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Laser clad coatings consisting of a Ni-based matrix and tungsten carbide (WC) hard phases are used for heavy wear protection of components in many industries. Matrix composition and WC type can be chosen to give these composites resistance to erosive/abrasive wear, as well as to the impacts of particles and corrosion. Maintaining ductility in the matrix is important, so that it can absorb impact energy and minimize cracking, thus avoiding penetration of corrosive media through cracks and subsequent corrosion in the substrate-cladding interface. Claddings were tested for their impact resistance by means of a single impact with a hard, spherical indenter. The depth of impact craters was measured, and the cracking behavior was analyzed. Corrosion resistance of Ni/WC composites, a HVOF WC/CoCr coating and a hard chrome reference coating were tested by electrochemical corrosion in a 3.5 wt.% NaCl solution and by exposure for up to 1000 h in a salt spray test. Refereed/Peer-reviewed

WOS: 000446711300053, The corrosion resistances of three different thermochemical coatings (grown using titanizing, boriding, and borotitanizing treatments) applied to STKM-13A steel surfaces were investigated. The coatings were characterized using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffractometry, 2D profilometry, and microhardness experiments. The corrosion tests were conducted using both electrochemical and static immersion methods, in 3.5 pct NaCl and 40 pct HF acid solutions, respectively. The corrosion resistance of STKM-13A steel was enhanced after the coating process. The specimens were exposed to more corrosion in the HF solution than in the NaCl solution. The best corrosion resistance was obtained in the borotitanized and borided specimens immersed in the NaCl and HF solutions, respectively. The borided STKM-13A steel sample showed even less cumulative weight loss than Inconel 625 in the static immersion HF acid solution test. This suggests potential use of the borided STKM-13A steel in the uranium production units of nuclear power plants as an alternative to more costly alternatives such as Monel, Inconel, and Hastelloy. (C) The Minerals, Metals & Materials Society and ASM International 2018

AISI 900 series stainless steels are considered as low-cost alternatives to nickel-based superalloys used for highly corrosive environments. However, in terms of mechanical properties, they have average strength and hardness similar to other austenitic stainless steel grades and this often limits their use. If a 900 series alloy were to be used under tribocorrosive conditions, its surface properties would have to be improved by a wear and corrosion resistant coating. In this study, AISI 904L steel was pack boronized in a solid medium at temperatures of 900, 1000 and 1100 °C for 2, 4 and 6 h with nano-sized boronizing powders. The grown boride layers were evaluated using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractometry, 2D profilometry, microhardness measurements, ball-on-disk type wear tests and electrochemical corrosion tests. Characterization studies revealed a complex boride layer consisting of FeB, Fe2B, CrB, Cr3B4, Ni3B and Mo2B phases with 2366–2396 HV hardness. Wear tests showed that the abrasive wear resistance of the AISI 904L steel was improved by up to 40 times. The corrosion resistance of boronized AISI 904L was inferior to untreated AISI 904L in 3.5% NaCl, but comparable to AISI 316L.

The utilization of Mg alloys for biomedical applications is so far underexplored due to the accelerated corrosion hampering patient recovery post implantation. Here, we explore the effectiveness of corrosion reduction of an AZ31 alloy in Simulated Body Fluid when coated with a 40 nm sputtered TiO2 layer and compare it to a similar coating made by Atomic Layer Deposition (ALD). Potentiodynamic polarization and hydrogen evolution experiments were performed on coated samples having different surface roughness and 3D topologies. Interestingly, ALD layers reduce corrosion current density by 94% on Ra = 118.6 ± 5.1 nm and 93% on Ra = 4794.3 ± 49.4 nm, whereas sputtered only by 84% on Ra = 118.6 ± 5.1 nm and 60% on Ra = 4794.3 ± 49.4 nm. Particularly on 3D aspects, the ALD coatings are superior, where a scaffold of 85% porosity with 1 mm pore sizes released 68% lower hydrogen compared to the sputtered counterparts. We relate these observations to the higher surface integrity, adhesion strength and lower line-of-sight restrictions of ALD compared to sputter deposition. The results can be interesting for researchers and practitioners aiming to make Mg alloys more commonplace as temporary metallic implant materials. This is an open access article distributed under the terms of the Creative Commons CC-BY license, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Electropolymerization of polypyrrole coatings in the presence and absence of sodium nitrate was applied on rheo-cast Al-4.5% Si alloy and pure aluminum. The results showed that the eutectic silicon phase and intermetallic particles in the alloy’s microstructure increase the electrodeposition rate in comparison to the pure aluminum substrate. The electrochemical and microstructural studies show that the polypyrrole coatings are able to protect the surface due to the barrier properties and the passivation protection provided by the reduction of the conductive polymer. The coating electrodeposited from sodium nitrate-containing electrolyte presented improved protection for longer immersion time. Localized formation of a thick oxide layer as a result of the drastic galvanic coupling at the polypyrrole/aluminum interface leads to blister formation and failure of the coating. It was revealed that the coating could be deposited into the porosities produced by the casting related defects, but in most cases, this affects the corrosion protection leading to imminent failure.

The present study investigates the initial corrosion behaviour of HVAF-sprayed NiCr, NiAl and NiCrAlY coatings in two different environments, O 2 + H 2 O and O 2 + H 2 O + KCl at 600 °C for up to 168 h in order to evaluate the possibility of utilizing such coatings in biomass- and waste-fired boilers. SEM/EDX analysis showed that all coatings displayed a protective behaviour in O 2 + H 2 O. Upon addition of KCl (O 2 + H 2 O + KCl), the corrosion behaviour of the NiCr coating drastically changed as it formed a thick oxide layer and displayed major chlorine diffusion down to the substrate. The NiCrAlY coating displayed a significantly better corrosion resistance with only minor oxide formation. The NiAl coating exhibited a protective behaviour similar to when exposed in the absence of KCl indicating that a thin protective oxide has formed on the coating surface. The performance of the NiAl and NiCrAlY coatings is promising for future studies with long-term exposures in more corrosive environments such as in a biomass- and waste-fired boiler. © 2019, The Author(s).

Fatigue crack formation is the most important factor affecting the damage in parts under subjected to the repeated stresses. In this context, intensive researches are continuing on the protective hard coatings in the aviation and automotive sectors in order to increase the fatigue life especially in aluminum alloy parts. Chromium oxide (Cr2O3) based coatings are widely used in many industrial applications due to high corrosion resistance, high hardness and high surface resistance. In this study, fatigue crack propagation on 7075–T651 alloy with chromium oxide coating by thermal spraying method was investigated. Test results show that the crack propagation is slowed down by the chromium oxide based coating and increase the surface resistance of the aluminum alloy. © 2019 Polish Academy of Sciences. All rights reserved.

Zeolites are crystalline nanoporous aluminosilicates. Thanks to their intrinsically nanoporous structure they are widely used as molecular sieves, for exchanging ions, or, also thanks to the high surface area of these structures, for catalytic applications. Furthermore, thanks to their thermal and chemical stability, in recent years zeolite coatings have been evaluated for application as anti-corrosion coatings. The non-toxicity of this class of coatings makes it possible that they will be an environmentally friendly alternative to conventional chromate-based coatings. This article provides a brief review of the anti-corrosion performance of zeolite coatings, applied by direct synthesis technique to several metals and alloys, as discussed in the literature. After a short description of the microstructure and properties of zeolites, the discussion addresses the research activities related to this topic, as reported in the literature. Comparative analysis of literature results supported the dry-gel conversion method as a promising approach that combines a simplified synthesis procedure with anti-corrosion coating performance. Based on these considerations, an evaluation of future trends is discussed along with the final remarks.

Alloy 600 and Alloy 800H are susceptible to metal dusting. Both alloys were thermally sprayed with two different corrosion resistant coatings: Ni50Cr and Ni31Cr11Al0.6Y. Laser remelting was used to enhance further the effectiveness of these coatings to resist metal dusting by eliminating interconnected porosity and improving coating adhesion. Uncoated, coated and laser-treated coated samples of Alloy 600 and Alloy 800H were exposed to a mixed gas atmosphere (20% H2, 80% CO at 650°C). Samples were examined in plan and cross-section using optical and scanning electron microscopy, electron probe microanalysis and X-ray diffraction. The extent of carbon deposition was tracked by mass difference measurements at intervals during exposure. The thermally sprayed coatings enhanced metal dusting resistance by acting as physical barriers to carbon ingress. The NiCrAlY coating performed well on both substrates. The NiCr coating itself underwent metal dusting and spalled from Alloy 800H due partly to CTE mis-match stresses. Laser treatment of both coatings successfully eliminated interconnected porosity and hence enhanced metal dusting resistance.

Thermally sprayed fluoropolymer, polyvinylidene fluoride (PVDF), ethylene chlorotrifluoroethylene (ECTFE), perfluoroalkoxy alkane (PFA) and fluorinated perfluoroethylenepropylene (FEP) coatings were produced by flame and plasma spraying processes. It was possible to produce high quality coatings by these techniques. These coating processes are one-step methods in which post heat treatments are not required, in contrast to conventional electrostatic deposition methods. Morphology and particle size distributions were determined for each fluoropolymer powder. The coatings prepared by plasma and flame spraying were studied by optical microscope, salt spray test and liquid immersion tests. Sprayed coatings were found to be poreless and very smooth. In salt spray test, no corrosion invasion was found. In both liquid immersion tests (pH values -0.7 and 14) only PFA coating performed well. The other coatings showed a little corrosion in liquid immersion tests. That was observed as darkening of the fluoropolymer coating in the areas that were exposed to corrosive liquid. According to sectioned micrographs, the coatings were fully dense.