Deposition induced corrosion is a popular attack associated with salt deposit formations on the metal or oxide surface in high temperature applications. This form of corrosion is common in systems and components used in the combustion products of fossil fuel including aircraft engines and gas turbines. Salt deposits encompass vanadates, sulfates and carbonates of calcium, sodium, and potassium.
Sodium sulfate is the most common deposit. It is formed when sodium chloride reacts with sulfur, which exists as an impurity in fuels. Severe corrosion is evident when the metal surface is wetted by molten sodium sulfate. This influences its oxidation behavior. When the deposits are absent, there would occur protective oxide scales formation on the surface of the base metal, and would serve as diffusion barriers to prevent further oxidation.
Nickel alloys that are normally used in high temperature gas turbines for power plants, marine applications and aircraft are susceptible to the deposit-induced corrosion owing to the formation of sodium sulfate-nickel sulfate eutectic. This leads to a reduction of their service life. For this reason, the deposit-induced corrosion resistance of the nickel alloys needs to be enhanced to fulfill operating requirements. This can be achieved using high temperature protective coatings. Aluminum and chromium are normally implemented as base elements for their ability to form chromium oxide and aluminum oxide for enhancing induced corrosion, carburization, and sulfidation resistance.
A team of researchers from Montana State University deposited binary and ternary thin film coatings from aluminum, chromium and carbide system implementing the magnetron sputtering system. Coated and uncoated specimens were then exposed to salt-induced corrosion in dry air/2ppm sulfate environment for 250 hours and their degradation analyzed. Their work is published in Journal of The Electrochemical Society.
The authors used pure nickel sheets as substrates. Magnetron sputtering system was then used to deposit thin films of graphite, which was 99.99% carbon, aluminum and chromium. They also monitored the thickness after deposition and recorded it as 1.5± 0.3µm. All the specimens were heat treated and taken for characterization analyses.
Uncoated and four coated samples were deposited with sodium sulfate. They were weighed periodically in the course of the process to ensure that the correct amount of sodium sulfate was deposited. These samples were exposed to air-2ppm sulfur dioxide gas flowing through a platinum catalyst to initiate the formation of sulfur trioxide.
The authors observed that, after 250hours for the uncoated nickel oxide samples, the resulting thickness was approximately 10µm. Nickel oxide reacted with the resulting sulfur trioxide forming a corrosion inducing sodium sulfate-nickel sulfate eutectic. This resulted in the formation of non-protective surface scales, which enhanced degradation of the base metal. Nickel oxide was the sole compound detected for chromium-carbide and aluminum carbide samples after exposure. The samples thickness was 14 and 10µ respectively.
Gravimetric results indicated that the mass gain was higher for aluminum carbide coated specimens as than for chromium-coated samples. Aluminum carbide was readily oxidized to aluminum oxide. However, chromium carbide oxidation proceeded through multiple decarburization processes before the formation of chromium oxide.
For chromium-aluminum and chromium-aluminum-carbide samples, the authors detected chromium and aluminum oxide indicating that both materials protected the base alloy for the 250hours. However, owing to the presence of nickel oxide, chromium-aluminum coatings were rendered less effective.
A thicker oxide scale was observed for the chromium-aluminum samples compared to chromium-aluminum-carbide samples. This suggested that the ternary Cr2AlC thin film coating offered better resistance.
Lik Ming Aw, Roberta Amendola, John W. Ryter, Madisen W. McCleary, Paul E. Gannon, McLain E. Leonard, and James L. Smialek. Investigation of Na2SO4 Deposit Induced Corrosion of Cr, Al, C Binary and Ternary Thin Film Coatings on Ni-201. Journal of The Electrochemical Society, 164 (6) C218-C223 (2017)Go To Journal of The Electrochemical Society