Cavitation Erosion Resistance Of Coatings Research Articles

Cavitation erosion and corrosion resistance of hydrophobic sol-gel coatings on aluminium alloy

Cavitation erosion and erosion-corrosion are the popular failure modes of hydronautics components namely propellers, valves, turbines etc which occurs due to mechanical destructions and electrochemical corrosion. Erosion corrosion is caused due to surge in the number of solid particles affecting the surfaces whereas cavitation erosion is caused due to steady collapse of cavities or bubbles. Aluminium alloys are widely used in marine renewable industries owing to its high strength, light weight and good corrosion resistance. Despite that, cavitation and erosion-corrosion are the limiting factors for these alloys. The aim of the present work is to produce a coating system capable of replacing chromate-conversion coatings on aluminium alloy by combining an anodised layer with additional deposition of superhydrophobic sol-gel coatings. Fundamental characteristics that affect the coating's corrosion and cavitation erosion namely adhesion and thickness was evaluated. Hardness and elastic modulus of the coatings was evaluated using a Nanoscratch mechanical tester. Electrochemical behaviour of the coatings was assessed using Potentiodynamic scanning (PDS) and electrochemical impedance spectroscopy (EIS). Prolonged performance was studied using neutral salt spray test (NSS). Cavitation erosion resistance of the coatings was investigated in laboratory using a standard ultrasonic test apparatus according to ASTM G32-16. Erosion rate of the coatings was evaluated based on cumulative mass loss v/s testing time. SEM/EDX was used to evaluate the surface damage caused by erosion-corrosion and cavitation erosion. The analysis was done aiming to decide if the developed coatings was a better alternative to protect the metals from corrosion and cavitation erosion.

Effects of microstructure and mechanical properties on cavitation erosion resistance of NiCrWMoCuCBFe coatings

NiCrWMoCuCBFe coatings were prepared by high velocity oxy-fuel (HVOF) spraying technology with the microstructures and mechanical properties characterized in detail. Besides, an ultrasonic vibratory apparatus was employed to investigate the cavitation erosion resistance of the coating in deionized water and its cavitation erosion mechanism was revealed. The results show that there were some inherent defects existed on the coating surface but its microhardness was more than triple that of 316 L stainless steel. In the initial stage of cavitation erosion, these inherent defects of coating surfaces were damaged at first, leading the incubation period of the coating was shorter than 10 min. With the prolongation of cavitation erosion time, many cracks interpenetrated and finally led to the exfoliation of coatings. When the cavitation erosion time extended to 10 h, the erosion rate of the coating was almost stable and reached maximum about 1.32 ± 0.08 μm/h, which was less than that of 316 L stainless steel (2.62 ± 0.18 μm/h). That is, the inherent defects influenced the cavitation erosion resistance in the incubation period of the coating but the mechanical properties played a dominant role in the long-term cavitation erosion of the coating.

Effect of annealing treatment on microstructures, mechanical properties and cavitation erosion performance of high velocity oxy-fuel sprayed NiCoCrAlYTa coating

The mechanical parts serving in the marine environment are up against the dual damage of corrosion and cavitation erosion, so the NiCoCrAlYTa coating with excellent corrosion resistance is successfully prepared by high velocity oxy-fuel (HVOF) sprayed technology to protect the equipment from deterioration. The chemical composition and mechanical properties as well as the microstructure evolution of the coating before and after annealing treatment are studied. At the same time, the influence of annealing treatment on the corrosion and cavitation erosion behaviors of the coating is investigated, as well. The NiCoCrAlYTa coating mainly contains γ-Ni, β-NiAl, and γ’-Ni3Al phases. During the deposition, the microcrystals or incomplete crystals generate in the interior of the coating because of the large temperature difference and impact force. Meanwhile, there is twin crystal structure in the as-spayed coating. After annealing treatment, the growth of grains and the segregation of reactive elements improve interface strength and inhibit the formation of micro-defects in the coating. The annealing temperature is of significance to the microstructure, mechanical properties, anti-corrosion and cavitation erosion resistance of the coating. This study provides a combined approach toward improving the corrosion and cavitation erosion resistance of NiCoCrAlYTa coatings.

Cavitation erosion behavior of HVOF CaviTec coatings

CaviTec® is an alloy known for its high resistance to cavitation erosion. Under cavitation, this material absorbs impact energies and undergoes a structural phase transformation. This attribute gives rise to a long incubation period before erosion and material loss take place. In this work, CaviTec powders were prepared by water atomization and mechanical alloying. Coatings were deposited on 304-type stainless steel substrates using the high-velocity oxy-fuel (HVOF) technique and their mechanical properties were evaluated using indentation. The cavitation erosion properties were evaluated using an ultrasonic cavitation erosion tester. The microstructure was studied using X-ray diffraction and scanning electron microscopy (SEM). The cavitation erosion resistance of coatings prepared from atomized CaviTec powder was enhanced when powders were milled prior to deposition. The resistance was further improved by optimizing deposition conditions reaching performances comparable to Stellite-6 and WC-CoCr HVOF coatings. Inspection using SEM micrographs of the damaged surface of CaviTec coatings indicates that surface degradation and material loss initiated at defect sites such as pores and inter-splat boundaries. This preferential erosion led to the removal of CaviTec particles before significant phase transformation took place in the material.

On the Cavitation Erosion Resistance of Coatings

The propellers of ships have often been observed as damaged as a result of cavitation erosion. Although several investigations have been made on the problem of cavitation erosion of coatings by way of prevention against damage of propellers, the cavitation tests have been conducted under different test conditions with different test methods. This is the report of test conducted of 6 coatings (neoprene, polyurethane, electro nickel plating, electrochromium plating, flame sprayed colmonoy (fused) and welded high Al-bronze) with a view to determining their comparative efficacy for cavitation erosion resistance. A magnetostrictive cavitation test apparatus, a rotating-blade cavitation test apparatus and a water-tunnel cavitation test apparatus were used for the purpose. These coatings were applied to base Al-bronze. The results obtained are summarized as follows:(1) In the magnetostrictive cavitation test, the electrochromium plating shows superior cavitation erosion resistance, but the flame sprayed colmonoy (fused) shows cavitation erosion 3 times as large as that in the electrochromium plating.(2) In the rotating-blade cavitation test, the electrochromium plating shows superior cavitation erosion resistance, but the welded high Al-bronze shows cavitation erosion 10 times as large as that in the electrochromium plating.(3) The ranking of cavitation erosion resistance of coatings in the water-tunnel cavitation test is the same as in the magnetostrictive cavitation test.