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Hot-dipping aluminizing of 45 steel was carried out in molten Al baths containing 0.0, 1.0, 3.0, and 5.0 wt.% Ni at 710°C for 10, 120, 300, and 600 s. The coatings were analyzed by x-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with energy-dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The coating hot-dipped in the Al-5Ni bath consisted of an outer Al-Ni topcoat (α-Al, θ-Al3Fe, Al3Ni, τ1-Al9FeNi), minor τ1-Al9FeNi, minor θ-Al3Fe, and major η-Al5Fe2layers, respectively, while no τ1-Al9FeNi layer was identified in the coating hot-dipped in the Al-1Ni and Al-3Ni bath. Diffusion path model was introduced to explain this phenomenon. Ni as an alloying element added into Al bath decreased the growth rate of η-Al5Fe2layer. The average thickness of η-Al5Fe2layer followed the parabolic law in hot-dipping in the Al-5Ni bath. Also, η-Al5Fe2had the largest growth rate among the intermetallic layers. [ABSTRACT FROM AUTHOR]

Aluminizing of nickel pre-plated AISI 316 is prepared by a high-activity pack at 1050 °C. The effect of initial nickel layer with different thicknesses on microstructure and oxidation behavior of coating is investigated. After aluminization, the surface microstructure of stainless steel mainly consists of β-(Fe, Ni)Al as matrix with β-FeAl + α -(Fe, Cr) precipitates and an inter-diffusion layer with γ phase. Aluminized coating on specimens with 10 and 20 µm primary thicknesses of the nickel layer includes three layers. The outer zone is made up of β-NiAl thin layer and a β-(Fe, Ni)Al layer. As the nickel layer increases to 50 and 100 µm, aluminide layers consist of outer and inner zone with β-NiAl and Ni3Al phase, respectively. Oxidation tests at 950 °C show that the oxidation resistance of aluminide coatings improves and oxidation kinetics follows a sub-parabolic rate law by increase in thickness of initial nickel layer. [ABSTRACT FROM AUTHOR]

Pack cementation is an attractive method for the formation of aluminum coatings on metal substrates. When applied to porous stainless steel substrates, the aluminum vapour species can diffuse into the pores and coat the interior surfaces. In this study, aluminum coatings were grown on commercial porous UNS 430 stainless steel and freeze cast UNS 440 stainless steel substrates by the pack cementation method. The effects of pack mixture composition, temperature, time, and the morphology of the substrates were characterized by scanning electron microscopy. It was shown that the degree of coverage of the aluminum coatings increased with porosity of the substrate. In addition, increased reaction time and temperature of the pack led to a more homogeneous distribution of aluminum across the substrate. [ABSTRACT FROM PUBLISHER]

Iron aluminide intermetallic compounds are well known for their high temperature oxidation resistance. In an effort to increase high temperature oxidation resistance of UNS 430 stainless steel, iron-aluminum intermetallic coatings with high iron content were grown on the substrate by the pack cementation technique. The microstructure and composition of the phases were characterized and the effects of pack cementation process parameters on the aluminide coatings were examined. Optoelectronic evaluation indicated that the formation of different stoichiometries of iron aluminide on the surface is controlled by outward diffusion of elements from the substrate. [ABSTRACT FROM AUTHOR]