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

Abstract

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 98Na₂SO₄-2NaCl 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 98Na₂SO₄-2NaCl 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 AlCl₃ in the cracks.

Fig. 1

Two-point bend test setup

Fig. 2

Schematic of two-point bend test. Modifying length is done by adjusting the bolt

Fig. 3

(a) Inward grown pack NiAl coating on CMSX-4 sample, (b) outward grown vapour NiAl coating on CMSX-4 sample, (c) pack aluminide coating on RR3010 sample, and (d) vapour aluminide coating on RR3010 sample

Fig. 4

Grit-blasted vapour NiAl coating on CMSX-4

Fig. 5

Collage of scanning electron micrographs of each coating type, with their corresponding EBSD phase map and Inverse Pole Figure map (Z-axis). (a) pack aluminide coated sample, (b) vapour aluminide coated sample and (c) vapour aluminide coated sample, grit blasted

Fig. 6

Cross-section image of (a) unsalted and (b) salted (98Na₂SO₄–2NaCl) samples of CMSX-4 tested at 550 °C coated with pack NiAl, with higher magnification at the crack tip. (c) unsalted and (d) salted RR3010. All samples pre-strained to 0.6 pct, then strained to 0.3 pct during heat treatment

Fig. 7

Cross-section image of a salted (98Na₂SO₄–2NaCl) sample of RR3010 tested at 550 °C coated with pack NiAl, with higher magnification at the crack tip. Samples pre-strained to 0.6 pct, then strained to 0.3 pct during heat treatment for 100 h (salted) and 124 h (unsalted). A large inclusion is present at the coating- alloy interface

Fig. 8

(a) Cross-section image of CMSX-4 coated with vapour aluminide, unsalted. (b) Cross-section image of salted CMSX-4 coated with vapour aluminide. Crack tip is shown on the right. (c) cross section of RR3010 unsalted sample. (d) Cross section of RR3010 salted sample

Fig. 9

Cross-section image of bare and grit-blasted samples of CMSX-4, both coated with vapour NiAl. Note that no cracks were observed in the grit-blasted sample. Samples pre-strained to 0.6 pct, then strained to 0.3 pct during heat treatment

Fig. 10

Cross-section image of salted grit-blasted samples of CMSX-4 coated with vapour NiAl. A few inclusions were observed at the coating-alloy interface. Note that one very thin crack, about 30 μm deep, was observed in the salted grit-blasted sample. Samples pre-strained to 0.6 pct, then strained to 0.3 pct during heat treatment at 550 °C

Fig. 11

Schematic showing the proposed reaction mechanism observed on vapour aluminide coated samples of RR3010 and CMSX-4 due to the addition of salt. (a) the alumina and sodium sulfate combine on the surface to create sodium aluminate and sulfur trioxide, (b) the combination of gaseous sulfur trioxide and sodium chloride then releases hydrochloric acid in the presence of moisture in the air, (c) the hydrochloric acid then reacts with the remaining alumina turning it into volatile aluminium chloride