WC-CoCr coatings sprayed by high velocity oxygen-fuel (HVOF) flame on AA7050 aluminum alloy: electrochemical behavior in 3.5% NaCl solution

Abstract

In the present work, the electrochemical behavior of WC-CoCr coatings with 10 (W10), 15 (W15) and 20 (W20) torch passes sprayed by High Velocity Oxygen-Fuel (HVOF) flame on AA7050 aluminum alloy substrate, evaluated in 3.5% NaCl solution, were compared using open-circuit potential (E OC) measurements, electrochemical impedance spectroscopy (EIS) and polarization curves. The coating surfaces and their cross sections were characterized by X ray diffraction and the Rockwell-C hardness test, and also by optical (OM) and scanning electron microscopy (SEM) before and after the corrosion tests. The electrochemical data showed that sample W10 presented higher corrosion resistance than the others in chloride solutions. In some tests, aluminum salts on the coating surface were identified by EDS, indicating the corrosion of the substrate. And using aluminon, aluminum ions were detected and analyzing the surface via stereomicroscopy, hydrogen bubbles were observed, both showing that the electrolyte reached the substrate and galvanic corrosion possibly occurred. The physical characterization showed that sample W10 presented a lower number of cracks and pores, justifying its higher corrosion resistance

Electrochemical impedance spectroscopy of HVOF-sprayed NiTi intermetallic coatings deposited on AISI 1045 steel

“…6). This behavior has been seen in several coating systems in which the coating is very dense and can form a passive layer such as HVOF-sprayed stainless steel coating [17], hydrothermal deposited TiO 2 [18] and HVOF-sprayed WC-CoCr coating [19] in 3.5% NaCl solution.…”

Comparative Study of Corrosion Performance of HVOF-Sprayed Coatings Produced Using Conventional and Suspension WC-Co Feedstock

“…Understanding the influence of carbon loss on the corrosion resistance for cases such as the one observed for DGS coating could be enhanced by further research. It has also been reported (Ref 13,14) that carbide particles can cause galvanic corrosion in the matrix, or crevice corrosion can take place at the carbide-matrix interface, especially when the metal matrix has passivation capabilities.…”

“…Previous studies have indicated that carbide size and distribution influences the mechanical properties of these coatings (Ref 7 -10). However, for service in corrosive environment, either Cr 3 C 2 -based compositions such as Cr 3 C 2 -NiCr, or compositions with chromium-alloyed binders such as WC-10Co4Cr and WC-NiCrBSi are the preferred technical solution (Ref [11][12][13][14][15]. In general, understanding the corrosion behavior of HVOF-sprayed WC-Co coatings is not trivial because of their complex microstructures comprising formation of new and nonequilibrium phases, as well as partial carbide dissolution into the metal matrix.…”

A comparative study on the microstructure and surface property evaluation of coatings produced from nanostructured and conventional WC–Co powders HVOF-sprayed on Al7075

“…The majority of the research efforts on HVOF spraying of cermets on Al-alloys deal with the fatigue (mainly) [40][41][42][43][44][45][46] and wear performance [43,[47][48][49][50] of the coated alloys and they refer to microcrystalline coatings. The works dealing with the corrosion behaviour of HVOF cermet coatings on Alalloys are also relatively few [12,47,49,[51][52][53][54][55], whilst they also refer to microcrystalline coatings. These works have produced quite encouraging results regarding the application potential of hardmetal coatings on Al-alloys.…”