Effect of NaCl and Na2SO4 on Low Temperature Corrosion of Vapour- and Pack-Aluminide Coated Single Crystal Turbine Blade Alloys CMSX-4 and RR3010

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 54; no. 8; pp. 3286 - 3299
• Main Authors: Tjandra, J., Ranjan, A., Ackerman, A. K., Appleton, M., Pedrazzini, S.
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.08.2023; Springer Nature B.V
• Subjects: Alloys; Aluminides; Aluminum chloride; Bend tests; Characterization and Evaluation of Materials; Chemistry and Materials Science; Corrosion effects; Crack propagation; Cracking (fracturing); Cracks; Grit blasting; Heat treatment; Interdiffusion; Intermetallic compounds; Low temperature; Materials Science; Metallic Materials; Nanotechnology; Nickel base alloys; Original; Original Research Article; Protective coatings; Single crystals; Spallation; Structural Materials; Superalloys; Surfaces and Interfaces; Thin Films; Turbine blades; Vapors
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-023-07099-5

The current work presents a systematic study of two alloy compositions (RR3010 and CMSX-4) and two types of coatings: inward grown (pack) and outward grown (vapour) deposited aluminides, exposed to 98Na 2 SO 4 –2NaCl mixture. Grit blasting was used on some of the samples, prior to coating, to mimic in-service procedures and remove oxides from the surface prior to coating. Two-point bend tests were then performed on the coated samples, with and without applied salt at 550 °C for 100 hours. Samples were pre-strained at 0.6 pct strain to deliberately pre-crack the coating and then strained at 0.3 pct for the heat treatment. Exposure to 98Na 2 SO 4 –2NaCl under applied stress of vapour-aluminide coated samples of both alloys, revealed significant coating damage in the form of secondary cracks in the intermetallic-rich inter-diffusion zone, although only CMSX-4 exhibited cracks propagating further into the bulk alloy while RR3010 proved more resistant. The pack-aluminide coating proved more protective for both alloys, with cracks propagating only into the coating and never into the underlying alloy. In addition, grit blasting proved beneficial in reducing spallation and cracking for both types of coating. The findings were used to propose a mechanism based on thermodynamic reactions, to explain the crack width changes through the formation of volatile AlCl 3 in the cracks.