer, the coatings have shown more excellent corrosion resistance.

Effect of arc spraying power on the microstructure and mechanical properties of Zn–Al coating deposited onto carbon fiber reinforced epoxy composites

This paper investigates the effect of arc spraying power on the microstructure and mechanical properties of Zn–Al coatings deposited on carbon fiber reinforced epoxy composites (CFRE composites). The bond strength between the Zn–Al coatings and the substrates was tested on a RGD-5 tensile testing machine. The microstructures and phase composition of the as-sprayed coatings were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results showed that both the melting extent of Zn–Al particles and the bond strength of the coatings were evidently improved by increasing the spraying power. Moreover, the content of crystalline Zn–Al coatings was slightly changed. Observation of fracture surfaces showed that the Zn–Al coatings could bond well with the carbon fiber bundles using 40 kW spraying power.

The effect of Al and Cr additions on pack cementation zinc coatings

Zinc is widely used as a protective coating material due to its corrosion resistant properties. The structure and oxidation resistance of Al and Cr mixed zinc coatings, deposited by pack cementation process, is thoroughly examined in this work. The morphology and chemical composition of the as-deposited and oxidized samples was accomplished by electron microscopy while the phase identification was performed by XRD diffraction analysis. The experimental results showed that the addition of aluminum or chromium in the pack mixture forms only Al and Cr rich phases on the surface of the specimens without affecting significantly the phase composition of the rest zinc coatings. In the case of Zn–Al coatings, the overlying layer contains high concentrations of Al together with lower amounts of zinc and iron and in Zn–Cr coatings this layer contains Cr, Fe and Zn atoms and has much smaller thickness. The presence of these additional layers promotes significantly the oxidation resistance of the zinc pack coatings and they preserve most of their initial thickness and chemical content when exposed to an aggressive environment while their oxidation mass gain was measured at low levels during the oxidation tests.

Electrodeposition of Al–Zn on Magnesium Alloy from ZnCl[sub 2]-Containing Ionic Liquids

Al–Zn codeposition on an AZ91D magnesium alloy from a Lewis acidic aluminum chloride-1-ethyl-3-methykimidazolium chloride (-EMIC) ionic liquid containing various contents of has been successfully established. The experimental results showed that the amount of Zn in the deposit increased with increasing concentration of in the electrolyte but decreased with increasing magnitude of cathodic potential. The presence of Zn in the deposit could cause a decrease in surface roughness as compared with that of plain Al deposit. The results of polarization curve measurement showed that Al and Al–Zn coatings could improve the passivation behavior of the AZ91D magnesium alloy in 3.5 wt % NaCl solutions.

Corrosion Performance of Aluminum-containing Zinc Coatings

Corrosion performance of zinc coatings containing aluminum in excess of 5% was assessed in modified salt-spray tests. X-ray diffraction studies indicated that the corrosion products consisted mainly of simonkolleite Zn5(OH)8Cl2·H2O, hydrozincite Zn5(OH)6(CO3)2, and zinc aluminum carbonate hydroxide hydrate Zn6Al2(OH)16CO3·4(H2O). The content of Zn6Al2(OH)16CO3·4(H2O) in the corrosion product increased as the aluminum content in the coatings increased. SEM-EDX analyses revealed that the microstructural features formed by the primary aluminum-rich α-phase frequently corroded first and at a faster pace than the zinc-rich β-phase in these coatings. The volume fraction and morphology of the zinc-rich β-phase existing in the coatings as degenerated eutectic are the two main factors which determine the corrosion resistance of Zn–Al coatings under development. The corrosion resistance of coatings peaked at about 12% Al in this study.

A new method for thermal spraying of Zn–Al coatings

This paper presents the development of the thermal spraying system built on the principles of the high velocity air flame (HVAF) process. HVAF sprayed coatings showed considerably higher bond strength than coatings obtained by the conventional methods, indicating the advantage of this method in areas where the adhesion strength is critically important. The highly dense structure of the coating obtained with HVAF eliminates a need for a top paint coat, which is typically applied on metal sprayed coatings to extend service life. The thermal sprayed coatings were characterized by the standard techniques, such as light microscopy, scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, salt spray and bond strength tests. The results show that thermal sprayed coatings have a dense structure, low presence of oxides and high resistance to corrosion. High spray rate and good coating quality make the HVAF thermal spray method a viable alternative to the conventional thermal spraying technologies, such as Wire Flame and Twin-Wire Arc.

Microstructures and Key Properties of Mechanically Deposited Zn–Al Coatings

Microstructures and Key Properties of Mechanically Deposited Zn–Al Coatings

Evaluating Zn, Al and Al–Zn coatings on carbon steel in a special atmosphere

This paper presents a comparative evaluation of Al, Zn and Al–Zn coatings on carbon steel, exposed to a coastal-marine atmosphere. It is a very aggressive atmosphere with high wind velocities (corrosion–erosion rate ≈ 1.4 mm/year (55.12 mpy) for ASTM 1029 steel). Two flame spraying zinc coatings with pore sealers were also evaluated. ISO and ASTM standards were used for the evaluation. After a 2-year exposure the best performance was achieved by the flame spraying Zn/15Al alloy (85% Zn–15%Al) with some damage of the coatings. But the one with a wash primer pore sealer did not show signs of damage.

Organic coatings degradation: Comparison between natural and artificial weathering

The protective properties of organic coated galvanised steel for outdoor applications are generally evaluated by means of accelerated laboratory tests, including electrochemical techniques. An open question is how these accelerated tests can be correlated with natural exposure in different environments. It is well known that many papers did not found any significant correlation between the natural exposure and laboratory tests. In this paper we propose a different approach. The starting point is the consideration to define the natural weathering by monitoring a few different environmental parameters affecting the organic coatings properties (energy from UV radiation, wet time, temperature, etc.), and not just considering the time of exposure. We studied two different galvanised materials: Zn and Zn–Al coatings, three different primers and one top coat. The samples were exposed for 12 months in three different natural environments: Trento (Italy), alpine region, Marseille (France) by the Mediterranean sea and Daytona (USA), a humid tropical environment. At the same time all the meteorological parameters were collected. The coatings weathering was evaluated in the laboratory after some different accelerated tests (salt spray chamber, UV exposure, cyclic test) and after natural exposure. A correlation between the “natural stresses” and the “artificial stresses” was investigated.

Effect of chromate on the electrochemical behavior of sintered Zn–Al coating in seawater

To improve the anticorrosion resistance of chromium-free Zn–Al sintered coating, characteristics of chromate on zinc–aluminum–chromium were studied by electrochemical methods. Depth profiles of elements were obtained by glow discharge spectrum (GDS) and X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical states of main elements in Zn–Al coatings. Corrosion behaviors of steel samples coated with chromium-free Zn–Al and zinc–aluminum–chromium exposed to seawater were investigated by using electrochemical impedance spectroscopy (EIS) measurement. Both in open circuit potential and EIS monitoring self-healing phenomena of chromate were obviously observed. The self-healing function endows zinc–aluminum–chromium coating with better anticorrosion performance. Electrochemical analysis show that self-healing action of coatings is missing when replace chromate with molybdate and silane.

Corrosion behavior of chromium-free dacromet coating in seawater

In order to overcome the environmental objection of chromium(VI) in dacromet, a novel chromium-free dacromet was developed. Electrochemical impedance spectroscopy (EIS) and d.c. potential measurements were employed to study the anticorrosion behavior of the sintered ceramic zinc–aluminum coatings in still seawater. Results of X-ray diffraction (XRD) indicate that the main corrosion products are composed of Al 2O 3, ZnCl 2, Zn(OH) 2 and Fe 2O 3. The scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDX) analysis were carried out to study the morphological appearance and the local chemical composition of run-out coatings. The results show that the evolution of open circuit potential can be divided into four stages during the lifetime of CFD/steel. The EIS behaviors of Zn–Al coatings immersion in seawater at the first day are similar to the ceramic CrN coating prepared by PVD, while the impedance data of the next days are parallel with multilayer protective coatings. The galvanic protection effect of the coating keeps active only in the first 3 days and then the physical shielding function play a dominant role.

軟Ｘ線放射光を用いた溶融Ｚｎ‐Ａｌ系めっき上腐食生成物の構造解析

Structure of the native oxide film and the thin corrosion product films, which formed in the early stage on hot-dip Zn-0.2%Al, Zn-5%Al and Zn-55%Al coatings in humid atmosphere under the presence of NaCl particles, were investigated by means of photoemission spectroscopy (PES) using synchrotron radiation (150 eV) and Al-K{alpha} radiation (1487 eV). In 150 eV, the detection depth of Al{sub 2p}, Zn{sub 3d}, Cl{sub 3p} and O{sub 2p} electron is below approximately 1 nm, and is a third or less of that in 1487 eV. As a result, the dominant surface components of the native oxide films were aluminum oxide independent of Al content in Zn coatings. It was found that the dominant component in 1nm depth of thin corrosion product films on Zn-Al coatings was different from those in 4 nm. Thin corrosion product films were composed mainly of zinc oxide, zinc chloride, aluminum oxide, aluminum chloride, and the ratio of those components was changed by the Al content in Zn coatings and the depth from surface. (author)

The Influence of Aluminium Addition on the Microstructure of Zinc Pack Coatings

This work aims to investigate the feasibility of Zn-Al deposition on low alloy steels at temperatures from 400 up to 440oC by pack cementation process aiming to increase their corrosion resistance. A series of experiments were undertaken to investigate the effects of pack powder composition and the deposition temperature of the process. It was observed that the parameters of zinc content and temperature affect only the coating deposition speed, but not the phase composition of the as produced coating. Al forms an overlying layer that seals the zinc coating. In any case, the deposition of successive layers of Zn and Al is feasible with pack cementation. The corrosion performance of Zn-Al coatings formed with alternative methods is already studied and proved to be resistant in harsh environments. So the herein studied coatings are expected to be corrosion resistant. Furthermore as Al is much more resistive than Zn, these coatings are more effective than pure Zn ones.

An 18-Year Exposure Test of Thermal-Sprayed Zn, Al, and Zn-Al Coatings in Marine Environment

Abstract The thermal spray committee of the Japan Association of Corrosion Control (JACC) has been conducting a marine corrosion test of thermal-sprayed Zn, Al, and Zn-Al coatings since 1986. The coating was applied to twelve steel pipes by arc- and flame-spraying to varied thicknesses and subjected to various post-spray treatments. The samples were set vertically into seawater at a port located approximately 80 km south of Tokyo. The corrosion performance of these coatings has been inspected annually by recording their appearance and thickness at the sea-air, splash, and tidal zones. No significant changes were observed for the first several years of exposure. After 7 years, however, Zn coatings with and without sealing started to suffer degradation in the immersed portion. Contrary to this, most of the Al and Zn-Al coatings still exhibit superb corrosion performance. In order to place the test in a proper perspective, a special team of about 20 members was formed to conduct a survey on corrosion prevent...