Magnetic Property Changes of CoNiCrAlY Coating Under Cyclic Oxidation and Hot Corrosion

• Published in: Oxidation of metals Vol. 93; no. 1-2; pp. 75 - 86
• Main Author: Rahmani, Kh
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.02.2020; Springer Nature B.V
• Subjects: Aluminum oxide; Chemistry and Materials Science; Coatings; Corrosion and Coatings; Depletion; Destructive testing; Gas turbine engines; Hot corrosion; Inorganic Chemistry; Intermetallic compounds; Low pressure; Magnetic permeability; Magnetic properties; Magnetism; Materials Science; Metallic Materials; Nickel aluminides; Nickel base alloys; Nickel compounds; Nondestructive testing; Original Paper; Oxidation; Room temperature; Sodium sulfate; Superalloys; Thickness; Tribology; Turbine blades; CoNiCrAlY coating; Cyclic oxidation; Hot corrosion; Magnetic properties
• ISSN: 0030-770X; 1573-4889
• DOI: 10.1007/s11085-019-09945-7

In this paper, a non-destructive method for evaluation of a CoNiCrAlY coating applied to gas turbine blades has been studied. The specimens from cast Ni-base superalloy IN738LC were coated with Co–34Ni–18.6Cr–8.7Al–0.5Y by low pressure plasma spray (LPPS) technique. The XRD peaks indicated that CoNiCrAlY coatings are composed of β-NiAl and γ-Co (Ni, Cr) matrix with small amounts of Al 2 O 3 and γ′-Ni 3 Al. The surfaces of the coated specimens were covered with thin salt mixture of Na 2 SO 4  + l0 wt% NaCl. Afterward, the specimens were maintained at 850 °C for 24 h, cooled to room temperature (as one cycle) and then reintroduced in the furnace. The magnetic susceptibility of coating, as one of the magnetic properties, was periodically determined after each cycle. The results showed that the magnetic susceptibility of Co–34Ni–18.6Cr–8.7Al–0.5Y coating is related to the phase compositions and microstructures. The magnetic susceptibility typically increases with depletion of β-NiAl phase and decrease in the coating thickness during cyclic oxidation and hot corrosion. Therefore, measurement of magnetic susceptibility can be employed as a non-destructive test (NDT) method to estimate the thickness or residual lifetime of CoNiCrAlY coatings.