Improvement on the Corrosion Resistance of AZ91D Magnesium Alloy by Aluminum Diffusion Coating

By combination of magnetron sputtering deposition and vacuum annealing, an aluminum diffusion coating was prepared on the substrate of AZ91D alloy to improve its corrosion resistance. The microstructure and composition of the diffusion coating was investigated by scanning electron microscopy and X-ray diffraction. The diffusion coating was mainly comprised of β phase-Al12Mg17. The continuous immersion test in 3.5 wt pct neutral NaCl solution indicated that the specimen with diffusion coating had better corrosion resistance compared with the bare AZ91D alloy specimen. The potentiodynamic polarization measurement indicated that the diffusion coating could function as an effectively protective layer to reduce the corrosion rate of AZ91D alloy when exposed to 3.5 wt pct NaCl solution.

Na_2SO_4- and NaCl- Induced Hot Corrosion Behaviors of a Nickel-Base Superalloy with Aluminide Diffusion Coating

Hot-corrosion behaviors of nickel-base superalloy and aluminide diffusion coating have been investigated in conditions of contents of Na2SO4 and NaCI molten salts at 900℃ by means of XRD and SEM. Hot-corrosion scale of the superalloy and aluminide diffusion coating were analyzed and their surface morphologies were observed. The results demonstrate that both coated and uncoated specimens are not susceptible to various contents of NaCI. That may be resulted from the AI2O3 scale formation. Growth stress was characterized by the formation of convoluted scales.

ANALYSIS ON KINETICS OF CARBIDE DIFFUSION COATING

A kinetic model for carbide diffusion coating has been proposed bused on the theory of steady diffusion and the sublattice model of solid solution.It is supposed that the growth rate of car- bide layer is controlled by the volume diffusion coefficient of carbon (?)s in carbide layer and the activity of carbon in steel substrate.The calculated results for V_4C_3 coating treatment of commercial steels T10.45,Crl2MoV,4Cr5MoVSi are in good agreement with the exper- imental data.

EFFECT OF CARBON ACTIVITY IN MATRIX ON DYNAMICS OF CARBIDE DIFFUSION COATING

The increase of thickness of carbide layer during carbide diffusion coating is proportional to the root of carbon activity in various steels. Thus it was demonstrated that the controlling fac- tor of the dynamics of carbide layer growth is carbon activity in steel matrix, but not carbon concentration.

A method for preparing composite diffusion coating alloy on ceramic surface

Metallization of the ceramic surfaces of Si3N4 and Al2O3 was carried out in a composite diffusion coating vacuum furnace using a Ti-Cu composite target. The experimental process and influencing factors were discussed. Optical microscope （OM） , energy dispersive spectroscopy （EDS） , scanning electron microscopy （SEM） , X-ray diffusion （XRD） and sound emissive scratch test （SEST） were applied to evaluate the alloy layer formed on the ceramic surface. It was indicated that the diffusion coating alloy layer contained Cu, Ti, Fe, Al and Si etc. XRD result indicated that the diffusion coating alloy layer was composed of CuTi2, Cu, Si2 Ti and CuTi, Al2 TiO5 , Ti3O5. It was found that the diffusion coating alloy layer got bonded with ceramic well, and no spaUation occurred under the maximum load of 100 N. Deposited Si3N4 ceramic was welded with Q235 and the joining quality was examined. Robust joint was formed between Si3N4 ceramic/Q235. This present method has advantages in high efficiency and low cost and provides a new approach for producing ceramic and metal bond.

Al diffusion coating on Mg alloy by a surface nanocrystallization enhanced CVD process

Surface nanocrystallization by mechanical attrition was used to enhance the chemical vapor deposition process.An aluminum(Al)diffusion coating was produced on AZ91 Mg alloy surface.This process was conducted at a relatively low temperature(400◦C)for a short time of 120 min.The results indicated that a continuous and dense Mg17Al12 intermetallic coating with a thickness of∼8µm formed on the Mg alloy substrate.Almost no corrosion was observed after the coated samples were immersed in 3 wt%NaCl solution for 6 h,reflecting a relatively good corrosion resistance.The formation mechanism of the Al diffusion coating is discussed based on the experimental results.

Mechanistic Investigation into the Role of Aluminum Diffusion in the Oxidation Behavior of Cryomilled NiCrAlY Bond Coat

High temperature oxidation behavior of the bond coat layer is a critical factor that controls the failure mechanism of thermal barrier coatings（TBCs）.Previous work reveald that TBCs with cryomilled NiCrAlY bond coats exhibited an improved oxidation behavior compared to equivalent TBCs with conventional bond coats.The cryomilled NiCrAlY bond coats contributed to a slower growth rate of thermally grown oxides（TGO） with a final thinner thickness and enhanced homogeneity in TGO composition.To better understand the improved oxidation behavior,a mechanistic investigation based on diffusion theory and quantum mechanics is performed to elucidate the role of aluminum diffusion in the oxidation behavior and how the microstructural features of the cryomilled NiCrAlY bond coats,i e,the creation of a thermally stable,uniform distribution of ultrafine Al-rich oxide dispersoids,affect the diffusion kinetics of Al and the migration of free electrons.It is revealed that these Al-rich oxide dispersoids result in a uniform diffusion of Al and slow migration of free electrons within the NiCrAlY bond coat,consequently leading to the improved oxidation behavior.

Oxide coating mechanism during fluidized bed reduction: solid-state reaction characteristics between iron ore particles and MgO

Experiments on the solid-state reaction between iron ore particles and MgO were performed to investigate the coating mechanism of MgO on the iron ore particles＇ surface during fluidized bed reduction. MgO powders and iron ore particles were mixed and compressed into briquettes and, subsequently, roasted at different temperatures and for different time periods. A Mg-containing layer was observed on the outer edge of the iron ore particles when the roasting temperature was greater than 1173 K. The concentration of Fe in the Mg-containing layer was evenly distributed and was approximately 10wt%, regardless of the temperature change. Boundary layers of Mg and Fe were observed outside of the iron ore particles. The change in concentration of Fe in the boundary layers was simulated using a gas–solid diffusion model, and the diffusion coefficients of Fe and Mg in these layers at different temperatures were calculated. The diffusion activation energies of Fe and Mg in the boundary layers in these experiments were evaluated to be approximately 176 and 172 k J/mol, respectively.

Progress in corrosion-resistant coatings on surface of low alloy steel

Low alloy steels are widely used in bridges,construction,chemical and various equipment and metal components due to their low cost and excellent mechanical strength.Information in the literature related to the preparation,advantages and disadvantages,and applications along with research progress of various types of protective coatings suitable for low-alloy steel surfaces is reviewed,while a conclusive and comparative analysis is also afforded to the numerous factors influencing the protective ability of coatings.The characteristics of coatings drawn from the latest published literature are discussed and suggest that the modification of traditional metal coatings and the development of new organic coatings under the consideration of environmental protection,low cost,simplicity and large-scale industrial application are simultaneously proceeding,which holds promise for improving the understanding of corrosion protection in related fields and helps to address some of the limitations identified with more conventional coating techniques.

Microstructure and Wear Resistance of in situ Formed Duplex Coating Fabricated by Plasma Nitriding Ti Coated 2024 Al Alloy

In this study, plasma nitriding was carried out on pure titanium film coated 2024 Al alloy to improve its surface mechanical property. Ti film with the thickness of 3.0 mm was firstly fabricated by means of magnetron sputtering method. Then, the Ti coated specimen was subjected to plasma atmosphere comprising 40% N2e60% H2 at 430 C for 8 h. The microstructures of the nitrided specimens were characterized by X-ray diffraction and scanning electron microscopy. Microhardness tester and pin-on-disc tribometer were used to test the mechanical properties of the untreated and nitrided specimen. The results showed that the surface of the nitrided specimen was composed of three layers（i.e. the outside nitride Ti N0.3layer, the middle Al3 Ti layer and the inside Al18Ti2Mg3 layer）. The surface hardness and wear resistance of 2024 Al alloy were increased simultaneously by duplex treatment. The untreated specimen exhibited severe adhesive wear while the nitrided one behaved in middle abrasive wear.