Zn-Al Alloy Coatings Research Articles

Effects of Al content on microstructure and corrosion behavior of Zn-Al alloy coatings

Alloying Zn with other metals is an alternative method to create advanced alloys with better corrosion properties, in this work the electrodeposition of Zn-Al binary alloy in sulphate-based acidic bath and with different Al2O12S3 content from 0.03 to 0.1M on a treated copper substrates was studied, the structure and microstructure of the coatings were analysed by X-ray diffraction (XRD) and scanning electron microscope (SEM) supported by (EDX) analysis, the microhardness was measured, the corrosion resistance was evaluated by potentiodynamic polarization (Tafel), the effect of Al ions was visible on the structure, (XRD) spectra showed zinc phase (η-phase), and Al phase (α-Al phase) with Al peak intensity increasing along with the increase of Al concentration, the microhardness also enhanced gradually to 252.33 HV, the corrosion current density decreased by almost 13 times and the corrosion resistance was drastically improved at 0.1M Al.

Effect of Al Content on the Long-Term Corrosion Behavior of Arc-Sprayed ZnAl Alloy Coatings

The corrosion of steel structures in aggressive marine environments is a vital issue that induces significant degradation of their performance and lifespan. Herein, three arc-sprayed ZnAl coatings with varied Al contents of 0 wt.%, 15 wt.%, and 50 wt.% were deposited onto a hull steel substrate. The effect of Al content on the long-term corrosion protection performance of ZnAl coatings left in a chloride-containing solution for 840 h was systematically investigated. The evolutions of open-circuit potential, polarization curves, and electrochemical impedance spectroscopy of different ZnAl coatings during the long-term immersion test were examined. The morphologies and phase constitutions of the corrosion products were characterized. The results indicated that the corrosion rate of ZnAl coatings decreased as the Al content increased, and the ZnAl50 coating exhibited the most superior long-term corrosion protection performance. Moreover, for the three ZnAl coatings with an Al contents varying from 0 to 50%, their corrosion rate increased with immersion time in the initial 360 h due to the formation of the unstable and porous corrosion product ZnO; after 360 h immersion, their corrosion rate decreased with the prolonging of immersion time. This was revealed to be related to the formation of different corrosion products. ZnO and stable Al2O3 were the main corrosion products for the pure Zn coating and ZnAl15 coating, respectively. Al2O3 and powerful layered double hydroxide Zn6Al2(OH)16CO3·4H2O were found to be the dominant corrosion products of the ZnAl50 coating, which was responsible for its remarkable long-term corrosion protection performance.

The Effect of Ammonium Ions on the Corrosion Resistance of Zinc - Aluminium and Zinc - Aluminium - Magnesium Coated Steel

Galvanizing is the process of applying a protective zinc or zinc alloy coating to steel or iron which acts as a sacrificial anode to protect the underlying steel or iron. These coatings are mainly used in the automotive and construction industry where corrosive environments will be encountered. There has been a drive to improve the performance of galvanised coatings through alloy additions, initially Al (5 – 55 wt%) and more recently through the development of Zn - Al - Mg coatings. Here, Al and Mg are added as minority additions (typically 1 - 3 wt %) to improve corrosion resistance allowing thinner coatings to be used thus saving material, weight and cost. The assessment of the corrosion performance of such coatings is often evaluated through standardised tests. An industry standard test known as Prohesion or ASTM G85 Annex A5 is carried out on metallic coatings as it is thought to better represent long-term natural exposure than a regular salt spray test. The Prohesion test is a cyclic accelerated corrosion test using an electrolyte of 0.4% Ammonia sulphate and 0.05% sodium chloride (NaCl).This investigation aims to determine the effects on the corrosion performance of zinc-aluminium alloys and zinc-aluminium-magnesium alloys of varying compositions immersed in Prohesion solution. Previous work on this topic suggests that magnesium containing alloys were particularly susceptible to accelerated corrosion in Prohesion solution. Here, several zinc based alloys were immersed in Prohesion solution as well as a control 0.45% sodium chloride solution and subjected to electrochemical testing. The electrochemical techniques used to test said substrates are linear polarisation resistance (LPR), scanning vibrating electrode technique (SVET) and time-lapse optical microscopy (TLM) all for 24 hours. The alloys that underwent testing are shown in Table 1. Table 1: Table showing the composition of substrates used.Sample number Composition 1 Zn-<0.2wt%Al 2 Zn-5wt%Al 3 Zn-55wt%Al 4 Zn-1.2wt%Al-Zn1.2wt%Mg 5 Zn-3.5wt%Al-3wt%Mg 6 Zn-6wt%Al-3wt%Mg The results from the TLM showed differences in the corrosion mechanism that is dependent on the alloy composition and electrolyte. Generally, all substrates immersed in the control 0.45% NaCl showed localised corrosion with distinct anodes developing surrounded by a ring of corrosion product and white rust is observed on the surface of the substrate. The initial site of anodic attack in the microstructure was dependent on the alloy composition. However, when immersed in the Prohesion solution significant differences were observed dependent on the alloy composition. For Zn-Al-Mg coatings rapid attack of zinc-magnesium phases was observed especially in the eutectic regions followed by aggressive attack of the entire exposed surface of the alloy within 6 hours. Additionally, no corrosion product rings formed. For Zn-Al alloy coatings, the zinc phases were preferentially attacked with the aluminium containing phases appearing to be resistant to corrosion during the duration of the tests. No white rust was seen on the surface of the substrates and instead a dark surface/corrosion product film was observed. The corrosion rate from the LPR data measured at hourly intervals over 24 hours increases when substrates are immersed in Prohesion solution compared to NaCl, with the most noticeable increase seen when there are zinc-magnesium phases present within the alloy. The largest increase in corrosion rate mm per year (mmpy) was seen in sample 5 with it increasing from 0.15 mmpy after 24 hours in the control solution to 0.55 mmpy in the Prohesion solution after 24 hours. The same is observed with the SVET results showing that the ammonium ions are having a significant damaging effect on the Zn-Al-Mg metallic coatings. The proposed corrosion mechanism for Zn-Al-Mg alloy coatings in Prohesion solution is preferential dissolution of MgZn2 in eutectic phases due to reactivity of Mg producing Mg(OH)2 which is a highly basic hydroxide. Due to the basic nature of Mg(OH)2, Ammonium (NH4 +) cations react with Mg(OH)2 producing Mg2+ and Ammonia (NH3).Mg(OH)2+2NH4 +=Mg2++2NH3+2H2OZn(OH)2 does not undergo this reaction to the same extent as it hydrolyses in water (producing H+) therefore is not basic enough to support much NH3 production. Neutral NH3 reacts with metallic Zn or Zn2+ compounds to produce highly soluble zinc (II) tetra-ammine complex cations. Therefore, the protective Zn and Mg compounds forming at the metal surface will dissolve and the surface will become activated. This is demonstrated by the lack of white rust shown from samples immersed in the Prohesion solution. Al phases are resistant to NH3 hence the better performance of Zn-Al coatings and the preferential attack of the Zn in the Zn-Al timelapse microscopy.

Peritectic Structure Evolution and Corrosion Behavior in Hot-dip Zn-Al Alloy Coatings

Peritectic Structure Evolution and Corrosion Behavior in Hot-dip Zn-Al Alloy Coatings

Microstructure and properties of arc sprayed Zn-Al alloy coatings

In this study, zinc-aluminum alloy coatings were deposited on low carbon steel substrates by arc thermal spraying with different current and spraying distances. Optical microscope (OM) was used to characterize the deposited coatings. From the experiment, it is found that the samples that use different electric currents and spraying distances result in the different microstructure of the coating surfaces and different mechanical properties. The sample that uses higher current tends to have a higher hardness than the sample with a lower electric current at the same spraying distance.

Effects of Al contents on microstructure and properties of hot-dip Zn-Al alloy coatings on hydrogen reduced hot-rolled steel without acid pickling

AbstractA new hot-dip galvanizing method was employed on hot-rolled low carbon steel. The effects of Al contents on microstructure, micro-hardness and corrosion resistance of Zn-Al alloy coatings were systematically investigated. Phase composition, microstructure and element distribution in Zn-Al alloy coatings were analyzed using X-ray diffraction (XRD) and electron probe micro analysis (EPMA), respectively. It is found that Al content (0. 6–6. 0 wt. %) in galvanizing zinc affects surface quality and adhesion between coatings and matrix in the newly developed method. In addition, with increasing Al content, micro-hardness significantly increased due to the increase in Zn-Al eutectoid phases. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) also revealed that increase in Al plays a noticeable role in improving the corrosion resistance of Zn-Al alloy coatings.

Wettability Behavior of Zinc-Based Alloys on Cast Iron

The wettability of two Zn–Al alloy coatings (called “McGill 2” and “McGill 3” with 27 and 10 wt% Al, respectively) on the compacted graphite cast iron substrate was studied using the static sessile drop method. The effects of wetting time and coating composition on the macrostructural and microstructural evolution of the system were investigated. It was found that although the contact angle was obtuse for both coating materials over the time, the interfacial reactions occurred between the drop and the substrate from the first stage of contact. The most significant result was the formation of a uniform and thick reaction layer (interlayer) with the diffusion-controlled property at the interface. The results showed that “McGill 2” coating is superior to “McGill 3” alloy as an intermediate material for the Al–Fe cast joining application. The “McGill 2” interlayer revealed a composite nature as it consisted of different layers of solid, in the form of stable AlxFeySiz intermetallic phases from the initial steps of the reactions.

Corrosion Resistance of Two Thermal Sprayed Zn-Al Alloy Coatings in Seawater at Low Temperatures

Corrosion Resistance of Two Thermal Sprayed Zn-Al Alloy Coatings in Seawater at Low Temperatures

Corrosion Behaviour of Zn-Al Pseudo-Alloy Coating on Carbon Steel in Chloride Environments

In this work, Zn-Al alloy and Zn-Al pseudo-alloy coatings are deposited on carbon steel by arc spray technology. The corrosion behaviour of the coatings has been studied by salt spray test and electrochemical experiment. The morphology of the coatings exposed to salt spray test for 400 h was analyzed using SEM and EDS. Results show that corrosion rate of Zn-Al alloy coating is faster than that of Zn-Al pseudo-alloy coating and the steel substrate is attacked. The corrosion products formed on the surface of Zn-Al alloy coating were loose, with a large number of cracks and holes, but the Zn-Al pseudo-alloy coating is converted into a stable Al-rich oxide layer, which protects the steel substrate against corrosive attacks and enhances the corrosion resistance of the Zn-Al pseudo-alloy coating. Electrochemical tests show the Zn-Al pseudo-alloy coating as a sacrificial protection coating has the capability of providing sacrificial protection for the steel substrate.

State‐of‐the‐Knowledge on Coating Systems for Hot Stamped Parts

AbstractIn the present contribution, recent developments of coatings for hot stamped steels are reviewed. The use of bare steel in the initial hot stamping technology is discussed, including the application of lubricant oils which are used as oxidation inhibitors on bare steel surfaces. The aluminized coatings are introduced, focusing on the microstructure evolution of aluminized coatings during the hot stamping process. An analysis of the cracking of the coating, caused by the formation of brittle Fe–Al intermetallic phases and their high temperature deformation, is presented. The development of a ductile aluminide coating formed during the diffusion treatment of an aluminized coating is discussed. This aluminide coating can endure both high temperature oxidation and severe plastic deformation. The recently developed galvanized and galvannealed coatings are also reviewed and the influence of the gas atmosphere during the heating cycle on the coating stability is emphasized. The solutions which have been proposed to avoid liquid Zn‐induced embrittlement are analyzed. The use of Zn–Ni alloy coating, which is characterized by a higher melting temperature, is reviewed. The behavior of sol–gel hybrid coatings on hot stamped steels is discussed. The possible use of the recently developed Al–Zn alloy coatings, dual layer Zn–Al and Zn–Al–Mg coatings is also introduced. The application of Zn–Al–Mg post‐process galvanizing is also discussed. In each case, all available information related to the weldability, paintability, and corrosion resistance of the coating systems is also reported. Finally, the advantages and technical challenges associated with each type of coating are reviewed.

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&lt;sub&gt;2&lt;/sub&gt;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.

Formation and stability of organophosphonic acid monolayers on ZnAl alloy coatings

Abstract The formation and stability of octadecylphosphonic acid (ODPA) layers have been studied on ZnAl alloy coatings. The passive film covered alloys were characterized by means of scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), time-of-flight second ion mass spectroscopy (ToF-SIMS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) before and after the adsorption of ODPA layers. The thickness and ordering of adsorbed ODPA films as well as their stability in aqueous solutions were further studied by means of static water contact angles reflecting the hydrophobicity of the surface. It was observed that self-assembled monolayers (SAMs) of ODPA form on ZnAl alloys with higher Al-content (≥ 5%) such as Zn–5%-Al and Zn–55%-Al. For small concentrations of Al (Zn–0.5%-Al) the formed Al-oxide containing passive film was not stable enough to prevent dissolution and precipitation of a phosphonate salt film. The results show that organophosphonic acids can potentially be used for temporary corrosion protection, and as molecular adhesion promotors or inhibitors on ZnAl alloy coatings with Al-concentrations (≥ 5%) covered with of Al-oxide rich passive films.

Acridine as Effective Corrosion Inhibitor for Hot Dipped Coatings on Steels in Diluted HCl Solution

The environmental and health hazards associated with chromium-containing solution have aroused wide public concern, so attempts have been made to find alternatives for metal finishing. In the present paper, inhibition effect of acridine on corrosion of hot dipped Zn and Zn-Al alloy coatings on steels in diluted hydrochloric acid was investigated using weight loss test, electrochemical measurement, and quantum chemistry analysis. The results showed that inhibition efficiency increased with inhibitor concentration, and the highest inhibition efficiency was up to 99%. And acridine adsorbed on coating surface by chemisorption. Quantum chemistry calculation results indicated that acridine possesses planar structure with a number of active centers, and showed a good ability of electron exchange with the coating surface.

Nicotinic acid as a nontoxic corrosion inhibitor for hot dipped Zn and Zn–Al alloy coatings on steels in diluted hydrochloric acid

The inhibition effect of nicotinic acid for corrosion of hot dipped Zn and Zn-Al alloy coatings in diluted hydrochloric acid was investigated using quantum chemistry analysis, weight loss test, electrochemical measurement, and scanning electronic microscope (SEM) analysis. Quantum chemistry calculation results showed that nicotinic acid possessed planar structure with a number of active centers, and the populations of the Mulliken charge, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) were found mainly focused around oxygen and nitrogen atoms, and the cyclic of the benzene as well. The results of weight loss test and electrochemical measurement indicated that inhibition efficiency (IE%) increased with inhibitor concentration, and the highest inhibition efficiency was up to 96.7%. The corrosion inhibition of these coatings was discussed in terms of blocking the electrode reaction by adsorption of the molecules at the active centers on the electrode surface. It was found that the adsorption of nicotinic acid on coating surface followed Langmuir adsorption isotherm with single molecular layer, and nicotinic acid adsorbed on the coating surface probably by chemisorption. Nicotinic acid, therefore, can act as a good nontoxic corrosion inhibitor for hot dipped Zn and Zn-Al alloy coatings in diluted hydrochloric acid solution. (c) 2007 Elsevier Ltd. All rights reserved.

The kinetics and mechanism of cathodic oxygen reduction on zinc and zinc–aluminium alloy galvanized coatings

A rotating disk electrode technique is used to investigate the kinetics and mechanism of O 2 reduction as it occurs at the surface of various hot-dip Al–Zn alloy coatings (on steel) immersed in weakly alkaline (pH 9.6) aqueous sodium chloride. The zinc component of coatings behaves electrochemically as though it were free zinc and the O 2 reduction pathway is determined by the potential dependent state of zinc. A 2e − reduction to H 2O 2 predominates at potentials near the free corrosion potential, where zinc is (hydr)oxide covered. A 4e − reduction to OH − predominates at potentials where zinc is bare. Tafel slopes (∂ E/∂log i) of 0.058 V dec −1 and 0.132 V dec −1 are determined for 2e − and 4e − O 2 reduction on pure zinc, respectively. Aluminium is virtually inert and varying aluminium content between 0.1% and 55% exerts little influence on O 2 reduction kinetics. However, all the Zn–Al alloy surfaces give very much higher O 2 reduction currents at low polarization than does pure zinc and it is proposed that this arises through an electrocatalysis of 2e − O 2 reduction by traces of substrate derived iron.

XPS study of the surface chemistry of conventional hot-dip galvanised pure Zn, galvanneal and Zn–Al alloy coatings on steel

This study uses X-ray photoelectron spectroscopy (XPS) to analyse the outer surfaces of galvanised, galfan and galvanneal metallic coatings obtained by hot dipping. Considerable aluminium segregation towards the surface, principally as Al 2O 3, has been seen in all the studied coatings. The aluminium content on the surface was surprisingly similar to or even greater than the zinc content, which is the major element in the chemical composition of these coatings. The formation of Al 2O 3 seems to prevent the oxidation of the zinc in the metallic coating.

Characterization of black rust staining of unpassivated 55% Al–Zn alloy coatings. Effect of temperature, pH and wet storage

Wet storage staining is a phenomenon that occurs on both zinc coated and 55% Al–Zn coated steel sheets during shipment or storage in damp conditions. Whereas zinc coated sheets form white corrosion products, 55% Al–Zn coated steel sheets form black corrosion products. The effect of temperature, pH and wet storage on the occurrence of black rust staining of unpassivated Aluzink samples has been investigated in the laboratory in terms of corrosion product formation and composition. A characterization of corrosion products formed has been performed mainly based on scanning electron microscopy with X-ray microanalyses (SEM/EDS) for morphological and quantitative analyses and X-ray diffraction techniques (XRD) for crystalline phase identification. Black rust formation is strongly related to alkaline pH regions and is enhanced by the temperature. All black panels show the presence of Bayerite (Al(OH) 3), mainly formed on the aluminum rich dendrite branches and a basic zinc aluminum carbonate (Zn 6Al 2(OH) 16CO 3·4H 2O) formed in the zinc rich interdendritic alloy regions in contact with air. Blackening of Aluzink surfaces is connected to differences in optical properties of embedded metallic zinc and/or aluminum particles of different shape and size in the corrosion layer.

Influence of substrate bias on the microstructure of sputtered Al-Zn alloy coatings

The influence of energetic particle bombardment in the modification of vacuum deposited coatings is reasonably well understood. However, the ion assisted deposition of alloy coatings is considerably more complex, and is often influenced by the re-emission of deposited material. In this work, unbalanced magnetron sputtering is used to study the effect of substrate bias on the composition and microstructure of coatings deposited with ion assistance. The coatings were sputtered from targets of Zn and two Al-Zn alloys onto silicon substrates, over a range of bias voltages. The composition, microstructure, and morphology of the coatings were characterised by energy dispersive X-ray spectrometry, Bragg-Brentano X-ray diffraction, and scanning electron microscopy respectively. The Zn coatings were highly preferred with the close packed plane parallel to the substrate surface. The residual stresses of the deposited films were highly dependent on the substrate bias. The Zn coatings were also affected by resputtering and re-evaporation of the deposited material due to energetic particle bombardment. The microstructures of the Al-Zn alloy coatings were influenced largely by the relatively high T/ T m, which favoured the segregation of Zn and Al into discrete grains. Energetic ion bombardment during the growth of the alloy coatings significantly affected the coating composition by preferential resputtering and/or re-evaporation of Zn. Furthermore, the bias voltage required to reject the Zn completely from the coating varied with its concentration in the target. It is argued that this effect occurs when the Zn re-emission flux is equal to the Zn flux arriving at the substrate, so that only Al is deposited when the bias voltage is sufficiently high.