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The analysis of the influence of the parameters of electrospark alloying with an aluminium electrode on the quality (roughness, microstructure of the coating, its continuity, phase composition, and microhardness) of the aluminized layer is presented. The effect of finishing methods after aluminizing is evaluated. The heat resistance of the obtained coatings is studied. Metallographic analysis shows that the coating consists of three sections: a ‘white’ layer, a diffusion zone, and the base metal. With an increase in the discharge energy, such quality parameters of the surface layer as thickness, microhardness of both a ‘white’ layer and a transition zone, and roughness are increased. The continuity of a ‘white’ layer at the discharge energy Wp = 0.52 J is low (of 50–60%); with a subsequent increase in the discharge energy, it increases and, at Wp = 6.8 J, it is of 100%. An increase in the discharge energy during electric-spark alloying (ESA) leads to a change in the chemical and phase compositions of the layer: at low discharge energies, a layer is formed, consisting mainly of α-Fe and aluminium oxides. As Wp increases, the layer consists of iron and aluminium intermetallic compounds, as well as free aluminium, that is confirmed by the data of local x-ray microanalysis. For practical application, it is possible to recommend the process of aluminizing by the ESA method, using the modes (discharge energy in the range of 4.6–6.8 J and productivity of 2.0–3.0 cm2/min). Such process provides the formation of a ‘white’ layer with a thickness of 70–130 µm, microhardness of 5000–7500 MPa, roughness (Ra) of 6–9 µm, and continuity of 95–100%. In order to increase the thickness of the aluminized layer, it is recommended to preliminarily apply grease containing aluminium powder to the steel surface and, without waiting for it to dry, carry out ESA with an aluminium electrode. In this case, the coating continuity is of 100%, the layer thickness is of up to 200 µm, and the microhardness is of 4500 MPa. The paper presents the results of a study of the quality parameters of multicomponent aluminium-containing coatings of Al–S, Al–C–S, and Al–C–B systems. Replacing the aluminium electrode with graphite one leads to a decrease in the thickness and continuity of a ‘white’ layer, respectively, to 50 µm and 30%. In turn, the microhardness on the surface increases to 9000 MPa. The addition of 0.7 boron to the consistency substance leads to an increase in the thickness and continuity of a ‘white’ layer, respectively, up to 60 µm and 70%. The microhardness on the surface rises to 12000 MPa. In order to reduce the roughness of the surface layer and to obtain continuous coatings, it is recommended to carry out ESA with an aluminium electrode, but at lower modes.

The development of high-performance aero-engines raises the urgent requirement of coatings deposited on blades, containing both thermal barrier coatings (TBCs) on the external surface and aluminide coatings in the internal winding cooling channels. A TBC system with a low activity aluminizing process before deposition of conventional TBC was developed here to evaluate the feasibility of co-deposition technology. The thermal cyclic oxidation behavior of co-deposited “Al-modified TBC” was examined at 1100 °C and compared to that of conventional TBC. The cross-sectional microstructure evolution and elemental diffusion behavior of the modified TBC were also investigated. Even though the underlying aluminizing layer can serve abundant Al for the upper NiCoCrAlYHf layer to eliminate “Al depletion” in conventional TBC, which is a main reason for spallation, the Al-modified TBC system exhibited shorter thermal cycling life than conventional TBC. The spallation mechanism was discussed. Cracks caused by the fast growth rate of thermally grown oxide (TGO), Cr-rich precipitates resulted from the decreased γ/γ′-phase in the coating, and Re-rich precipitates originated from outward diffusion through the aluminide layer were mainly responsible for the failure.

Chromium-molybdenum steel (16Mo3) is widely used in petroleum, gas, automotive, and construction industries due to its good oxidation resistance and mechanical properties at moderately elevated temperatures. The aim of the research was to evaluate the corrosion susceptibility of 16Mo3 steel in hot rolled and aluminized states. Aluminization was performed by diffusion pack aluminization process at 900°C/2h and 730°C/4h, respectively. Electrochemical corrosion testing included measuring open circuit potential (EOCP), linear polarization resistance (LPR), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in potassium phosphate buffer (KH2PO4, pH = 7). Optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used for surface layer microstructure characterization before and after corrosion tests. It was demonstrated that corrosion resistance of aluminized steel increased substantially. Corrosion properties were related to the structure and properties of intermetallic phase (FeAl, FeAl2 and Fe2Al5) that formed on the surface of 16Mo3 steel.

The growing interest in intermetallic and metal–intermetallic materials and coatings is based on the number of favorable properties they possess, primarily mechanical. However, the lack of data on their corrosion resistance has largely limited their scope of application. In this study, the corrosion destruction mechanisms of coatings formed on substrates made of AISI 321 steel and Aluchrom W (fechralloy) were investigated. The coatings were created by alloying in an aluminum melt followed by diffusion annealing to form the ultimate intermetallic structure. Corrosion resistance was studied under cyclic exposure to a humid marine atmosphere simulator and potentiostatic tests in an aqueous NaCl solution. Corrosion destruction parameters were determined, and mechanisms for each type of coating were revealed. The conducted studies allowed us to determine the electrochemical parameters of the corrosion destruction process and its mechanisms. It was shown that the corrosion rates during potentiostating for coatings on substrates Cr15Al5 and 12Cr18Ni10Ti differed by almost twofold. Two different mechanisms of corrosion are proposed. The first is associated with the formation of Al2O3 and MgO oxide films, which at the initial stage protect only local areas of the coating surface on Cr15Al5. The second is determined by the diffusion of titanium atoms during annealing to the coating surface on a 12Cr18Ni10Ti steel substrate with the formation of TiC carbide at the grain boundaries.

Platinum-modified-βNiAl is a bond coat (BC) material for thermal barrier coatings (TBCs) applications applied on aeroengine hardware to reduce their surface temperatures. However, it is desirable to minimize its production and material costs by the low-cost alternatives of similar performance. It has been acknowledged that the small concentration of the reactive elements (REs), such as Zr, Hf, and Y, could tremendously enhance the oxide adhesion even in some cases better than Pt modified counterparts. The aim of this study was to design and fabricate the Zr-modified-βNiAl bond coat on CMSX-4 superalloy using an aluminizing method. Moreover, the study focused on the development of a systematic understanding of underlying mechanisms behind the beneficial effects of REs. Initially, three sets of BCs were prepared: Zr-free βNiAl (undoped), Al and Zr co-deposited in a single-step process (1SP), and Zr and Al, which were individually deposited in two processing steps (2SP): zirconizing and aluminizing. Such three sets of BCs helped to understand the processing, as well as Zr and Al effects on scale adhesion. In particular, 1SP/2SP BCs showed uniformity of Zr in the form of precipitates and networks that caused hardness enhancement. All BCs were isothermally oxidized at 1150°C for 100 hours wherein 2SP revealed the best spallation resistance, microstructural stability, and its Zr-oxide pegs were extended to substrates. In addition to the Zr effect, BC Al content was found to affect the oxide adhesion equally. Under identical Zr contents (of 1SP and 2SP = 1at %), the higher Al showed the better spallation resistance while lower Al caused the inverse effect of Zr owing to its reactive nature that was termed as over doping. Moreover, it was established that over-doping either local or into entire BC, accelerated the Al depletion that destabilized the βNiAl into γ’-Ni3Al phase. An extensive discussion is presented in the light of the observed results.

Steel that has been aluminized said as hot dipping aluminizing has better protection against corrosion and can protect against temperatures as high as 800°C. In hot dipping aluminizing, Steel is immersed into a molten aluminium for certain dipping time. The research aims to know the effect of preheating and dipping time on the toughness of low carbon steel. The method research was started by cutting the low carbon steel plate, according to ASTM E23 (Charpy test sample) into 16 pieces samples. Then the samples were grouped into four groups. Group-1 was initial samples. The Group-2 was directly immersed into molten aluminum 700 0C, for dipping time 5 minutes. The Group-3 was preheated at 700 oC for 30 minutes and then to be aluminized (700 oC) for dipping time 5 minutes. The Group-4 was preheated at 700 oC for 30 minutes and then to be aluminized (700 oC) for dipping time 10 minutes. Finally, all groups were tested by the Charpy test at room temperature. The results show that the aluminizing increases the toughness of low carbon steel from 228.125 KJ/m2 to 312.5 KJ/m2. The preheating process before aluminizing increases sharply the toughness of low carbon steel from 228.125 KJ/m2 to 512.5 KJ/m2. The increasing dipping time from 5-minute to 10-minute increase gradually the toughness from 512.5 KJ/m2 to 556.25 KJ/m2.

In the present work, the tensile properties and microstructure of pack cementation preparing aluminized AISI 321 stainless steel with subsequent annealing treatment was investigated. The results reveal that the coatings of aluminized AISI 321 stainless steel are mainly composed of outermost Al2O3 layer, Fe-Al compound intermediate layer and Fe(Al,Cr) solid solution diffusion layer. The cross shape precipitate (NiAl) and strip precipitate (Ni3Al) are observed in Fe(Al,Cr) layer. After annealing, no new phase in aluminized coating is detected, meanwhile, the thickness of the aluminized coatings is increased and the porosity of Fe-Al layer is increased as well. In addition, the size of NiAl precipitates is decreased. The strength and plasticity of stainless steel are degraded by aluminizing treatment. However, after annealing, the aluminized steel exhibits a lower strength but a higher ductility. The crack initiation region of both aluminized steel with or without annealing treatment comprise coarse columnar grains and cleavage planes with river patterns. The fracture model of aluminized steel is cleavage fracture, while a mixed of intergranular and transgranular fracture is observed in aluminized annealed steel.

Aluminum-based coatings were prepared by flame spraying. After remelting and diffusion treatment, the phase, composition, friction and corrosion resistance were analyzed experimentally by using scanning electron microscopy, X-ray diffraction, energy dispersive spectrometer, wear tester. The results indicate that the surface phase consists of Al and FeAl3, inner layer phase mainly contains FeAl 3 and Fe2 Al5. The coating has higher micro-hardness than the underlying material, with a maximum of up to HV 950. In an experiment of high-temperature oxidation at 800℃ and 900℃, the spray-aluminized samples show almost no weight gain, and the antioxidant properties of cast iron improve substantially (by dozens of times). In addition, the samples show excellent wear and corrosion resistance.

The present technical solution refers to the field of electrophysical and electrochemical processing of parts, in particular, to the electroerosion alloying (EEA) of the surfaces of steel parts with aluminum (aluminizing) and sulfur (sulfidizing), and it can be used to treat the surfaces of heat-treated steel parts in order to increase their hardness, wear resistance, to prevent frictional seizure and improve the resistance to atmospheric corrosion. When aluminizing steel parts with the use of the method of electroerosion alloying (EEA) by aluminum electrode at discharge energy Wp = 0.52–6.8 J and productivity of 1.0–3.0 cm2 / min, before the EEA process by an aluminum electrode, to the surface of the part to be aluminized, there is applied a consistency substance containing sulfur and aluminum powder, and thereafter, not having waited for drying of the consistency substance, the process of aluminizing by the EEL method with an aluminum electrode is carried out, and the consistency substance should have the aluminum powder content of not more than 56 %. There have been carried out metallographic and durametric analyses of the features of the surface layers made of carbon steels after simultaneous aluminizing and sulfidizing them by the EEA method. It is shown that the structure of the layer consists of three portions, namely, a “white” layer, a diffusion zone and a base metal. Such qualitative surface layer parameters as thickness, “white” layer and transition zone microhardness values, and also roughness increase with increasing discharge energy. The “white” layer continuity for all the investigated discharge energies of Wp = 0.52, 2.60 and 6.80 J is 100 %.

The coating thickness is an important factor to evaluate the coating quality and determining the properties of the hot-dip aluminizing (HDA)coating. In the present work, a hot dipping pure aluminum (99%) on stainless steel (AISI 303) rods was carried out for different diameters of rods (8, 10 and 12mm) and different lengths (250 and 500 mm) at different aluminizing conditions of temperature and time. The dipping temperature was set to 700, 740, 780, 820 and 860℃ .The dipping time was set to 1, 2, 3,4 and 5 minutes. A response surface methodology (RSM) using a central composite rotatable design (CCD) for a 2³ factorial, with 5 central points and α = ±2 approach, based on the experimental data, was used to obtain the optimum modelto get the best thickness of coating and the best conditions of dipping. A 2nd polynomial model was obtained with a confiding percentage of 95%.Analysis of the experiments using RSM indicated that 807℃ and3minare optimum dipping conditions for hot-dip aluminizing process with corresponding thickness of coating layers of 134 μmto Al layer,62.9 μm to intermetallic compound (IMC) layer and 197μm to total coating layer.

This study presents a duplex treatment (aluminazing and nitrating) applied to some steels aiming atimproving the properties of the superficial coating, increasing hardness and resistance against wear. Thus, therewere executed 8 parts having cylindric form with a diameter of 20mm and thickness of 8mm using 15Cr8 and40Cr10 as material and a sample with the same sizes from 39CrAl6 and 34CrAlNi7. The samples made of15Cr8 and divided in two groups of four parts each were labelled, participating in the first group only for tincoatingand the second group both for aluminazing and nitriding. The sample made of 39CrAl6 and34CrAlNi7 was labelled and participated only in nitrating. On the samples were done visualization using themetalic graphical microscope and also micro-hardness measurements. The obtained results were eloquentconfirming the possibility of modifying the superficial coatings, improving their properties.

In this work a pack cementation of germanium-doped aluminum and silicon coatings on low alloy steel type-T21 has been applied. This gives significant improvement in the oxidation. Steel-T21 was coated with germanium-doped aluminizing-siliconizing. Diffusion coating was carried out at 1050oC for 6 h under an Ar atmosphere by simultaneous germanium-doped aluminizing-siliconzing process. Cyclic oxidation tests were conducted on the coated steel-T21 alloy in the temperature range oxide 300-900oC in air for 102 h at 3 h cycle. The results showed that the oxidation kinetics for coated system in air can be represented by parabolic curve .Oxide phases that formed on coated system are SiO2 and Cr2O3. A neural network model of oxidation kinetics has been proposed to model the oxidation kinetic. The neural model shows good agreement with the experimental data.

High power pulsed Neodymium doped Yttrium Aluminum Garnet- (Nd– YAG)- laser with 1J power was used to fuse coatings containing a mixture of FeAlO and Al. This mixture produced by aluminizing pack cementation treatment and thermal spray of low alloy steel, That is for Studying the mechanical , surface and metallurgical properties for each coating system , and to make a comparison before and after irradiation , and also the specified properties including the coating thickness . Aluminum Coated samples were made by two advanced techniques pack cementation and thermal spray methods in order to compare the resulted coating layer properties before and after irradiation, and also to determine the technique role from one hand ,also fusing effect and thermal residual stresses role which resulted from irradiation in the variation of studied properties on the other. Results showed that pack cementation coating layer has roughness, Micro hardness and wear resistance higher than thermal spray coating layer. Irradiation results was showed an improvement in studied properties, this was attributed to the fusing, thermal residual stresses and laser rays role in refining the grains which are in conjunction with finer microstructure . After irradiation a formation of a hard phase (martensite) was recorded, this was attributed to the sufficient percentage of carbon (0.4%) in the selected alloy, this was enshured by microstructure images taken by optical microscopy. X-ray diffraction showed, for laser surface irradiated coating layers, presence of the phase due to interaction between the base metal constituents. A modification of coating phases composition could be concluded for both coating techniques. Also the two techniques illustrate a formation of coating layer with a different thickness.

Coatings are seen a promising way to improve the corrosion resistance of relatively cheap power plant steels to enable higher steam temperatures than currently in use. In this research, 9–12% Cr steels P91 and HCM12A are coated with aluminium diffusion coating by a slurry method and exposed for 336 hours at 833 K and 883 K to atmospheres containing varying amounts of O2, H2O, HCl and SO2. Corrosion behaviour of the coated steels is compared to that of those steels in an uncoated condition. Characterization is performed by weighing, SEM + EDS and XRD. The results show that corrosion resistance of P91 and HCM12A is significantly improved by the aluminium diffusion coating at high temperatures in atmospheres containing HCl and SO2. The corrosion rate of the aluminized specimens slightly increases with increase in test temperature but remains virtually the same irrespective of the composition of the atmosphere. On the other hand, the corrosion rate of the uncoated specimens is dependent on both the atmosphere and the temperature. The steels undergo active oxidation that results in formation of non-protective, thick and layered scales in HCl containing atmospheres. SO2 addition slightly decreases the corrosion rate although it is anyway higher than that in SO2 containing atmosphere without HCl.

Formation mechanism of silicon modified aluminide coating applied on a nickel base super alloy IN-738 LC by pack cementation process was the subject of investigation in this research. Study of the microstructure and compositions of the coating was carried out, using optical and scanning electron microscopes, EDS and X-ray diffraction (XRD) techniques. The results showed that due to low partial pressure of silicon halide in Pack process, the amount of soluble silicon in the coating can not exceed 1 wt % of the total coating composition, although the Si content of the particles present within the outer coating sub-layer could reach as far as 5 wt%. Thus, the small amount of soluble Si within the coating could not provide the necessary conditions for formation of any intermetallic and it seems that the formation and growth behavior of various sub-layers in Si-modified aluminide coating is similar to that of simple aluminide coating. Three sub-layers were detected in the coating structure after being subjected to diffusion heat treatment. They were an outer Ni-rich NiAl sub-layer a middle Ni-rich NiAl and an inter diffusion sub-layers. The details of formations and growth mechanism of these sub-layers has been discussed in this research.

The isothermal oxidation resistance of Al0.2Co1.5CrFeNi1.5Ti0.3 high-entropy alloy is analyzed and the microstructural evolution of the oxide layer is studied. The limited aluminum, about 3.6 at %, leads to the non-continuous alumina. The present alloy is insufficient for severe circumstances only due to chromium oxide that is 10 μm after 1173 K for 360 h. Thus, the aluminized high-entropy alloys (HEAs) are further prepared by the industrial packing cementation process at 1273 K and 1323 K. The aluminizing coating is 50 μm at 1273 K after 5 h. The coating growth is controlled by the diffusion of aluminum. The interdiffusion zone reveals two regions that are the Ti-, Co-, Ni-rich area and the Fe-, Cr-rich area. The oxidation resistance of aluminizing HEA improves outstandingly, and sustains at 1173 K and 1273 K for 441 h without any spallation. The alumina at the surface and the stable interface contribute to the performance of this Al0.2Co1.5CrFeNi1.5Ti0.3 alloy.