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

Abstract

Cr 3 C 2 -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 Cr 3 C 2 -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., Cr 3 C 2 -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 (∆E) 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 > Cr 3 C 2 -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 Cr 3 C 2 -NiCr top layer due to their capabilities to show high OCP. The corrosion resistance (R p ) of the bilayer coatings was ranked (from high to low) as follows: NiCoCrAlY/Cr 3 C 2 -NiCr > NiCr/Cr 3 C 2 -NiCr > NiAl/Cr 3 C 2 -NiCr > Ni/Cr 3 C 2 -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.

In Vitro Corrosion of Titanium Nitride and Oxynitride-Based Biocompatible Coatings Deposited on Stainless Steel

“…Based on the superior anticorrosive properties of TiN, we also investigated the corrosion resistance of TiNO coatings, as the oxygen content determines more proficient the creation of passive layers under the saline solution corrosive attack [49]. To prove the aforementioned expectation on the two isostructural coatings, we studied two bilayer structures composed of TiNO and TiN layers, considering the superior properties of bilayer structures [32,[57][58][59][60][61], in order to find the best solution for our goal, obtaining coatings adherent to SS substrate with superior resistance to corrosion [14].…”

Comparison of Aqueous and Gelled 3.5% NaCl Electrolytes for Assessing the Corrosion Resistance of Thermal Spray Stainless-Steel Coatings in Electrochemical Corrosion Tests

“…The salt spray test or the Corrodkote test are specifically designed to induce sub-corrosion in order to test the apparent density of the coatings. In research, studies often deal with electrochemicalinstrumented tests such as potentiodynamic polarization, electrochemical impedance, and immersion tests in 3.4% to 5% sodium chloride (NaCl) solutions [14][15][16][17]. For these corrosion tests, the following observations can be summarized: the open circuit potential (OCP) and the corrosion potential (E Corr ), respectively, decreases with immersion time due to electrolyte infiltration of the coating toward the substrate.…”