A Comparative Study on Ni-Based Coatings Prepared by HVAF, HVOF, and APS Methods for Corrosion Protection Applications
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E. Sadeghimeresht, N. Markocsan, and P. Nyle´n (Submitted June 8, 2016; in revised form October 16, 2016) Selection of the thermal spray process is the most important step toward a proper coating solution for a given application as important coating characteristics such as adhesion and microstructure are highly dependent on it. In the present work, a process-microstructure-properties-performance correlation study was performed in order to figure out the main characteristics and corrosion performance of the coatings produced by different thermal spray techniques such as high-velocity air fuel (HVAF), high-velocity oxy fuel (HVOF), and atmospheric plasma spraying (APS). Previously optimized HVOF and APS process parameters were used to deposit Ni, NiCr, and NiAl coatings and compare with HVAF-sprayed coatings with randomly selected process parameters. As the HVAF process presented the best coating characteristics and corrosion behavior, few process parameters such as feed rate and standoff distance (SoD) were investigated to systematically optimize the HVAF coatings in terms of low porosity and high corrosion resistance. The Ni and NiAl coatings with lower porosity and better corrosion behavior were obtained at an average SoD of 300 mm and feed rate of 150 g/min. The NiCr coating sprayed at a SoD of 250 mm and feed rate of 75 g/min showed the highest corrosion resistance among all investigated samples.
Keywords
corrosion, OCP, process optimization, potentiodynamic polarization, thermal spraying
1. Introduction Ni-based coatings such as Ni, NiCr, and NiAl coatings are extensively used to repair or protect structural materials in corrosive environments (Ref 1-5). Corrosion protection requires coatings with dense microstructure and high adherence in order to insulate the substrate material from the surrounding corrosion environment (Ref 6-8). Addition of alloying elements such as Cr or Al to these materials facilitates a passivation layer formation that improves the corrosion resistance of the coatings (Ref 9, 10). A dense coating with uniform microstructure and less defects is not only hindering the corrosive agent penetration, but also helps to a quicker passive layer formation as pores are considered as barriers for rapid diffusion of the passive layer-forming elements. Typically, a pore, particularly if connected to other pores, allows much faster penetration of the electrolyte solution inside the coating and toward the substrate and acts as a crevice to hinder proper passivation of the metal alloy along its surfaces (Ref 11). It is well known that the thermal spray methods are mostly defined by their in-flight particle E. Sadeghimeresht, N. Markocsan, and P. Nyle´n, Department of Engineering Science, University West, 461 53 Trollha¨tan, Sweden. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
characteristics, i.e., temperature and velocity, which in turn have the highest influence on coatings microstructure. Comparing high-velocity air fuel (HVAF), high-velocity oxy fuel (HVOF),
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