Electrochemical Behavior of Bilayer Thermal-Spray Coatings in Low-Temperature Corrosion Protection

Esmaeil Sadeghimeresht,Nicolaie Markocsan

doi:10.3390/coatings7100162

Abstract

Cr3C2-NiCr coatings are greatly used to protect critical components in corrosive environments and to extend their lifetime and/or improve functional performance. However, the pores formed during spraying restrict the coating’s applicability area for many corrosion protection applications. To overcome this technical challenge, bilayer coatings have been developed, in which an additional layer (the so-called “intermediate layer”) is deposited on the substrate before spraying the Cr3C2-NiCr coating (the so-called “top layer”). The corrosion behavior of the bilayer coating depends on the composition and microstructure of each layer. In the present work, different single-layer coatings (i.e., Cr3C2-NiCr, Fe- and Ni-based coatings) were initially sprayed by a high-velocity air fuel (HVAF) process. Microstructure analysis, as well as electrochemical tests, for example, open-circuit potential (OCP) and polarization tests, were performed. The potential difference (ΔE) had a great influence on galvanic corrosion between the top and intermediate layers, and thus, the coatings were ranked based on the OCP values (from high to low) as follows: NiCoCrAlY > NiCr > Cr3C2-NiCr > NiAl > Fe-based coatings (alloyed with Cr) > pure Ni. The Ni-based coatings were chosen to be further used as intermediate layers with the Cr3C2-NiCr top layer due to their capabilities to show high OCP. The corrosion resistance (Rp) of the bilayer coatings was ranked (from high to low) as follows: NiCoCrAlY/Cr3C2-NiCr > NiCr/Cr3C2-NiCr > NiAl/Cr3C2-NiCr > Ni/Cr3C2-NiCr. It was shown that splat boundaries and interconnected pores are detrimental for corrosion resistance, however, a sufficient reservoir of protective scale-forming elements (such as Cr or/and Al) in the intermediate layer can significantly improve the corrosion resistance.

Highlights

Cr3 C2 -NiCr coatings are thermally sprayed on structural components which undergo severe wear, for example, pump impellers, piston rings and hydraulic equipment, as a hard chrome alternative [1].most of the applications in which the hard Cr3 C2 -NiCr coatings are employed generally comprise environments that are intrinsically corrosive [2,3,4].The corrosion behavior of the Cr3 C2 -NiCr coatings in alkaline and acidic environments has been extensively studied [4,5,6,7,8,9,10,11]
The potential difference (∆E) had a great influence on galvanic corrosion between the top and intermediate layers, and the coatings were ranked based on the open-circuit potential (OCP) values as follows: NiCoCrAlY > NiCr > Cr3 C2 -NiCr > NiAl >
It was shown that splat boundaries and interconnected pores are detrimental for corrosion resistance, a sufficient reservoir of protective scale-forming elements in the intermediate layer can significantly improve the corrosion resistance

Summary

Introduction

Cr3 C2 -NiCr coatings are thermally sprayed on structural components which undergo severe wear, for example, pump impellers, piston rings and hydraulic equipment, as a hard chrome alternative [1].most of the applications in which the hard Cr3 C2 -NiCr coatings are employed generally comprise environments that are intrinsically corrosive [2,3,4].The corrosion behavior of the Cr3 C2 -NiCr coatings in alkaline and acidic environments has been extensively studied [4,5,6,7,8,9,10,11]. Showed that defects such as pores and cracks play a key role in the integrity of the coating–substrate system and can negatively affect the corrosion resistance, as they act as main paths for the electrolyte to pass the coating and reach the substrate. In this case, if the substrate is more active than the coating, a galvanic pair is formed between the coating and substrate which leads to substrate corrosion, damage of the component and a decrease in the component’s lifetime [9]

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Results

Conclusion