Al-Zn Bath Research Articles

Growth process and mechanism of hot-dip galvanised Zn–5Al coating on steel

ABSTRACT In this paper, the growth process and mechanism of hot-dip galvanised Zn–5Al coating on steel at different bath temperatures and immersion times were investigated by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The result shows that the growth of Zn–5Al coating is approximately divided into three stages: initial incubation stage, rapid thickening stage and steady detaching stage. The thickening and outbreak of the intermetallic layer is responsible for the thickening of the coating. When immersed into the Zn–Al bath, an Fe–Al intermetallic layer forms on the interface of liquid and steel and prevents the coating growing at the initial time; with the immersion time increasing, the solid-soluble Zn in the Fe–Al intermetallic layer makes the intermetallic layer unstable, the liquid Zn is discharged outward from the intermetallic layer and many pores and cracks occur, making the coating growth enter the rapid thickening stage. As the liquid phase areas keep expanding, the intermetallic layer will start to detach outward and produce a large amount of dross, the coating with linear growth finally enters the steady detaching stage. Increasing the immersion temperature will accelerate the growing rate of the coating and shorten the initial incubation stage of the coating.

Structure and Corrosion Resistance of Coatings Obtained by the Batch Double Hot Dip Method in Eutectoid ZnAl Bath with the Addition of Mg and Si

The article presents the results of the research determining the synergistic effect of Mg and Si additives in the ZnAl23 bath on the microstructure (SEM), growth kinetics, and corrosion resistance of coatings obtained by the batch double hot dip method. On the basis of the research on the chemical composition in micro-regions (EDS) and the research on the phase composition (XRD), the structural components of the coatings with a content of 3 and 6 wt.% Mg and 0.2–0.4 wt.% Si were identified. The corrosion resistance of the coatings was compared with that of the reference coating obtained in the ZnAl23Si0.4 bath in the neutral salt spray test (EN ISO 9227), and the test with sulfur dioxide in a humid atmosphere (EN ISO 6988). The parameters of electrochemical corrosion of the coatings were determined. This allowed for the conclusion that the addition of Mg and Si to the ZnAl23 bath improved the microstructural uniformity of the coating. In the area of the diffusion layer, the presence of the FeAl3 intermetallic phase was found, while in the outer layer the dendrite of the Al-rich solution with interdendritic spaces, where it locates areas of Zn-rich solution, eutectic Zn/MgZn2 or phase separation MgZn2 and Mg2Si are located. Additions of Mg and Si stabilize the kinetics of the coating’s growth, and its course is parabolic. The conducted corrosion tests showed better corrosion resistance of the coatings obtained in the bath with Mg addition compared to the reference coating obtained in the ZnAl23Si bath. The increase in corrosion resistance results from the presence of a more anodic MgZn2 phase, which, however, may be weakened by the presence of Mg2Si phase precipitates.

Recovery of aluminum-zinc alloy from 55%Al-Zn dross by supergravity separation.

As a by-product of hot-dip 55%Al-Zn coating processing, the dross contains high levels of Al and Zn. However, no cost-effective recycling methods have been reported to date. Therefore, this work aims at developing a novel method for Al-Zn alloy recovery from an industrial 55%Al-Zn dross using supergravity separation. The separation efficiency was analyzed as a function of separation time and temperature as well as the gravity coefficient (G). Al-Zn alloy was recovered at G values above 70. Separation at higher temperatures benefited the recovery of Al-Zn alloy but decreased the removal rate of Fe impurities due to the increased Fe solubility. With the optimal conditions at 590 °C and G > 500, over 86 wt. % of Al-Zn alloy, containing only 0.4 wt. % of Fe, was recovered. According to the FactSage software calculation, the solubility of Fe increases from 0.06 to 1.11 wt. % when the temperature rises from 550 to 650 °C. This is consistent with the experimental results. The purified alloy could then be further used in the hot-dip 55%Al-Zn bath for production. This study demonstrates the feasibility of supergravity separation as a promising process efficient for recovering Al-Zn alloy from 55%Al-Zn dross.

Effect of Mg on the Formation of Periodic Layered Structure during Double Batch Hot Dip Process in Zn-Al Bath

The article presents the results of studies on the influence of Mg on the formation of the periodic layered structure of the Zn-AlMg coatings. These coatings were produced by the double batch hot dip method in a Zn bath and then in a Zn-Al(Mg) bath with a content of 15, 23, 31 wt.% Al and 3, 6 wt.% Mg. The microstructure of the coatings (OM, SEM) was revealed and the phase composition (XRD) obtained in two-component Zn-Al baths and Zn-AlMg baths were determined. The periodic layered structure was found to consist of alternating FeAl3 phase layers and a bath alloy (Zn + Al + Mg). Moreover, it was observed that the addition of 3 wt.% Mg reduces the thickness of the coating in baths containing 23 and 31 wt.% Al. However, the addition of 6 wt.% Mg causes complete disappearance of periodic layered structure in a bath with 23 wt.% Al. In a bath with a content of 31 wt.% Al the addition of 6 wt.% Mg creates a compact layer consisting of the FeAl3 phase containing the precipitation of the MgZn2 phase and Fe2Al5 phase. Such a structure of the coating transition layer limits the growth of the periodic layered structure in the coating.

Effect of Al on phase evolution in Prior-Ni coated galvanized steel: Thermodynamic and kinetic assessment

Coating of advanced high strength steel and ultra-high strength steel is a burning issue due to the problem of surface oxidation of alloying elements and liquid metal induced embrittlement (LMIE). In last decade, prior coating of Ni before galvanizing has gained much interest as it has the potential to suppress the surface oxidation of advanced high strength steel with added benefits. Recently, the galvanized ultra-high strength steel with a prior-Ni coating also has been proved as a solution for hot stamping application. It has been observed that prior-Ni reacts with Zn at the stage of galvanizing and forms different Ni–Zn alloy phases with high melting temperatures and resist the LMIE at the time of hot stamping. Al is one of the most important alloying element in commercial galvanizing bath. However, effect of Al on the Ni–Zn phase formation in such coating has not been researched in detail. In the present study, phase formation in prior-Ni coated galvanized steel in the presence of Al in Zn bath has been thoroughly investigated. The influence of Al on the Ni–Zn phase formation has been assessed by thermodynamic and kinetic calculation and then validated by experiments. Al3Ni2 with dissolved Zn precipitated in the Zn overlay and of Ni–Zn alloy phases such as NiZn8 and gamma phase formed on prior-Ni layer in a Zn–Al bath up to 0.01 mass fraction of Al. However, Ni–Zn alloy phases such as NiZn8 and gamma phases were absent in coating achieved by Zn–Al bath above 0.01 mass fraction of Al. Instead a thin compact layer consisting high amount of Ni, Al and Zn formed on prior-Ni layer and hindered the formation of Ni–Zn alloy phase in coating. The mechanism of the phase formation in the prior-Ni coated galvanized steel in presence of Al has been described with the help dissolution, phase fraction, driving force calculations and experimental observations.

Influence of Cold Rolling Process on Mini Spangle Formation of 55%Al-Zn Alloy Coating Steel Sheet

According to that mini spangle is the most common defect affecting the appearance quality of hot-dip 55%Al-Zn alloy coated steel sheet, industrial experiments and statistical analysis were done to investigate the influence of cold rolling process on the formation of mini spangle. The results show that, with the decrease of rolling oil concentration, the increase of rolling time, and the increase of rolling pass, the probability of mini-spangle formation increases. Due to the different equipment conditions, the probability of mini-spangle formation on the upper and lower surfaces of steel strip is different. The reason of mini-spangle formation lies in the presence of carboxylates (R-COO-Fe) result from the residual emulsion on the surface of cold rolled steel strip. The carboxylates may interfere with the interfacial reaction between the steel substrate and Al-Zn bath, and result in more convex Fe5Si2Al20 phases formed on the surface of intermetallic compound layer. The Fe5Si2Al20 phases may provide more heterogeneous nucleation sites for the formation of Al-rich dendrites and lead to the formation of mini spangle.

Formation of periodic layered structure during hot-dip galvanizing in Al-Zn-Mg bath

Abstract This study investigated the effect of Al and Mg content and immersion time on the formation of periodic layered structure during hot-dip galvanizing in Al-Zn-Mg bath. The steel substrates were immersed in Al-Zn bath for 10 s to 120 s, with five levels of Al content in the range of 15 to 55 wt.%, and four levels of Mg additions, in the range of 0.5 to 3.0 wt.%. The microstructure and constituent phases of the periodic layered structure were characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The coating with periodic layered structure contains a thin light gray Fe2Al5Znx layer near substrate and outer periodic layered structure. The periodic layered structure consists of alternating light gray layer (FeAl3Znx) and black gray layer (Al dendrites and Zn-rich interdendritic regions). The formation of periodic layered structure requires an incubation period t1, while if the immersion time is over t2, the periodic layered structure is destroyed. In addition, t1 and t2 varies with the Al content and Mg addition.

The Impact of Al Addition to a Zn-Al Bath upon Growth Kinetics and Structure of Coatings Obtained in the Batch Double Hot Dip Process

In the paper the results of tests on obtaining coatings by use of double hot dip in Zn-Al baths with various Al contents are presented. The coatings were produced in ZnAl baths with the following Al contents: 7, 15, 23, 31 % wt. Al. Based on Zn-Al alloy solidification curves the optimum temperature of the metallization process has been determined. The growth kinetics has been defined and the structure of coatings has been developed. It has been established that an increase in Al contents to 23 and 31% in the bath leads to excessive growth of the coating thickness and its porosity. Smaller contents of Al (7 and 15 % wt.) in the bath allow to obtain coatings of a favourable structure and correct thickness.

Effect and controlling mechanism of vanadium on Fe–Al interface reaction in Al–Zn bath

Abstract The influence of vanadium on Fe–Al interface reaction of steels during hot-dip process was investigated by immersing low carbon steel plates into six different Al and Al–Zn baths at 730 °C and 620 °C. The microstructure and phase constituent of the formed coatings were analyzed with SEM–EDS and XRD. The intermetallic layers formed on the steel plate surfaces in Al bath consist of Fe2Al5 and FeAl3 phases. As 0.05 wt.% V was added to the Al bath, the Fe2Al5 layer became thinner. Whereas the steel plates were immersed into 55%Al–Zn bath, a periodic laminar structure, containing FeAl3 and Al–Zn phases, formed. However, the periodic laminar structure disappeared and a continuous thin Fe2Al5 layer appeared as V was added into Galvalume bath, and the whole intermetallic layer consists of Fe2Al5, Fe2Al8Si and FeAl3 phases. The presence of 0.05 wt.% V in the bath promoted the formation of Fe2Al5 layer. As it covers the whole steel plate surface, it retards the further Fe–Al reaction in the bath, leading to a thin intermetallic layer. The influence of V on Fe–Al interface reaction was studied by means of Vienna Ab-initio Simulation Package (VASP) based on first-principles method. The exchange energy, bonding energy, charge density differences and electronic density of states of Fe2(Al,V3)5 and Fe(Al,V12)3 are analyzed. Both the experimental results and theoretical calculation results show that V could occupy the vacancies of Fe2Al5 lattice which could promote the nucleation of Fe2Al5, and therefore it suppresses the growth of Fe–Al phase.

600 °C isothermal section of the Al–Cr–Zn ternary phase diagram

600 °C isothermal section of the Al–Cr–Zn system has been determined by Scanning Electron Microscopy-Energy Dispersive X-ray spectrometry (SEM-EDS), and X-ray Diffraction (XRD). Eleven three-phase regions have been identified experimentally at 600 °C. The τ3 and τ4 ternary compounds were identified in this isothermal section and the crystal structures of both phases are cubic. The lattice parameters of τ3 and τ4 are a = 2.1536 nm and a = 1.8323 nm, respectively. The formerly reported τ1 phase was not found. The formerly reported τ2 phase is an extension of Al7Cr. The highest Zn content in γ2 and ν phases is 7.1 at at.% and 6.7 at.%, respectively. The Zn solubility in Al7Cr phase can be up to 10.4 at.%, while that in Al4Cr phase is less than 4 at.%. The clearly phase relation of the Al–Cr–Zn system can lead us to better understand the effect of Cr on the corrosion behavior of metals in the Zn–Al bath and the Hot-dip galvanizing process.

Development of smart oxidation and corrosion resistance of multi-doped complex hybrid coatings on mild steel

The microstructure, mechanical and thermal treatment behavior of Zn–Al–SnO2/TiO2 (Zn–Al–Sn/Ti) produced through chloride deposition system was studied. 7.0–13.0wt% TiO2 and SnO2 was added to chloride Zn–Al bath. A thermal treatment characteristic was done for 2h at 200°C, 400°C and 600°C. The ageing behaviors of the co-deposited alloys were evaluated using (SEM/EDS) and XRD. The hardness and wear value of the solid coatings were examined with micro-hardness and UMT-2 sliding tester respectively. The corrosion properties were investigated by linear polarization method in 3.65% NaCl environment. From the obtained results, the deposited alloys revealed excellent stability. The even distribution of the particulate on the produced coating and thermal-treatment were observed to cause the improvement on mechanical, tribological and electrochemical properties. The overall best coating was obtained at Zn–Al–7Sn–Ti–0.3V–Cl for as-coated and Zn–Al–7Sn–Ti–0.3V–Cl at 400°C for the thermo-mechanical treated samples. The hardness, corrosion and micro-mechanical resistance performance against the working substrate were depended on the development of coherent and regular precipitation from the incorporated strengthening particulate.

The Influence of Si Content in Steel on Growth Kinetics and Structure of Hot Dip ZnAl23Si Coatings

In the paper the results of tests on obtaining coatings in a ZnAl bath of eutoctoid composition on steel with various contents of silicon by use of the batch hot dip method are presented. The growth kinetics of coatings obtained in a ZnAl23 bath with the addition of 1% and 2% of Si on steel with low silicon content (0.01% Si), on Sandelin steel (0.035% Si) and on high-silicon steel (0.27% Si) have been defined. The structure of coatings on tested steels was developed. It has been established that coatings obtained on the Sandelin steel and on high-silicon steel have similar thickness and structure as compared to coatings obtained on low-silicon steel.

Effect of thermal treatment on the interfacial reaction, microstructural and mechanical properties of Zn–Al–SnO2/TiO2 functional coating alloys

In this paper, the microstructure and thermal treatment behavior of Zn–Al–SnO2/TiO2(Zn–Al–Sn/Ti) produced through co-deposition route was investigated. 7.0–13.0wt% TiO2 and SnO2 was added to Zn–Al bath. Thermal treatment was done for 2h at 200°C, 400°C and 600°C. The ageing characteristics of the coated alloys were evaluated using (SEM/EDS) and XRD. The hardness and wear value of the thin film were examined with micro-hardness and UMT-2 sliding tester respectively. The corrosion properties were evaluated by linear polarization in 3.65% NaCl. From the result obtained, the coating exhibited good stability. The uniform distribution of the composites on the fabricated alloy and age-treatment were the major factor responsible for the mechanical and electrochemical improvement. The optimum value was obtained at Zn–Al–7Sn–Ti–0.3V–S for as-coated and Zn–Al–7Sn–Ti–0.3V–S at 400°C for the thermal-aged samples. The stabilities in hardness and corrosion resistance against the working substrate were attributed to the formation of coherent and uniform precipitation in the metal lattice.

A sessile drop setup for the time-resolved synchrotron study of solid-liquid interactions: Application to intermetallic formation in 55%Al-Zn alloys

We introduce a dedicated setup for measuring by synchrotron diffraction in-situ crystallographic and chemical information at the solid–liquid interface. This setup mostly consists of a double-heating furnace composed of a resistive heating for the solid surface and an inductive heating to produce a liquid droplet. The available high energy and high flux beams allow the rapid reaction kinetics to be investigated with very good time resolution down to 1 ms. An application of this setup is illustrated for the growth mechanisms of intermetallic phases during the hot-dipping of steel in a 55%Al-Zn bath. Results show that the three η-Al5Fe2, θ-Al13Fe4, and α-Al8Fe2Si phases grow at different times and rates during the dipping process, whereas the face-centered cubic AlFe3 phase is not formed.

The reactivity of 316L stainless steel in 55%Al-Zn bath

Static immersion experiments were conducted to assess the reactivity of austenitic stainless steel grade 316L in contact with molten 55%Al-Zn alloys. The static immersion tests were carried out at bath temperatures of 600 to 650 °C and continuous immersion times of 24 to 645 h. The level of reactivity was assessed by measuring the alloy layer thickness that formed on the test coupons after the immersion experiments. The reaction of 316L in contact with molten 55%Al-Zn bath results in the formation of two distinct alloy layers on the surface of the steel: one alloy layer forms adjacent to the steel surface (designated “alloy layer 1”) and the other formed between alloy layer 1 and the bath (designated “alloy layer 2”). Characterization of the nature of the alloy layers was carried out by optical and scanning electron microscopy. Analyses of the chemical composition of alloy layer 1 by SEM/EDS (and taking into account the Al/(Fe + Cr + Ni) ratio) suggest that the alloy layer is most likely based on FeAl2 intermetallic phase. Alloy layer 2, on the other hand, consists of modified o5c (the normal dross intermetallic phase modified by the substitution of Fe by Cr). After 27 days of continuous immersion at 600 °C, the 316L sample immersed in 55%Al-Zn bath had a total alloy layer thickness of 423 ¯m. The alloy layer growth is diffusion controlled and follows a parabolic rate law: at bath temperatures of 600 to 650 °C and continuous immersion times of up to 27 days, the alloy layer thickness increases approximately with the square root of immersion time. The parabolic rate constants for the total alloy layer thickness growth were: 2.84 × 10-7 m.s−1/2 at 600 °C, 3.34 × 10-7 m.s−1/2 at 615 °C and 6.76 × 10-7 m.s−1/2 at 650 °C. Growth of the alloy layers takes place predominantly at the interface between alloy layer 1 and alloy layer 2, and in part at the substrate/alloy layer 1 interface. The growth of alloy layer 2 is dominated by the inward diffusion of aluminium, while both inward diffusion of aluminium and outward diffusion of iron are believed to play a role in the formation of alloy layer 1.

Effect of temperature on the reactivity between a CoCrMoSi alloy and 55 wt% AlZn baths

The reactivity of metallic alloys with Al has important impacts in industrial components and in Co-based components has been associated with the Al–Co reactions. The reactivity between Al and a CoCrMoSi alloy was assessed by immersion tests in a 55wt% AlZn bath and DSC tests of multilayered pure Al/CoCrMoSi specimens. Phases were identified by XRD analysis. Results showed that the synthesis of Al–Co compounds occurs at a temperature as low as 380°C. Despite the higher corrosion resistance of the Laves phase the performance of CoCrMoSi component is compromised by the reactivity between Al and elements from the Co solid solution regions.

The Fe–Zn–Al–Cr system and its impact on the galvanizing process in chromium-added zinc baths

The zinc rich corner of the Fe–Zn–Al–Cr at 460 °C is of interest for galvanizing because Al is a usual addition element in zinc bath, whereas Cr is naturally present because it is supplied by the stainless steel roller dipping in the Zn bath during the process. Indeed, it is used to understand the formation and growth mechanisms of the solid phases during galvanizing in Al and Cr-added Zn bath. By using additional experimental results in the Al–Cr–Zn and Fe–Zn–Al–Cr systems, the zinc rich corner of the Fe–Zn–Al–Cr system at 460 °C was determined with more accuracy. Thus, new equilibria between the liquid and quaternary phases have been pointed out, namely Al2Cr3 stabilized by Zn and enriched with Fe and τ1, the latter being isotypic with δ-FeZn9. This quaternary system was assessed with the CALPHAD method using the PARROT module of the Thermo-Calc Software. The liquid and solid solutions are described by the Redlich-Kister-Muggianu equations. All the modeled phases are considered as stoichiometric in the binary systems.

Microstructural characteristics and corrosion resistance in molten Zn–Al bath of Co–Mo–Cr–Si alloys

Co-base alloys Tribaloy T-800 and Stellite 6 are widely used bearing materials in the galvanizing industry. Tribaloy alloys are strengthened by Laves intermetallic phase while Stellite alloys depend on carbide dispersed strengthening. The chemical composition influences the amount and mechanical properties of the strengthening phases in these alloys, which consequently influence the overall performance of the materials. Currently preferred compositions of alloys Tribaloy T-800 and T-400 are hypereutectic with primary Laves phase dendrites and interdendritic eutectics. A new hypoeutectic alloy Tribaloy T-401 with primary Co-rich dendrites and interdendritic eutectics is obtained by adjusting Cr, Mo, Si and C contents. The new alloy Tribaloy T-401 has better ductility than alloys Tribaloy T-800 and T-400. Tribaloy alloys T-800, T-400 and T-401 as well as Stellite 6 have been dipped in an Fe-saturated Zn bath containing about 0.22 wt.% Al at 470 °C for various lengths of time up to 604,800 s (168 h) to study their corrosion resistance. It is found that all four alloys have reacted with the Zn–Al baths. Tribaloy alloys outperform Stellite 6 in corrosion resistance in molten Zn–Al bath and Tribaloy T-401 performs best in terms of the thickness of reaction layer.

Sliding wear performance in molten Zn–Al bath of cobalt-based overlayers produced by plasma-transferred arc weld-surfacing

The purpose of this investigation was to improve the wear resistance of part materials (bush and sleeve in sink roll) by applying plasma transferred arc (PTA) surfacing technique in the continuous hot-dip galvanizing facilities. Candidate alloys from the preliminary investigation were cobalt-based Stellite 6 and Tribaloy 800 with WC, WC–Co, TiC, NbC, and Cr 3C 2. The resultant deposits were subsequently characterized using X-ray diffraction, scanning electron microscopy, and the microhardness tests. Microstructural development during the solidification of each deposit is discussed. Pin-on-disk sliding wear tests were conducted in a molten Zn–0.18%Al (wt.%) bath at 470 °C to evaluate the wear performance of the PTA weld-surfaced cobalt-based alloys. Based on microstructural observation and the microhardness measurement, sliding wear performance of the PTA weld-surfaced cobalt-based alloys in molten Zn–Al bath is reported and compared with pin and disk combination.

溶融Ｚｎ‐６ｍａｓｓ％Ａｌ浴に浸漬した炭素鋼の界面反応挙動に及ぼす炭素濃度の影響

The phases and microstructure of the alloy layer formed during hot dipping of carbon steels in molten Zn-Al bath were investigated. Pure iron, 0.2%C and 0.82%C steels and 2.45%C cast iron were used. A plate shape specimen of 300μm in thickness was firstly hot dipped in a molten Zn bath for 3s and then hot dipped in a molten Zn-6mass%Al alloy bath for certain times, and quenched with water. The microstructure of the quenched specimen was observed by using a SEM, and change in thickness of the specimen was measured by using a laser microscope. The pure iron specimen was entirely changed to an intermetallic compound of Fe2Al5(Zn) phase after 1.2ks hot dipping. The thickness of 0.2%C steel specimen was decreased linearly with the dipping time after an early non-reaction period, and 2/3 of the specimen was changed to the Fe2Al5(Zn) phase after 3.6ks hot dipping. It was observed that primarily precipitated ferrite phase preferentially reacted and pearlite structure scarcely reacted. The reacted thickness of the 0.82%C steel and cast iron specimens were only several μm after 86.4Ks hot dipping. In the cast iron specimen, cementite reacted with Al and an Al3C4 phase was formed. The growth of the alloy layer was restricted by the Al3C4 phase. A heat-treated 0.2%C steel specimen with sorbite structure also exhibited the same low reaction behavior as the 0.82% C steel due to homogenized ferrite + cementite structure.