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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. [ABSTRACT FROM AUTHOR]

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.

Modification of the surface is an effective approach for increasing the corrosion resistance of Mg alloys without influencing their bulk properties. Among numerous surface modification techniques, aluminizing is a low-cost and effective approach that is applied on many alloys for improved resistance to wear and corrosion. Based on the previous research, the enhancement by aluminizing can be further increased by the addition of rare-earth elements such as Y. In the current work, the effect of the addition of yttrium to the aluminizing mixture on the corrosion and tribological behavior of Mg-3%Al alloy was studied. Ball-on-disk tribometer and electrochemical testing apparatus were used to test wear and corrosion behavior and scanning electron microscopy was utilized to study the microstructures and worn surfaces. The results demonstrated that the aluminizing process is beneficial to the resistance of the alloy to wear, corrosion, and corrosive wear. The addition of Y makes the benefits larger. By adding Y, the wear volume loss decreased from 5.6×107 to 3.5×107 μm3 and the corrosive wear volume loss decreased from 4.1×106 to 3.5×106 μm3. In addition, incorporating Y in the aluminizing mixture resulted in a decrease in corrosion current from 4.59×10-2 to 6.17×10-3 μA. The improved properties are mainly attributed to the improved passivation capability of the modified surface layer. [ABSTRACT FROM AUTHOR]

Surface modification is an effective approach for improving Mg alloys to corrosion and wear. Among various surface techniques, aluminizing is an economical one, which has been applied to various metallic materials for increased resistances to oxidation, corrosion, wear, and corrosive wear. The effectiveness of aluminizing could be further improved by surface nanocrystallization. In this work, effects of surface nanocrystallization on the resistance of aluminized Mg-3Al alloy to corrosion, wear, and corrosive wear were investigated. The results demonstrate that aluminizing increases the resistance of Mg-3Al alloy to wear and corrosion, resulting from introduced passivation capability and hard β-Mg 17 Al 12 phase. Surface nanocrystallization maximizes the benefits, largely attributed to further strengthened surface layer with stronger passivation capability and a more protective film. [ABSTRACT FROM AUTHOR]

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.

Highlights: [•] Slurry aluminized and uncoated 9–12wt.% Cr steels exposed to KCl deposit in various atmospheres. [•] Corrosion rates and mechanisms at 560 and 610°C are studied and compared. [•] The materials are degraded by active oxidation process. [•] The coating does not provide the steels with protection against corrosion beneath KCl deposit. [ABSTRACT FROM AUTHOR]

Alloys of Fe-25.8 %Cr-19.5 %Ni (SUS310 stainless steel) were either chromized or aluminized via pack cementation, and corroded at 800 °C for 100 h in 1 atm of (0.9448 atm of N + 0.031 atm of HO + 0.0242 atm of HS)-mixed gases. The chromized layer consisted primarily of CrFe and some CrC. Its corrosion resulted in the formation of CrS and some FeS and FeNiS. The aluminized coating consisted primarily of FeAl. Its corrosion resulted in the formation of α-AlO, AlS, and CrS. Aluminizing was more effective than chromizing in increasing the corrosion resistance of the substrate, due mainly to the formation of α-AlO. [ABSTRACT FROM AUTHOR]

Abstract: Aluminizing is often used to improve steel''s resistances to corrosion, oxidation and wear. This article reports our recent attempts to further improve aluminized carbon steel through surface nanocrystallization for higher resistances to corrosion and corrosive wear. The surface nanocrystallization was achieved using a process combining sandblasting and recovery heat treatment. The entire surface modification process includes dipping carbon steel specimens into a molten Al pool to form an Al coat, subsequent diffusion treatment at elevated temperature to form an aluminized layer, sandblasting to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. The grain size of the nanocrystallized aluminized surface layer was in the range of 20–100nm. Electrochemical properties, electron work function (EWF), and corrosive wear of the nanocrystalline alloyed surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface of carbon steel exhibited improved resistances to corrosion, wear and corrosive wear. The passive film developed on the nanocrystallized aluminized surface was also evaluated in terms of its mechanical properties and adherence to the substrate. [Copyright &y& Elsevier]

Abstract: Aluminum-plated or aluminized steel has been used in the exhaust system of vehicles, which exhibits a reasonably high corrosion resistance in the exhaust gas condensate environment. However, due to the inhomogeneous element distribution the outmost surface layer, which is directly exposed to the exhaust gas, is more like an aluminum layer (with little intermetallic compounds) that, though corrosion-resistant, may not be strong enough to effectively resist erosion. In this work, efforts were made to increase the service life of Al-plated or aluminized steel surface, particularly to enhance the outmost surface layer by nanocrystallization for improved resistances to corrosion, wear and corrosive wear. The entire process includes dipping steel specimens into a molten Al pool to make an Al coat, sandblasting the surface coat to generate dislocation network or cells, and recovery treatment to turn the dislocation cells into nano-sized grains. Electrochemical properties, electron work function (EWF), wear and corrosive wear of the nanocrystalline surfaces were investigated. It was demonstrated that the nanocrystalline aluminized surface exhibited improved resistances to corrosion, wear and corrosive wear. [Copyright &y& Elsevier]

Abstract: Laser surface alloying (LSA) of mild steel AISI 1050 using aluminum alloy (AlFeSi) was attempted. LSA was conducted by a two-step process: the alloy powder was preplaced on the surface of the substrate by flame spraying and the preplaced layer was subsequently remelted using a 2.5-kW CW Nd:YAG laser beam. The microstructures of the alloyed layers were studied by scanning electron microscopy and X-ray diffractometry. The corrosion and cavitation erosion characteristics in 3.5% NaCl solution at 23 °C were studied by potentiodynamic polarization technique and by means of a 20-kHz ultrasonic vibratory facility, respectively. The laser-aluminized layer was found to consist of a ferritic matrix reinforced with the intermetallic phases Fe3Al, FeAl, and tiny amount of Fe2Al5. The maximum hardness achieved was 595 Hv. The cavitation erosion resistance of the laser-aluminized specimens was much higher than that of the substrate (about 17 times) due to the presence of Fe3Al, FeAl, and solid solution hardened α-ferrite. Although alloying of aluminum to the steel led to an active shift in the free corrosion potential from −610 mV to −880 mV, the specimens showed passivity with pitting and protection potentials of −650 mV and −717 mV, respectively, while the steel substrate did not. The corrosion resistance of the specimens was significantly improved after laser aluminization as evidenced by a much lower corrosion current density. [Copyright &y& Elsevier]

The skutterudite CoSb 3 is a well-known thermoelectric material widely used to produce electricity from heat. This material is commonly employed under vacuum but in a near future it is expected to be used at high temperatures under oxidative atmospheres, e.g. in air. The lifetime of the material may be affected by the oxidative environment, considerably limiting the use of the thermoelectric equipment. The present research describes the degradation mechanisms of CoSb 3 from oxidation experiments carried out under a flow of synthetic air at 800 K. The skutterudite material is progressively oxidized after 15 h, 50 h, 100 h and 1000 h of treatment, producing three oxides on the CoSb 3 surface (CoSb 2 O 4 /CoO·Sb 2 O 3 , Sb 2 O 4 and CoSb 2 O 6). These three oxides have different growth kinetics and they are produced in various amounts as a function of the oxidation time. Next, as a solution for an appropriate oxidation protection, this work explores the chemical vapor deposition (CVD) process named pack cementation to synthesize a protective coating on CoSb 3. This process produces a surface layer made of aluminum antimonide (AlSb) and cobalt aluminide (Al 9 Co 2). The oxidation experiments carried out on the coated CoSb 3 highlight the protective properties of this innovative surface layer. The coating is a protective barrier against oxygen that keeps the CoSb 3 substrate unaffected by the flow of air at 800 K for 1000 h. Consequently, pack cementation is an efficient process to synthesize a protective surface layer that makes CoSb 3 usable under oxidative environments. Unlabelled Image • The oxidation behavior of CoSb 3 in air at 800 K is explored for 1000 h. • The progressive degradation of CoSb 3 produces a surface oxide layer made of several phases. • A surface layer is synthesized by aluminum pack cementation to protect CoSb 3. • The surface layer is made of AlSb and Al 9 Co 2 and protects CoSb 3 from oxidation. [ABSTRACT FROM AUTHOR]