Sodium vanadate induced corrosion of MCrAlY coatings – Burner rig studies

Abstract

Dedicated to Professor Dr. Alfred Rahmel on the occasion of his 60th birthdayThe corrosion of several MCrAIY-coatings (M = Ni, Fe and Ni + Co) has been studied in a high velocity burner rig at 650, 800 and 950°C. The fuel used 'was diesel oil with additions of 3% sulphur, 200 ppm vanadium and 100 ppm sodium.The deposits formed on the specimens mainly consisted of sodium vanadates which were molten at the test temperature. Sodium sulphate was only found at and below 650°C. The corrosion mechanism involved was vanadate-induced hot corrosion. This corrosion mechanism is characterized by the formation of an oxide layer adjacent to the metal, the dissolution of oxide in the molten deposit, and precipitation of vanadates or oxides near the outer surface of the deposit. The continuous deposition of fresh vanadate on the surface served to maintain high corrosion rates for extended exposures. Das Korrosionsverhalten einiger MCrAIY-Schutzschichten(M = Ni, Fe und Ni + Co) wurde mit Hilfe eines Hochgeschwindigkeits-Brennerprufstandes bei 650, 800 und' 950 "C untersucht. Als Brennstoff wurde Diesel01 verwendet, dem 3% Schwefel, 200 ppm Vanadium und 100 ppm Natrium zugesetzt wurde.Die Ablagerungen auf den Proben bestanden hauptsachlich aus Natriumvanadaten, die bei den Versuchstemperaturen im geschmolzenen Zustand vorlagen. Natriumsulfat wurde nur bei und unterhalb 650 "C gefunden. Die abgelagerten Vanadate induzierten die HeiRgaskorrosion. Der Korrosionsmechanismus kann wie folgt beschrieben werden: Bildung einer Oxidschicht auf dem Metall, Auflosung des Oxides in der geschmolzenen Ablagerung und Ausscheidung von Vanadaten oder Oxiden nahe der auReren Oberflache der Ablagerung.

Degradation of free-standing air plasma sprayed CoNiCrAlY coatings by vanadium and phosphorus pentoxides

“…V 2 O 5 is a strong acidic oxide, and acidic dissolution of protective oxide scale in the presence of V 2 O 5 melt had been given greater importance in earlier studies [10,12]. Trivalent protective oxides such as Cr 2 O 3 and Al 2 O 3 would act as basic oxides in the presence of a strong acidic medium, and would readily be dissolved in an acidic medium such as V 2 O 5 melt.…”

“…This clearly warrants an understanding of interaction of such corrosive molten deposits with the underlying metallic bond coat, which also can be applied as stand-alone ORCs. Investigations of bond coat degradation by corrosive molten deposits have been seldom documented [10][11][12], where MCrAlY (M = Ni and/or Co) coatings were reported to degrade by fused sodium vanadates through dissolution of Ni and Co oxides [12]. Fused salt was also found to hinder the formation of protective TGO layer on NiCrAlY coatings and enhanced the Al depletion through accelerated internal oxidation [11].…”