Crack initiation and propagation of galvanized coatings hot-dipped at 450 °C under bending loads

Crack initiation and propagation behaviors in the intermetallic layers of galvanized coatings subjected to bending loads are characterized and numerically simulated. Coating structure of galvanized steel prepared by hot dipping at 450 °C is a laminate composite consisting of δ, ζ, and η phases, with an infinitesimal layer between the coating and steel article speculatively representing a Γ phase. The specimens were deformed in a four-point bending configuration, and the evolution of cracks was investigated as a function of bending angles. Through-cracks were found to develop in the δ layer of the coatings after thermal cooling due to thermal stresses and propagate toward the outer surface under increments of bending loads. Finite element simulations of galvanized steels were subsequently developed with an initial crack tip located in the δ layer to determine the controlling parameters of the crack propagation and to assess the coatings' fracture parameter, critical far field stress, and stress distributions. The analysis highlights the enhancement of fracture resistance of the galvanized coatings owing to the presence of the ζ layer.     

Hot-dip galvanization with pulse-electrodeposited nickel pre-coatings

The galvanizing process whereby steel is electrodeposited with nickel using pulsed current waveforms and hot-dipped in a molten zinc bath at 450 °C is investigated here as a potential route to mitigate the coating overgrowth problem. The influences of processing parameters, including electroplating and galvanizing durations, on the evolutions of microstructure and phase structures, and polarization characteristics of galvanized steels are explored. The results from the polarization study and salt spray tests indicate that the galvanized coating prepared with a nickel pre-coating, comprising zinc–nickel intermetallic layers, exhibits significantly better corrosion resistance than the conventionally-galvanized steel.     

Effect of vanadium on galvanizing Si-containing steels

In the present study, the effect of V additions to the galvanizing bath on the microstructure and growth kinetics of the galvanized coating on Si-containing steels was experimentally investigated. Three different steels with effective Si contents of 0.158, 0.242 and 0.363 wt.%, were immersed for 0.5 to 8 min in Zn baths with four levels of V, ranging from 0 to 0.16 wt.%. The results indicated that V additions could adequately suppress the Si reactivity of all three steels. The potency of V increased with its increasing content in the bath. Crystallites of a ternary compound were found at the ζ/liquid interface.Based on the isotherms of Zn-Fe-V and Zn-Fe-Si-V systems at 450 °C, a diffusion path model was proposed to interpret the effect of V on galvanizing pure Fe or Si-containing steels. V in the bath introduced a new equilibrium state between the liquid and a ternary phase T and made the equilibrium state between the liquid-ζ phases metastable. Furthermore, the greater solubility of Si in T made the diffusion path more difficult to enter the (ζ + T + Liq. + FeSi) region and, thus, protected the integrity of a normal coating.     

Effect of Fe-Zn alloy layer on the corrosion resistance of galvanized steel in chloride containing environments

In this study, the effect of Fe-Zn alloy layer that is formed during galvanizing process on the corrosion behavior of galvanized steel has been investigated. The galvanostatic dissolution of galvanized steel was carried out in 0.5 M NaCl solution to obtain the Fe-Zn alloy layer on the base steel. The alloy layer was characterized to be composed of FeZn₁₃, FeZn₇ and Fe₃Zn₁₀ intermetallic phases, which constitute the zeta, delta1 and gamma layers of galvanized steel, respectively. It was observed that the alloy layer has similar cathodic polarization behavior but different anodic polarization behavior compared to galvanized steel. The anodic current plateau of alloy layer was up to 100 times lower than that of galvanized coating. Corrosion test performed in wet-dry cyclic condition has shown that the alloy layer has lower corrosion rate as compared to galvanized steel. From the results of corrosion test of alloy layer and base steel, it was concluded that Zn²⁺ has positive effect on the protectiveness of the zinc corrosion products. The measurement of surface potential over the alloy/steel galvanic couple has confirmed the galvanic ability of alloy layer to protect both the alloy layer itself and the base iron during initial stage of atmospheric corrosion.     

Reactive wetting during hot-dip galvanizing of high manganese alloyed steel

The present study discusses reactive wetting during hot-dip galvanizing of high Mn alloyed steel (X-IP1000, 23 wt.% Mn) and is focused on investigating the influence of the metallic Mn concentration in the steel bulk composition on phase formation at the interface steel/coating. Samples were in-line bright annealed (1100 °C/ 60 s in N2-5%H2 at DP −50 °C) prior hot-dipping to avoid external MnO on the steel surface. This approach was applied to avoid influencing the wetting reaction by an aluminothermic MnO reduction, because this is considered to lead to an unwanted zeta-phase (FeZn13) formation in the coating by hot-dipping of Mn alloyed steels (< 5.0 wt.% Mn). The influence of hot-dipping parameters, which are contributing to the kinetics of the wetting reaction, was examined in terms of varying bath-Al content (0.17 and 0.22 wt.%), bath temperature (440-500 °C) and strip entry temperature (420-520 °C). The structure and chemical composition of both galvanized coating and interface steel/coating were characterized. While external MnO was verifiably avoided, brittle zeta-phase distinctively appeared at the interface steel coating together with the typical Fe₂Al₅ phase. This shows that the model of aluminothermic MnO reduction failed in the present case. This study suggests an alternative model explaining the appearance of zeta-phase with the removal of bath-Al by metallic Mn, which is dissolved out of the steel bulk into the Zn bath. The present investigation shows that alloying elements in the steel bulk may influence coating quality not only “indirectly” by external formation of nonwettable oxides, but also “directly” by influencing phase equilibria and kinetics of the wetting reaction. Understanding these phenomena will improve processing of (high) alloyed steel concepts as well as industrial Zn bath management.     

Corrosion of AZ91D magnesium alloy with a chemical conversion coating and electroless nickel layer

A chemical conversion treatment and an electroless nickel plating were applied to AZ91D alloy to improve its corrosion resistance. By conversion treatment in alkaline stannate solution, the corrosion resistance of the alloy was improved to some extent as verified by immersion test and potentiodynamic polarization test in 3.5 wt.% NaCl solution at pH 7.0. X-ray diffraction patterns of the stannate treated AZ91D alloy showed the presence of MgSnO₃ · H₂O, and SEM images indicated a porous structure, which provided advantage for the adsorption during sensitisation treatment prior to electroless nickel plating. A nickel coating with high phosphorus content was successfully deposited on the chemical conversion coating pre-applied to AZ91D alloy. The presence of the conversion coating between the nickel coating and the substrate reduced the potential difference between them and enhanced the corrosion resistance of the alloy. An obvious passivation occurred for the nickel coating during anodic polarization in 3.5 wt.% NaCl solution.     

Effect of fluxing chemical: An option for Zn-5wt.%Al alloy coating on wire surface by single hot dip process

The corrosion of wire is one of the primary causes for premature failure. The ideal way to overcome this problem is to provide corrosion protection right at the time of manufacturing. It is well established fact that Zn-5 wt.% Al alloy coating on steel surface provides much better protection against corrosion than the conventional pure Zn coating. Conventional fluxing operation is done on wire surface using zinc and ammonium chloride mixture before dipping in molten zinc bath. Galvanization bath temperature of about 415 °C for Zn-5 wt.% Al alloy coating on wire surface develops black spots of AlCl₃ when conventional flux is used. Double dip process is being followed for Zn alloy coating on wire surface due to non availability of suitable flux. An effort has been made to develop a suitable flux to obtain Zn-5 wt.% Al alloy coating on wire surface by single hot dip process. A salt mixture (containing zinc, ammonium, sodium, potassium, cobalt and lead chloride) was formulated based on the decomposition temperature of individual chloride salts. Differential thermal and thermo gravimetric analysis indicate the temperatures for complete decomposition of conventional and formulated flux are 445 and 410 °C, respectively. The lower decomposition temperature of formulated flux is ensured black spot free Zn-5 wt.% Al alloy coating. Alloy coated wire consists of alternative layers of zinc rich and aluminium rich phases. The performance of alloy coated wires has been evaluated by salt spray and Tafel tests. The alloy coated wire shows around 4 times improvement of corrosion performance against aggressive chloride environment compared to pure zinc coated wire. This can be attributed to the fact that aluminium rich phase prevents dissolution of zinc rich phase.     

Hot dip fine Zn and Zn-Al alloy double coating for corrosion resistance at coastal area

A new hot dip Zn-7Al alloy coating was performed on a structural steel by double coating of fine Zn and Zn-7 wt.% Al alloy, to prevent severe corrosion in coastal area. The alloy-coated steels were exposed to seaside, quasi-industrial, and rural districts to compare with conventional Zn coating. Double coating was significantly effective in preventing corrosion, particularly in a seaside. It was estimated from the exposure test for 10 years that the life of the Zn-7Al alloy-coated steel would be almost four times that of the Zn-coated steel in the seaside. A bending test showed that no exfoliation occurred at the interface between the coated alloy and substrate steel. TEM observation revealed that the excellent adhesiveness of the doubly coated fine Zn and Zn-7Al alloy to the steel substrate was due to formation of the interface region consisting of heterogeneous fine phase mixture of zinc, aluminium and iron.     

Formation of an aluminum-alloyed coating on AZ91D magnesium alloy in molten salts at lower temperature

An aluminum-alloyed coating was formed on an AZ91D magnesium alloy in molten salts containing AlCl₃ at a lower temperature of 380 °C. The microstructure and phase constitution of the alloyed layer were investigated by optical microscopy, scanning electron microscopy, energy dispersive spectrum and X-ray diffraction. The nano-hardness of the coating was studied by nanoindentation associated with scanning probe microscopy. The corrosion resistance of the coated specimen was evaluated in a 3.5 wt.% NaCl solution by electrochemical impedance spectroscopy and cyclic potentiodynamic polarization. The results show that the aluminum-alloyed coating consists of Mg₂Al₃ and Mg₁₇Al₁₂ intermetallic layers. The formation of the coating is dictated by the negative standard free energy of the reaction: 2AlCl₃ + 3 Mg = 3MgCl₂ + 2Al. This process is associated with a displacement reaction mechanism and diffusion process that takes place during the molten salt treatment. High activity of Al elements in molten salts contributes to the lower temperature formation of the Al-alloyed coating. The alloyed coating markedly improves the hardness as well as the corrosion resistance of the alloy in comparison with the untreated AZ91D magnesium alloy, which is attributed to the formation of the intermetallic compounds.     

A novel high-strength,high-ductility and high-corrosion-resistance FeAlMnC low-density alloy

The as-quenched Fe–8.68 wt.% Al–30.5 wt.% Mn–1.85 wt.% C alloy is plasma-nitrided at 500 °C for 8 h. The nitrided layer obtained is 40 μm thick and composed predominantly of AlN, with a small amount of Fe₄N. The resultant surface hardness (1860 Hv), substrate hardness (550 Hv), ductility (33.6%) and corrosion resistance in 3.5% NaCl solution in the present nitrided alloy are far superior to those obtained previously in optimally nitrided high-strength alloy steels, as well as martensitic and precipitation-hardening stainless steels.     

低碳钢表面热浸镀锌层的组织

利用传统的溶剂工艺在低碳钢表面热浸镀锌。采用X射线衍射仪和扫描电镜研究了低碳钢热浸镀锌层的显微组织及相组成。结果表明,热浸镀锌层的组织由基体向表面依次为Γ(Fe5Zn21)相、1δ(FeZn7)相、ζ(FeZn13)相和η(Zn)相;镀层最表层出现异常的Γ相,而1δ相合金层存在η相;与基体结合处的Γ相呈白亮的线状,靠近表面则呈齿状;镀层不同部位的块状1δ相的致密度不同,远离基体处比较疏松;棒状ζ相分散在η相中。     

Study on the role of PVD TiN coating in improving the performance of electroplated monolayer superabrasive wheel

The monolayer grinding wheels, coated with a physical vapour deposited (PVD) coating (viz. TiN, HfN, TiN + ZrN etc.), have been reported to outperform their uncoated counterparts as claimed in some patented literatures. The present work aims at exploring the mechanism how PVD TiN augments the performance of nickel electroplated monolayer superabrasive wheels. This study also includes the effect of negative substrate bias voltage on performance of TiN coated electroplated cBN wheels during grinding of hardened bearing steel. TiN was deposited by pulsed DC closed-field unbalanced magnetron sputtering (CFUBMS) technique in an in-house PVD coating system. The structure of the TiN coating and post-grinding condition of the wheels were observed using scanning electron microscopy (SEM). Energy dispersive X-ray (EDX) line scan, Electron probe micro analysis (EPMA) and secondary ion mass spectrometry (SIMS) depth profiling at the junction of TiN and nickel layer indicated the occurrence of inter-diffusion between them and grazing incidence X-ray diffraction (GIXRD) confirmed the formation of Ni-Ti intermetallic phases at their interface. The scratch test revealed a significant increase in cohesive and adhesive strengths of nickel layer when TiN was deposited at a bias voltage of −60 V or beyond that. The uncoated cBN wheel exhibited large number of grit fracture at the bond level and some grit pull-out. Such failures of grit were significantly arrested with TiN coating deposited at the bias voltages of −60 V and −90 V.     

Effect of pulsed DC CFUBM sputtered TiN coating on performance of nickel electroplated monolayer cBN wheel in grinding steel

The present research involves the deposition of pulsed DC CFUBM sputtered TiN on nickel plated steel discs and electroplated monolayer cBN wheels at seven different target frequencies and ten different bias voltages separately. The coating microstructures and the interaction between TiN and nickel were studied using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and electron probe micro analysis (EPMA). Phase detection was carried out using grazing incidence X-ray diffraction (GIXRD) technique. The cohesive and adhesive strengths of nickel layer were assessed by scratch test. After grinding of low carbon steel (AISI 1020) and hardened bearing steel (AISI 52100), the conditions of the uncoated and coated cBN wheels were observed under Stereo Zoom Microscope and SEM.Average column size of TiN was found to decrease with increase in both target frequency and negative bias voltage. The structure of the coating gradually transformed from porous and open columnar (at 0 V bias) to very compact, dense and featureless (at − 80 V bias). EDX line scan and EPMA confirmed the cross-diffusion between TiN and nickel and GIXRD indicated the formation of nickel-titanium intermetallic phases at their interface. The cohesive strength of nickel layer was not effectively enhanced with increase in target frequency, whereas the same was significantly improved with increase in negative bias voltage. Seemingly, TiN coated wheel could not perform better than the uncoated wheel in grinding AISI 1020 steel due to high wheel loading. However, the uncoated wheel was found to undergo fracture wear, which was remarkably absent in the coated wheels. On the other hand, many fractured grits and some grit pull-out were observed in the uncoated wheel when grinding AISI 52100 steel, whereas almost no pull-out along with much less fractured grits were observed in the wheels coated at bias voltages like − 60 V and − 90 V.     

The effect of hot-dipped aluminum coatings on Fe-8Al-30Mn-0.8C alloy

The morphology and microstructure of an intermetallic layer formed on the surface of Fe-8Al-30Mn-0.8C alloy by hot-dip aluminization treatment have been examined in detail. The phases present in the coating are unambiguously identified by means of transmission electron microscopy. After aluminization, a two layer coating was formed consisting of an external Al layer and a (Fe, Mn)₂Al₅ intermetallic on top of the substrate. The (Fe,Mn)₂Al₅ compound has an orthorhombic structure with lattice parameters a = 0.752 nm, b = 0.667 nm and c = 0.417 nm. The activation energy (E_FeMnAl) for the growth of such an intermetallic layer is calculated to be 52.7 kJ/mol. These results are different from those observed in aluminized low-carbon steel (E_Fe). The difference between E_FeMnAl and E_Fe is attributed to the alloying elements (Mn) in the present alloy. Environmental salt fog corrosion and high temperature oxidation tests were carried out to examine the corrosion and oxidation resistance. The results indicated that both the corrosion and oxidation resistance of the Fe-8Al-30Mn-0.8C alloy treated by hot-dip aluminization can be significantly increased.     

Synthesis and properties of open-cell Ni–Cr–Fe–Al alloy foams by pack co-deposition process

A pack cementation process for the co-deposition of Cr, Fe and Al onto open-cell nickel foam was developed. The reticulated open-cell Ni–Cr–Fe–Al foams were annealed to homogenize the material with 18.8 wt.% Cr, 11.3 wt.% Fe and 7.7 wt.% Al. The microstructure and phase composition of the Ni–Cr–Fe–Al foams were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive analysis (EDS). The results show that the coating is uniform and dense along the perimeter of the Ni strut, and consists of three layers: a Cr–Fe outer layer, an inner layer containing mostly Al and a transition zone. After homogenization annealing, the alloyed foams retain the hollow struts structure of the original pure nickel foams and the low relative densities. The Ni–Cr–Fe–Al alloy foams exhibit enhancement in absolute strength as compared to the pure nickel and Ni–35.2Cr foams. Furthermore, the Ni–Cr–Fe–Al alloy foams show excellent oxidation resistance and outperform the chromia-forming Ni–35.2Cr alloy foam after oxidation at 900 and 1000 °C, which is mainly due to its high aluminum and chromium content leading to the formation of a continuous and adherent duplex oxide layer.     

Modelling the relationship between microstructure of Galfan-type coated steel and cut-edge corrosion resistance incorporating diffusion of multiple species

This paper describes a considerable extension to a previously documented [S.G.R. Brown, N.C. Barnard, 3D computer simulation of the influence of microstructure on the cut edge corrosion behaviour of a zinc aluminium alloy galvanized steel, Corrosion Science 48 (2006) 2291-2303], first-order model used to simulate the localized degradation experienced in Zn-4.5 wt% Al steel coatings exposed to 5% NaCl aqueous solution. The temporal localization and intensity of discrete corrosion effects are predicted using established relationships and, in contrast to earlier models, the evolution of multiple concentration fields is included and calculated using straight-forward finite difference techniques. Changes in composition are included in the quantification of both anodic and cathodic processes involved in the corrosion of steel coatings in contact with aerated saline solutions. Reported [J. Elvins, J.A. Spittle, D.A. Worsley, Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion, Corrosion Science 47 (2005) 2740-2759] and modelled performances of typical Galfan composition coated steels are evaluated for different coating microstructures undergoing so-called cut-edge corrosion. In summary, this latest model successfully matches measured rates of metal loss during localized corrosion. Additionally, the inclusion of multiple species diffusion functionality has greatly improved the simulation of the cathodic reaction in particular and the overall form of the current density distribution near the corroding surface.     

Fluxless laser brazing of aluminium alloy to galvanized steel using a tandem beam – dissimilar laser brazing of aluminium alloy and steels

Tandem beam brazing with aluminium filler metal (BA4047) was conducted in order to develop the fluxless laser brazing technique of aluminium alloy (AA6022) to galvanized steels (GA and GI steels). Laser powers of tandem beam and offset distance of preheating beam from the root to the steel base metal were varied. Sound braze beads could be obtained by optimizing the preheating and main beam powers under the offset distances of 0–1 mm. A small amount of zinc remained at the braze interface between galvanized steels and the braze metal. The reaction layer consisting of Fe–Al intermetallic compounds was also formed at the steel interface, and the thickness of reaction layer could be predicted during the laser brazing (thermal cycle) process based on the growth kinetics with the additivity rule. The metal flow analysis of the melted filler metal on joints revealed that wettability and spreadability of the filler metal on the GI steel joint were superior to those on the GA steel joint. The fracture strength of the lap joint attained approx. 55–75% of the base metal strength of aluminium alloy. It was concluded that fluxless laser brazing could be successfully performed by using a tandem beam because the zinc coat layer acted as the brazing flux.     

Synergistic effect of Cu and Si on hot-dipping galvalume coating

The coating microstructures and thicknesses of the iron panels galvanized in galvalume baths containing 0.0, 0.1, 0.5 and 1.0 wt.%Cu for 10 s, 30 s, 60 s, 180 s, 300 s and 600 s have been studied detailedly. The results indicate that Cu can effectively control the Fe–Al reactivity by the synergistic effect with Si. The addition of Cu makes Si be enriched in the reaction region during the hot-dipping. It promotes the formation of the τ₅ phase and hinders the growth of the Fe₂Al₅ phase. The diffusion path model was introduced to studying the effects of Cu and Si in the present study. The addition of 0.5–1.0 wt.%Cu in galvalume bath forms a stable diffusion path, iron substrate/Fe₂Al₅/FeAl₃/τ₅/overlay. The violent reaction between the iron substrate and the Al–Zn liquid is under control by the compact intermetallic layer, and it decreases the thickness of the intermetallic layer.     

Measuring secondary phases in duplex stainless steels

The use of duplex stainless steels is limited by their susceptibility to the formation of dangerous intermetallic phases resulting in detrimental effects on impact toughness and corrosion resistance. This precipitation and the quantitative determinations of the phases have received considerable attention and different precipitation sequences (σ phase, χ phase, and carbides) have been suggested. This study investigates the phase transformation during continuous cooling and isothermal treatments in commercial duplex stainless steel grades and the effects on alloy properties, and compares the most common techniques of analysis.     

热镀锌层在模拟湿热酸性大气环境中的耐蚀性研究

目的研究Q420钢表面热镀锌工艺中,Zn和Zn-Al-Ni-RE合金镀层在酸性铜离子加速盐雾试验条件下的耐蚀性能。方法 Q420钢表面预处理后进行热镀锌,根据GB 6460—1986进行铜加速醋酸盐雾腐蚀试验,对比纯Zn镀层与Zn-Al-Ni-RE合金镀层的耐蚀性。结果 Ni,RE等元素的加入使镀层表面光亮,组织更加细密。在酸性铜离子加速实验进行到192 h时,纯锌镀层的腐蚀质量损失是合金镀层的2.7倍;72 h后纯锌镀层出现红锈,120 h后合金镀层出现红锈,说明Zn-Al-Ni-RE合金镀层比纯Zn镀层更耐腐蚀。结论通过适量添加Al,Ni与稀土元素,能使Q420钢合金镀层的耐蚀性能大幅度提高。