Hot-Dip Coating of Lead-free Aluminum on Steel Substrates with Ultrasonic Vibration

Hot-dip coating has been practically employed in manufacturing zinc alloy coated steel sheets.However,it is difficult to coat aluminum alloy on a bulky steel substrate without sufficient preheating,because a rapidly solidified layer containing gas babbles is formed on a substrate surface.A variety of iron-aluminides are also formed at the interface of a steel and aluminum hot-dip coating system,which is the main difficulty in joining of steel with aluminum.Ultrasonic vibration was applied to a steel substrate during hot-dip coating of aluminum alloy to control a rapidly solidified layer and a brittle reaction layer.Hot dipping of columnar steel substrates into molten aluminum alloy （Al-2.7 mass fraction Si-4.6 mass fraction Sn） was carried out through the use of a Langevin oscillator with resonant frequency of 19.5 kHz.The application of ultrasonic vibration is quite effective to control a rapidly solidified layer and a surface oxide layer from a substrate surface by the sonocapillary effect based on a cavitation phenomenon,so that the intimate contact is achieved at the beginning of hot-dip coating.The application of ultrasonic vibration to hot-dipping is effective to control a reaction layer with less than 5 #m in thickness.An impact test exhibits that the good adhesive strength is approved in hot-dipped aluminum coatings with a thin reaction layer of approximately 5μm.

Analysis of Coating Microstructure of Hot-Dip Aluminum of Deformed Low-Carbon Steel Containing Rare Earth

The coating microstructure of hot-dip aluminum (HDA) of deformed low-carbon steel containing RE was analyzed by metallography microscopy, TEM and XRD, and the forming mechanism was also discussed. The results show that, the Fe_2Al_5 phase, on whose subcrystal boundaries, Al particles with the size of 7～30 μm existing on parallel linear are, grows a strong orientation. And the spread activation energy of Al is 155.22 kJ·mol -1. In addition, the effects of deformation on coating microstructure of hot-dip aluminum and the function of RE were preliminarily analyzed.

A sandwich-like structure hybrid coating of cold galvanizing coating/polydopamine on hot-dip galvanized steel with enhanced adhesion and corrosion resistance

This study aims to enhance the adhesion strength and anti-corrosion performance of the cold galvanizing coating(CGC)applied on the hot-dip galvanized steel(HDG).Polydopamine(PDA)is deposited on the HDG surface with different time ranges and as an interlayer between CGC and HDG through covalent immobilization.The surface morphology and the covalent interaction between PDA/HDG are exhibited by scanning electron microscope(SEM),atomic force microscopy(AFM)and X-ray photoelectron spectroscopy(XPS).The pull-off adhesion tests before and after neutral slat spry tests show an enhanced dry adhesion strength and less adhesion loss of the hybrid CGC/PDA coated HDG compared with the direct CGC coated HDG.In addition,open circuit potential(OCP)reveals that the corrosion protection performance of the hybrid CGC/PDA coated HDG increases by 200%(up to 201 d)and the corrosion density icorrattaining about 4.45×10^(-7)A/cm^(2).Electrochemical impedance spectroscopy(EIS)measurements and X-ray diffraction(XRD)analysis confirm that the precipitate of the stable chelation formed by PDA and Zn^(2+)between CGC and HDG substrate can also improve the corrosion protection performance.Such a strategy of strengthening adhesion and forming the chelate compound at the HDG surface promises a new route to corrosion protection of CGC on HDG.

Influence of Si content on interface reaction of iron-based hot-dip aluminizing on Fe sheet

Based on the diffusion channel,the influence of Si content on the microstructure evolution of iron-based hot-dip Al-χSi coating was analyzed(χ=0,1.5 wt%,3.0 wt% and 7.0 wt%).The results show that the introduction of Si makes the reaction interface change from the lingual-tooth interface of hot-dip Al to the flat interface of hot-dip Al-Si.It also reduces the thickness of the alloy layer in the coating,especially the Fe_(2)Al_(5) layer.When the Si content is 1.5 wt%or 3.0 wt%,the diffusion channel crosses the conjugate line of the two-phase region(FeAl_(3)+liquid phase)into the FeAl_(3) single-phase region,and then moves to the region with higher Si content.Next,the diffusion channel cuts off the conjugate line of FeAl_(3)phase,τ_(1)/τ_(9) phase,and Fe_(2)Al_(5)phase,which promotes the form ofτ_(1)/τ_(9) phase.The formedτ_(1)/τ_(9) phase inhibits the diffusion between Fe and Al atoms.When the Si content is 7.0 wt%,the diffusion channel passes through the two-phase region(liquid phase+τ_(5))and enters theτ_(5) single-phase region.The form ofτ_(5) single-phase region has a strong inhibitory effect on the interatomic diffusion of Fe and Al,thereby reducing the thickness of the coating,especially the Fe_(2)Al_(5)layer.

Evolution of appearance profile of hot-dip galvanized car-body steel sheet during deformation process

In recent years, the waterborne free intermediate coating process has been widely used in the automotive industry. Because the baking times and coating thickness are decreased, the surface covering capability of the painting process is reduced, which directly affects the appearance quality（ long-and short-wave values） of the body paint. Thus, there are correspondingly higher requirements for the white body surface profile prior to painting. The surface profile of the white body is mainly affected by the plate material, the surface profile, and the deformation process. So,the change rule for the surface profile during deformation of the steel plate is a key factor in coating appearance optimization. In this paper, we first analyze the typical deformation of the outer cover of a car body. Then ,we examine the change tendency of the surface profile of steel plates with respect to different deformation rates, specifically for a steel plate comprising a hot-dip galvanized bake-hardened steel sheet. Based on our analysis of the influence of the deformation on the coating appearance,we selected 3% ,5% ,and 8% deformation rates in this research. We found the roughness （Ra） value in the typical deformation range （3% -8% ） of the car body to exhibit a decreasing trend at first and then an increasing trend. The Ra value of the 8% deformation is not more than the original plate test value. When the Pc value of the original plate is in the lower range （ about 60）, it exhibits a slight increasing trend in the deformation process （3 % -8 % ）. And when the Pc value of the original plate is in the higher range （ about 120 ）, it exhibits no increasing trend in the deformation process （ 3% -8% ）. In contrast,the waviness （WCA） value in the car body＇s typical deformation range （3%-8%） shows a significant growth trend.

Microstructure of Hot-Dip Galvanized Zn-Al-Mg Alloy Coating

The microstructure of hot-dip galvanized Zn-ll%A1-3%Mg-0.270Si alloy coating was studied in this article. X-ray diffraction analysis revealed the coating is composed by Zn, A1 and MgZn2 phase. Optical microscope （OM） and scanning electron microscope （SEM） observations showed the coating is occupied by snowflake-like dendrite, double hexagonal organization and eutectic. The coating backbone was the dendrite considered to be a phase of hexagonal close-packed （HCP） structure judging form its morphology according to the crystal growth way in the coating. Transmission electron microscopy （TEM） and energy dispersive spectroscopy （EDS） researches on the dendrite suggested that an intermediate Zn-A1 phase was formed at high temperature, then decomposed into a type of Zn-A1 granular eutectoid after cooling down to room temperature, while the eutectoid Zn and A1 had a certain crystallographic relationship. The coating solidification process and the grain crystal structure were discussed.

Influence of Air-Knife Wiping on Coating Thickness in Hot-Dip Galvanizing

In hot-dip galvanizing process, air jet wiping control is so crucial to decide the coating thickness and uni- formity of the zinc layer on the steel strip. The mathematical models developed predict the zinc coating thickness as a function of pressure and shear stress. The required pressure and shear stress profile on the strip surface were calcu- lated using regression analysis, and carried out using numerical simulation as FLUENT, a finite element analysis software. The influences of the outlet pressure, the nozzle to strip distance, the slot opening, the edge baffle plate, as well as the tilting angle of air knife were discussed. Combining with these results and regression analysis on the practical data, four first-order polynomial multi-parameter models were established for different targeted coating thicknesses with better regression coefficients. The validated model was used to carry out sensitivity analysis to de- termine the favorable controlling regime for the air jet wiping process.

A Study of the Influence of Air-knife Tilting on Coating Thickness in Hot-dip Galvanizing

Gas wiping is a decisive operation in hot-dip galvanizing process. In special, it has a crucial influence on the thickness and uniformity in coating film, but may be subsequently responsible for the problem of splashing. The progress of industry demands continuously the reduction of production costs which may relate directly with the increase of coating speed, and the speed up of coating results in the increase of stagnation pressure in gas wiping system in final. It is known that the increase of stagnation pressure may accompany a harmful problem of splashing in general. Together with these, also, from the view point of energy consumption, it is necessary to design a nozzle optimally. And there is known that the downward tilting of nozzle using in air knife system is effective to prevent in somewhat the harmful problem of splashing. In these connections, first, we design a nozzle with constant expansion rate. Next, for the case of actual coating conditions in field, the effects of tilting of the constant expansion rate nozzle are investigated by numerical analysis. Under the present numerical conditions, it was turned out that the nozzle of constant expansion rate of p = having a downward jet angle of 5^0 is the most effective to diminish the onset of splashing, while the influence of small tilting of the nozzle on impinging wall pres- sure itself is not so large.

Development and application of a neural network based coating weight control system for a hot-dip galvanizing line

The hot-dip galvanizing line(HDGL) is a typical order-driven discrete-event process in steelmaking. It has some complicated dynamic characteristics such as a large time-varying delay, strong nonlinearity, and unmeasured disturbance, all of which lead to the difficulty of an online coating weight controller design. We propose a novel neural network based control system to solve these problems. The proposed method has been successfully applied to a real production line at Va Lin LY Steel Co., Loudi, China. The industrial application results show the effectiveness and efficiency of the proposed method, including significant reductions in the variance of the coating weight and the transition time.

Mechanism for colouring hot-dip zinc alloy coating

Hot-dip galvanizing is a widely used technique to prevent steel from oxidizing in air.However, only silver-grey zinc coating can no longer meet the need of people. At theWorld’s Fair in July 1989, Japanese first exhibited blue, red, yellow green andbrown hot-dip zinc coating sheets. These patent coatings are more incorruptible thantraditiona ones. This note tries to reveal the colouring mechanism of these coatings.

Berberine as an Environmental-Friendly Inhibitor for Hot-Dip Coated Steels in Diluted Hydrochloric Acid

The inhibition effect of an excellent environmental-friendly corrosion inhibitor--berberine on hot-dip coated steels in the diluted HCI has been investigated by using quantum chemistry analysis, mass-loss tests, elec- trochemical measurements and scanning electron microscopy （SEM） observation. Calculation results show that berberine has a nearly planar structure with a number of active centers. The value of Mulliken charge, and the distribution of the highest occupied molecular orbital （HOMO） and the lower unoccupied molecular orbital （LUMO） imply that berberine has a good ability of electron exchange with metal surface. The test results indicate that inhibition efficiency （IE%） increases with the inhibitor concentration and the highest IE can reach 99%. Adsorption of berberine on the coating surface follows Langmuir adsorption isotherm with a single molecular layer by chemisorption.

Relationship between La and Ce additions on microstructure and corrosion resistance of hot-dip galvanized steel

Scanning electron microscopy,X-ray diffraction,a neutral salt spray test,and electrochemical methods were applied to observe the microstructure and test the corrosion resistance of hot-dip galvanized steel before and after the addition of La and Ce.La/Ce mixture existed in the form of(La,Ce)Zn13 on the coated surface,decreased the coating thickness and accelerated the zinc dendritic growth.The corrosion current density of Zn-0.1La-0.1Ce coating was 63%less than that of pure Zn coating.This phenomenon can be explained that La/Ce mixture inhibited the transformation of Zn5(OH)8Cl2·H2O into Zn5(CO3)2(OH)6 or ZnO,reduced the time for appearance of red rust,and thus enhanced the stability of corrosion products and delayed the oxidation and corrosion processes of galvanized coating.La/Ce mixture improved the corrosion resistance compared to a single La or a single Ce addition.A competitive relationship between La and Ce was observed in the corrosion resistance improvement of hot-dip galvanized steel.

Study on the Wiping Gas Jet in Continuous Galvanizing Line

In the continuous hot-dip galvanizing process, the gas-jet wiping is used to control the coating thickness of moving steel strip. The high speed gas-jet discharged from the nozzle slot impinges on the strip, and at this moment, wipes the liquid coating layer dragged by a moving strip. The coating thickness is generally influenced on the flow characteristics of wiping gas-jet such as the impinging pressure distribution, pressure gradient and shear stress distribution on the surface of strip. The flow characteristics of wiping gas-jet mentioned above depends upon considerably both the process operating conditions such as the nozzle pressure, nozzle-to-strip distance and line speed, and the geometry of gas-jet wiping apparatus such as the height of nozzle slot. In the present study, the effect of the geometry of nozzle on the coating thickness is investigated with the help of a computational fluid dynamics method. The height of nozzle slot is varied in the range of 0.6mm to 1.7mm. A finite volume method (FVM) is employed to solve two-dimensional, steady, compressible Navier-Stokes equations. Based upon the results obtained, the effect of the height of nozzle slot in the gas-jet wiping process is discussed in detail. The computational results show that for a given standoff distance between the nozzle to the strip, the effective height of nozzle slot exists in achieving thinner coating thickness.