AISI 420 stainless steel is widely used in applications where wear and corrosion resistance are required. However, the heat treatment and nitriding process can drastically reduce the corrosion resistance of this stainless steel. This article focuses on investigating the influence of steel substrate and gas nitriding efficiency at two temperatures of 520 oC and 550 oC on some properties of CrN coating. The experiments were carried out to evaluate the surface hardness, microstructure and phase composition of nitrided layers. The coating adhesion and load capacity of the coating were performed according to VDI 3198 standard. Electrochemical testing was performed in a solution of 3.5% NaCl and then using the Tafel method to determine the corrosion current and corrosion potential. The thickness of CrN and CrN/CrN coating was 1.6 μm and 3 μm, respectively. The study showed that the corrosion resistance of coatings fabricated through duplex technology was affected not only by the normal defects but also by the porosity on the nitrided surface. The corrosion resistance of multilayer duplex coating was improved compare with mono-layer duplex coating due to its ability to cover and reduce pores and pitting defects.
A typical structure of thermal spray coatings consisted of molten particles, semi-molten particles, oxides, pores, and cracks. These factors caused the porosity of sprayed coatings, leading to a significant influence on the coating properties, especially their wear-corrosion resistance. In this study, a post-spray sealing treatment of Cr 3 C 2 -NiCr/Al 2 O 3 -TiO 2 plasma-sprayed coatings was carried out, and then, their corrosion properties were evaluated, before and after the treatment. For the sealing process, aluminum phosphate (APP) containing Al 2 O 3 nanoparticles (~10 nm) was used. The permeability of APP into the sprayed coating was analyzed by SEM-EDS. The treatment efficiency for porosity and corrosion resistance of sprayed coatings was evaluated by electrochemical measurements, such as the potentiodynamic polarization and electrochemical impedance spectroscopy. The wear-corrosion resistance of the coating was examined in 3.5 wt.% NaCl circulation solution containing 0.25% SiO 2 particles. The sealing efficiency was evaluated by the percentage of the treated open pores in the coating. The obtained results showed that APP penetrated deeply through the coating and the incorporation of Al 2 O 3 nanoparticles into APP sealant improved the sealing efficiency by 20% of open pores in comparison with the sealant without nano-Al 2 O 3 . The effect of the post-treatment on corrosion protection of the sprayed coating has been discussed.
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