Hot-dip Aluminizing Research Articles

Microstructure and phase composition of diffusion coating formed in NiCr alloys by hot-dip aluminizing

In this paper, the results of a study of the structure and phase composition of diffusion coatings formed on the surface of Ni80Cr20 and Ni60Cr15 alloys after hot-dip aluminizing and subsequent high-temperature heating are presented. It is shown that as a result of aluminizing on the surface of NiCr alloys a uniform coating without defects is formed.The coating consists of an Al matrix with CrAl7,Ni2Al9,FeNiAl9,NiAl3,Ni2Al3 intermetallic inclusions.It is found that heating of the coatings at 1100 °C leads to a diffusion redistribution of the elements over their thickness and the formation of a layered quasi-two-phase structure of Cr (in case Ni80Cr20 alloy) or Cr and Fe (in case Ni60Cr15 alloy) solid solutions in NiAl with different Al (Al-rich) and Ni (Ni-rich) saturations.At the interfacial boundary of NiAl phases with different composition precipitates are presented as a randomly distributed individual inclusions of solid solutions based on Cr and Cr3Si. At the ”coating - substrate” interface they are presented as a continuous secondary reaction zone, consisting of a large number of precipitates Cr(Ni) and Cr(Ni,Al) (Ni80Cr20 alloy) and a smaller amount of Cr(Fe,Ni) and Cr(Fe,Al,Ni) (Ni60Cr15 alloy). The formation of these heterogeneity elements is due to the limited solubility of Cr in NiAl.A protective oxide α−Al2O3 is formed on the coating surface as a result of heating. An increase in the exposure time leads to the formation of the NiAl2O4 spinel type oxide along with Al2O3 on the surface of the coating on the Ni80Cr20 alloy.

Long-term deterioration mechanism of hot-dip aluminum coating exposed to a coastal-atmospheric environment

This study focused on the long-term corrosion mechanism of hot-dip aluminum (HDA) coating applied to a steel bridge exposed to a coastal-atmospheric environment. Aging HDA samples were taken from the actual bridge, and the corrosion product composition and morphology of the deterioration stages were investigated. The localized environmental assessment indicated that the surface orientation of the structural members led to a significant difference in salt accumulation and corrosion rate, with the most important influence being the rain-washing effect. Besides, open circuit potential, critical pitting potential, and aging sequence of different metallic phases were clarified. In addition, the long-term deterioration mechanism and pitting growth kinetics of aging HDA were speculated and verified with electrochemistry and elemental analysis.

Improving the Corrosion Resistance of API X56 and API X70 Pipeline Steels by Hot Dip Aluminization

The external surface of the pipeline steels can be protected from corrosion by cathodic protection and a suitable coating system. But to protect the internal surface of steel pipelines from corrosion is always a challenge. In the current study, a protective aluminum coating was applied on the internal surface of steel pipeline grades API X56 and API X70 by hot dip aluminization process to minimize the cost of inhibitors. The steel substrates were dipped in the hot aluminum bath, whose temperature was maintained at 720 °C, for three different dipping times (5, 7.5 and 10 minutes) and then permitted to cool to room temperature. The coated specimens were characterized in terms of their structure and composition by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM micrographs analysis reveal that the hot dip aluminized specimens gave an unwavering and well adherent interface which is essential for corrosion resistance of the substrates. The corrosion behavior was evaluated by weight loss method in case of sweet corrosion (in 1.0 % NaCl + CO2 environment) and, by electrochemical potentiodynamic polarization tests for marine corrosion (in 3.5% NaCl environment). The sweet corrosion rates showed severe material loss (14.17 mpy for API X56 and 6.6 mpy in case of API X70) of uncoated samples as compared to the aluminized samples which showed no corrosion at all even after one week due to the passive layer of Al2O3. Whereas the marine corrosion of aluminized specimens was also reduced to 0.11 mpy (coated specimens) from 0.57 mpy (uncoated specimens).

Surface protective layers of iron aluminides developed on medium carbon steel by the hot-dip aluminizing (HDA) process

The investigation in this paper is based on the existence of carbon content and its role in the formation of Fe-Al intermetallic layer in unalloyed carbon steel C45 with a carbon content of 0.44 wt.%. Several sophisticated techniques such as PFIB SEM equipped with EDS and EBSD, GD-OES were employed for the in-depth surface analysis. The results of the metallographic examination reveal that the carbon would appear and could be well detected at the interface between the solidified aluminum and the solid iron-base steel substrate and got also incorporated in the top aluminum layer. Furthermore, due to the dissolution and outward diffusion of iron into the liquid aluminum melt during the HDA process, plus its involvement in the formation of the solid intermetallic surface layer, thus the carbon atoms gaining higher chemical affinity there will also be more likely to form carbide precipitates of different kinds like Fe3AlC and AlC inside these developing surface layers on the C45 type steel during the process of hot-dip aluminizing at 700 °C.

Microstructure evolution and hardness of hot dip aluminized coating on pure iron and EUROFER 97 steel: Effect of substrate chemistry and heat treatment

EUROFER 97 steel, a variation of reduced activation ferritic-martensitic (RAFM) steel, is considered to be a potential material for the first wall of fusion reactor. Aluminide diffusion barrier coatings offer an effective solution to mitigate corrosion and tritium permeation in the first wall. However, the performance of the coating is influenced by the nature of intermetallic phases and their microstructure. In order to critically understand the phase evolution during both hot dip aluminizing (HDA) process and subsequent heat treatment, experiments were performed on (i) pure iron and (ii) EUROFER 97 steel. The samples were hot dip aluminized at 973 K in an inert Ar-5% H2 atmosphere. Detailed metallurgical analysis using backscattered electron detector, electron probe microanalyzer and electron backscattered diffraction technique was performed to examine the evolution of phases and microstructure. It was noted that Fe2Al5 is the major phase forming in both the cases during HDA. Subsequent heat treatment involving solution treatment at 1253 K and ageing at 1033 K led to transformation of Fe2Al5 to FeAl2, FeAl(Cr) and α-Fe(Al, Cr) in HDA EUROFER steel due to interdiffusion of Fe, Al and Cr. However, no trace of FeAl2 could be detected in HDA iron. Thicker and wavy interdiffusion layers were formed in HDA iron, whereas in HDA EUROFER steel near-planar interfaces were present. Micro-pores were observed in all heat treated samples. It was found that the hardness of the phases determined using nanoindentation are in the order of FeAl2(Cr)>Fe2Al5(Cr)>Fe2Al5>FeAl(Cr)>FeAl>α-Fe(Al, Cr)>α-Fe(Al).

Hot corrosion behavior of aluminized and Si-modified aluminized coated IN-738LC produced by a novel hot-dip process

Hot-dip aluminum and Si-modified aluminum diffusion coatings were applied on the substrate of nickel-base superalloy IN-738LC to enhance its hot corrosion behavior for high-temperature applications. Aluminizing salt bath consisted of KCl, NaCl, NaF, Na3AlF6, and Al powder, while the Si-modified salt bath contained NaCl, NaF, KCl, Na2SiF6, and Na3AlF6. A temperature of 750 °C and an operating time of 30 min were applied for coating. At this condition, the thicknesses of about 48 and 35 µm were achieved for aluminum and Si-modified aluminum coatings, respectively. All coated samples were immersed in the molten salt of Na2SO4–25 wt%NaCl composition for 60 and 140 h for the hot corrosion test. It was found that the Si-modified coated samples had better corrosion resistance. The high hot corrosion resistance was believed to be due to the formation of the protective and continuous scale of Al2O3 and SiO2 and no sign of sulfidation was detected at 750 °C.

Evolution of the Microstructure of Intermetallic Compounds Formed on Mild Steel During Hot Dipping in Molten Aluminum Alloy Baths

Hot-dip aluminum coating technique has been applied to improve the high-temperature oxidation resistance of steels. This method is adopted widely due to the low cost and good performance. The purpose of this paper is to study the formation of intermetallic layers during the hot dipping of mild steel into a molten aluminum bath. Mild steel specimens were immersed into molten aluminum alloy baths at 750 °C for 1 h, 2 h and 3 h. Intermetallic compounds were analyzed by optical microscope, scanning electron microscope coupled with energy-dispersive spectroscopy and X-ray diffraction analysis. This study was completed by microhardness testing. The results showed that the hot-dip aluminized layer was divided into an outer pure aluminum or aluminum–silicon topcoat and an intermetallic layer. In the bath of pure aluminum, the intermetallic layer was the thickest and consisted of an outer FeAl3 layer and an inner Fe2Al5 layer adjacent to the steel substrate with tongue-/finger-like morphology, while in the Al–Si alloy bath, the thickness of the intermetallic layer decreased substantially and the interface intermetallic/steel substrate becomes flat. The ternary phases Al8Fe2Si and Fe3Al2Si3 are identified in addition to FeAl3 and Fe2Al5. The microhardness testing recorded high values of the intermetallic layers, up to 800Hv01, which confirms the high brittleness of the intermetallic compounds.

Microstructure evolution and high temperature corrosion behavior of FeCrBSi coatings prepared by laser cladding

In this study, an FeCrBSi coating was deposited on a stainless steel substrate by laser cladding with a 6 kW CO2 laser to improve the hardness and corrosion resistance. The phase composition, microstructure evolution and chemical composition of the coating were investigated via X-ray diffraction (XRD), confocal scanning laser microscopy (CSLM), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDX). Microhardness and high temperature corrosion tests in molten aluminum were also conducted for evaluating the properties of the coating. The XRD pattern suggested that the coating was particularly composed of martensite, (Fe, Cr)7C3 and CrFeB hard phases. The microstructure results exhibited that equiaxed dendrites, dendrite networks, columnar dendrites, cellular and planar grains were distributed from the surface to the bottom of the coating. The hardness of the substrate increased from 217 HV0.2 to 762 HV0.2. Moreover, the laser cladding FeCrBSi coating exhibited good corrosion resistance in molten aluminum, thereby making it a promising candidate in the surface modification and protection of hot dip aluminum equipment.

Effect of plasma processed Ti-Al coating on oxidation and tensile behavior of Ti6Al4V alloy

The investigation of the oxidation resistance and tensile behavior of aluminide coated and uncoated Ti6Al4V at room temperature and at 600 °C was carried out. Ti-Al coating was developed on Ti6Al4V by Hot-dip Aluminizing technique followed by solution treatment (900 °C/1 h), quenching in water and ageing treatment (500 °C/6 h). For a comparative analysis, the diffusion heat treatment was performed with and without oxygen plasma environment. The diffusion treatment without oxygen plasma is referred as thermal treatment while the diffusion treatment with oxygen plasma is referred as plasma treatment. The cyclic oxidation test was carried out for uncoated samples, thermally treated (THT samples) and plasma treated samples (PAHT samples) at 600 °C for 120 h. Significant improvement in the oxidation resistance was observed for coated samples. Weight gain during oxidation studies in PAHT samples is almost 100 times less than uncoated samples and 8 times less than THT samples. The tensile test conducted at room temperature and at 600 °C showed marginal reduction of Ultimate tensile strength (UTS) and Yield strength (YS) of substrate for PAHT samples and THT samples. Plasma processing yields nanocrystalline coatings which have been found to improve oxidation resistance and would help address oxidation issues in gas turbine applications.

Studies on the characterization and morphological features of the coating on interstitial free steel dipped in molten Al-Si-Mg alloy at 800 °C

Interstitial-free (IF) steel specimens were coated by hot-dip aluminizing process bearing bath composition as Al-6.9% Si-1.4% Mg and bath temperature at 800°C. Dipping time varied for 0.5, 1, 3 and 6min. The objective of the present study is to conduct a systematic investigation to identify and analyse the formation of various phases and compounds in the coating. Studies on aluminizing of steel had indicated the presence of an intermetallic layer adjacent to steel substrate. In this study, the morphological features obtained after hot dipping at 800°C were investigated. For the application of aluminized steel sheets in industry for manufacture of tubes and pipes, the adherence of the aluminized coating must be ensured. This property of the coating is influenced by the morphological features of the coating. Aluminizing being thought of a possible substitution of galvanizing owing to its promise to evolve lesser volume fraction of intermetallic phases and consequently known to generate a ductile coating. The entire coating consists of an outer Al-Si topcoat and a thin intermetallic layer which composed of an outer θ-FeAl3 and inner η-Fe2Al5 layers. The thickness of intermetallic layers was found to increase with the increase of dipping time in a near parabolic manner.The phases formed on the IF steel substrates were characterized by scanning electron microscopy with EDS, X-ray diffraction, EPMA and EBSD techniques.Interesting findings from XRD include the presence of Al3FeSi2 (τ4) and Al2FeSi (τ3) phases, which normally appear after annealing of aluminised sheet. A distinct elemental Si peak was found in XRD study. Presence of Mg in the form of Mg2Si was also found in XRD and supported by EPMA investigation. Presence of several ternary Al-Fe-Si (τ3, τ4, τ5) and binary FeAl3 and Fe2Al5intermetallic phases were identified by composition profile analysis from SEM based EDS and GDOES and further confirmed by EBSD. Presence of Si humps in the top and intermetallic layer attributed by the presence of Si-bearing phases or elemental pro-eutectic Si in Al-Si alloy were also indicated in SEM-EDS and GDOES study. Variation of surface topographical features of the HDA steel substrates with increasing dipping durations were examined and reported for the first time by using AFM. Formability tests were conducted by bending the samples 180° over a mandrel. None of the specimens showed any tendency of flaking exhibiting excellent bonding of the coating with IF steel substrate.

Low Cycle Fatigue Properties of Aluminizing Coating on Cold-Drawn AISI 1018 Steel

The mechanical and the low cycle fatigue (LCF) properties of cold-drawn AISI 1018 steel (CDS 1018) with and without hot-dip aluminizing (HDA) coating had been experimentally investigated at room temperature. The tensile properties and the LCF properties were determined from tensile test results and cyclic strain-fatigue test results, respectively. The aluminide coating on the CDS 1018 significantly decreases the mechanical properties and the strain-fatigue life of the steel. With increasing in strain amplitude levels, CDS 1018 experienced continuously cyclic softening behaviour after cyclic loading in few cycles. By contrast, the cyclic softening of HDA steel was observed in few cycles and HDA steel continuously exhibits the stably cyclic hardening behaviour until failure. CDS 1018 with aluminide coating results the longer transition fatigue cycles (Nt) than those of CDS 1018 without aluminide coating by a factor of ∼3.0. CDS 1018 with HDA coating can be used for material components which the components are safely designed under cyclic loading below the yield strength of the material.

Microstructure, Mechanical Properties and Fracture Behavior of Magnesium/Steel Bimetal Using Compound Casting Assisted with Hot-Dip Aluminizing

In this work, microstructure, mechanical properties and fracture behavior of the magnesium/steel bimetal using compound casting assisted with hot-dip aluminizing were investigated, and the interface bonding mechanism of the magnesium/steel bimetal were also analyzed. The results indicate that the magnesium/steel bimetal obtained without hot-dip aluminizing had larger gaps through the whole interface without reaction layers between magnesium and steel, leading to a poor mechanical bonding. After the steel substrate was hot-dip aluminized, an intermetallic layer along with an Al topcoat layer were formed on the surface of the steel substrate, and the intermetallic layer was constituted by Fe2Al5, τ10-Al9Fe4Si3, FeAl3 and τ6-Al4.5FeSi phases. In the case of the magnesium/steel bimetal obtained with hot-dip aluminizing, a compact and uniform interface layer with an average thickness of about 17 μm that consisted of Fe2Al5, τ10-Al9Fe4Si3, FeAl3 and Al12Mg17 intermetallic compounds was formed between the magnesium and the steel, obtaining a superior metallurgical bonding. The interface layer had much higher nano-hardnesses compared to the magnesium and steel matrixes, and its average nano-hardness was up to 11.1 GPa, while there were respectively 1.1 and 4.2 GPa for the magnesium and steel matrixes. The shear strength of the magnesium/steel bimetal with hot-dip aluminizing reached to 23.3 MPa, which increased by 8.59 times than that of the composites without hot-dip aluminizing. The fracture of the magnesium/steel bimetal with hot-dip aluminizing represented a brittle fracture nature, initiating from the interface layer.

High-temperature oxidation behaviors of plasma electrolytic oxidation coating on hot-dip aluminized HP40Nb alloy

In the current investigation, HP40Nb heat-resistant alloy was successfully aluminized. The procedure of aluminizing consisted of pack aluminizing and subsequent hot-dip aluminizing (HDA) process. Plasma electrolytic oxidation (PEO) ceramic coatings were also synthesized on the surface of hot-dip aluminized HP40Nb alloys. PEO coatings were made using aluminate (mixture of 2 g/L KOH, 5 g/L NaAlO2, and deionized water) and silicate (mixture of 2 g/L KOH, 5 g/L Na2SiO3, and deionized water) electrolytes at 440 V for 10 min. To evaluate the high-temperature oxidation behavior of PEO coatings, interrupted isothermal oxidation at 1100 °C for 100 h was employed. The surface and cross-sectional morphologies before and after high-temperature oxidation test, chemical composition, and phase identification of the coatings were characterized by field emission scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results revealed that the PEO coating using aluminate electrolyte could increase the oxidation resistance of HP40Nb alloy, more than pack aluminized, hot-dip aluminized samples, and also the PEO coating using silicate electrolyte. The PEO coating using silicate electrolyte contains relatively big surface cracks, which can be responsible for oxide spallation during high-temperature oxidation. The weight gain of samples shows that oxidation resistance of hot-dip aluminized specimen, improved 1.2 times by PEO treatment in aluminate electrolyte. The Kirkendall voids may have a deteriorated effect on the anti-oxidation performance of hot-dip aluminized and PEO coated samples.

Investigation of hot-dip aluminized Ti6Al4V alloy processed by different thermal treatments in an oxidizing atmosphere

Plasma assisted aluminide coating was formed on Ti6Al4V alloy by hot-dip aluminizing (HDA) followed by two-step diffusion heat treatment. The HDA coated Ti6Al4V alloys were subjected to solution annealing (900°C/1h) and aging heat treatment (500°C/6h) by plasma processing route and compared with thermal treatment. The resultant coated and heat-treated samples were characterized using SEM for surface morphology and microstructure analysis, EDS for elemental analysis and x-ray diffraction (XRD) for phase analysis. From the characterization, it was observed that after the diffusion heat treatments, the microstructure of aluminized coating consisted of an outer layer of Al2O3 followed by TiAl3, TiAl2, Ti-Al, and Ti3Al phases. In the diffusion zone, hardness variation in the range of 366–490HV0.05 and 342–525HV0.05 was observed in case of thermal and plasma-treated samples respectively. However, in the diffusion zone, the microhardness of plasma-treated samples was slightly higher than the thermally treated samples. The plasma ion bombardment appears to alter the crystalline size due to vacancy concentration which leads to increment in the hardness value. The details of experiments, characterization along with the interpretations and comparison of various microstructural aspects have been presented in this manuscript.

Influencing mechanism of pre-existing nanoscale Al5Fe2 phase on Mg–Fe interface in friction stir spot welded Al-free ZK60–Q235 joint

Al-free ZK60 magnesium (Mg) alloy sheet was selected as substrate material of Mg–steel pinless friction stir spot welding (FSSW), avoiding the effect of the Al element in the substrate on the alloying reaction of Mg-iron (Fe) interface. The sound FSSW joint of ZK60 Mg alloy and Q235 steel with a hot-dipped aluminum (Al)-containing zinc (Zn) coating was successfully realized. The detailed microstructural examinations proved that Al5Fe2 phase at the Mg-Fe interface came from the pre-existing Al5Fe2 phase in the coating and acted as the transition layer for promoting the metallurgical bonding of Mg and Fe. The interfaces with well-matched lattice sites among Fe, Al5Fe2 and Mg were formed during FSSW. A low energy interface with good match of lattice sites ((002)Al5Fe2//(110)Fe, [110]Al5Fe2//[1¯13]Fe) between Al5Fe2 and Fe was identified. For the interface between Al5Fe2 and Mg, an orientation relationship of (62¯2)Al5Fe2//(31¯1¯2¯)Mgand[15¯8¯]Al5Fe2//[24¯23]Mg was observed. The tensile–shear load of the ZK60–steel joint could reach 4.6 kN. Moreover, the joint fracture occurred at the interface between the Al5Fe2 layer and the Mg alloy substrate, suggesting the brittle fracture characteristic.

GTA Weld Brazing a Joint of Aluminum to Stainless Steel

The relationship between heat input, microstructures, and mechanical properties was studied in the gas tungsten arc (GTA) weld brazing process of aluminum alloy and stain-less steel dissimilar metals with a pure aluminum core wire (ER1100). The mechanisms involved were further revealed by hot-dip aluminizing experiments. The intermetallic compounds (IMCs) of the brazing interface consisted of a thin layer of -Fe2Al5 next to the steel, and a thick layer of -Fe4Al13 adjacent to the Al joint, surrounded by eutectic identified to be Al-FeAl6 and Al-Fe2Al9 distributed uniformly in the weld metal. With increasing heat input, the total thickness of the IMCs decreased first, reaching a minimum value of 3.5 m, and then increased. The decrease in IMC thickness with heat input was observed for the first time and had never been reported before in welding or brazing processes. The joint strength increased to a maximum value of 238 MPa and then declined, suggesting an inverse relationship between IMC thickness and joint strength. The results of the hot-dip aluminizing, regarding the relationship between IMC thickness and temperature, were consistent with the change of IMC thickness in the welds. A possible mechanism in action is that interfacial kinetics and thermodynamics play a role in the dissolution and decomposition of the thick layer of -Fe4Al13 into the FeAl6 and Fe2Al9 phase. Promoting the IMC dissolution or decomposition by adjusting welding procedures is a promising new way to control the IMC growth and improve the joint strength.

Morphology of composite coatings formed on Mo1 substrate using hot-dip aluminising and micro-arc oxidation techniques

A hot-dip aluminised layer was prepared on a Mo1 substrate and the obtained layer was oxidised in situ by a micro-arc oxidation (MAO) technique to form an oxide ceramic coating. The obtained hot-dip aluminised layer had a two-layer structure overall: the outer layer was a pure aluminium layer, and the inner layer was a diffusion layer. The hot-dip aluminising temperature directly influenced the structure and phase composition of the obtained diffusion layer. The diffusion layer of the hot-dip aluminised layer at 710 °C was divided into three layers. The inner, intermediate, and outer layers were mainly composed of Al8Mo3, Al4Mo, and Al12Mo phases, respectively. The diffusion layer of the hot-dip aluminised layer at 750 °C was divided into two layers: the inner layer was mainly composed of an Al8Mo3 phase, and the outer layer of Al4Mo. After the hot-dip aluminised layer was treated by MAO, a three-layer coating was obtained. The outer layer was a ceramic one obtained by MAO, which was mainly composed of Al2O3, mullite, and unoxidized aluminium. The middle one was pure aluminium obtained by hot-dip aluminising and without MAO. The inner layer was an Al–Mo diffusion layer.

Electrode insulation layer for electrochemical machining fabricated through hot-dip aluminizing and microarc oxidation on a stainless-steel substrate

Insulation of an electrode for use with an electrochemical machining (ECM) was achieved through hot-dip aluminizing and microarc oxidation techniques. To form an effective insulation layer on a stainless-steel substrate and achieve high precision in ECM, an electrode was processed through microarc oxidation, and the aluminum-rich layer was converted into an aluminum oxide insulating layer. The withstand voltage of the tool was evaluated through sodium chloride electrolysis, and the surface and cross-section were observed. The ECM performance of various tools with and without aluminum oxide insulating layer was examined by drilling on a stainless-steel workpiece. A precise and robust insulation layer must be produced. Precision can be achieved by reducing the stray effects of ECM.Experimental results indicated that the optimal parameters were aluminum dipping were 4 min, microarc voltage of 475 V for 10 min with a withstand voltage of 9.3 V. Electrochemical drilling was used to examine electrodes with and without an insulation layer. Improvement of variation between entrance and exit could be 64.58 %. The differences between the entrance and exit values indicated that the electrochemical insulating layer considerably reduces stray current and improves drilling accuracy.

Effect of Iron-Aluminide Coating on the Fracture Mechanism of Ferritic–Martensitic Steel in Coal-Fired Boilers Environment

Due to the coal combustion that generates halides, steel components can confront hot corrosion during applications at high temperature. The hot-dipping aluminum (HDA) was operated on the 9Cr–Mo steel (grade 91) to form the iron aluminide layer. A hot corrosion-loading test of aluminized grade 91 (HDA-91) was carried out by covering a salt mixture of NaCl/Na2SO4 under static load ranging from 75 to 100 MPa at 600 °C and 700 °C, respectively. The failure mechanism was assessed after various elongations using scanning electron microscopy and optical microscopy. The results showed that HDA-91 presented higher hot corrosion resistance than the uncoated grade 91. The aluminide layer formed a higher ductility oxide and prevented the substrate form grain-boundary oxidation at high temperatures, resulting in durability. The results also revealed a significant improvement in reduction of area during the hot corrosion-loading test for grade 91 that underwent HDA treatment.

The Influence of Nickel Additions to Liquid A359 on Hot-Dip Aluminizing of Solid S45C Steel

The properties of hot-dipping A359 aluminum alloys can be improved by strategically placing the S45C steel inserts to locally improve properties such as oxidation and corrosion resistance, but soft A359 aluminum alloy can be easily worn out and thus decreased the S45C steel’s reliability. In this study, we have been added 20 wt.%Ni to increases the hardness of the A359 aluminum alloy, then hot dipped the S45C steel into the molten A359-20 wt.%Ni aluminum alloy for 1 min and then heat treated at 400-600 °C. The effect of nickel on the interface characteristic (including kinetic and shear strengths of interface between the S45C steel and aluminum alloy) and the surface glossiness and wear property of the A359-Ni aluminum alloy is also discussed. The experimental results show that the Fe2Al5 intermetallic compound thus the activation energy of the compound formation in the A359-20 wt.%Ni aluminum alloy, and its thickness increased with an increase in heating temperature. Nickel hindered formation of the Fe2Al5 intermetallic compound thus the activation energy of the compound formation in the A359-20 wt.%Ni aluminum alloy (0.413 eV) is larger than A359 aluminum alloy. The nickel could increase the surface glossiness and anti-wear of the hot-dipped coating layer, but they would not deteriorate the interface shear strength.