Moisture-Induced Delayed Spallation and Interfacial Hydrogen Embrittlement of Alumina Scales

Authors

James L. Smialek Ph.D., NASA Glenn Research CenterFollow

Date of this Version

2006

Citation

JOM: The Journal of The Minerals, Metals & Materials Society, Volume 58, Issue 1, pp 29–35

Comments

U.S. Government Work

Abstract

While interfacial sulfur is the primary chemical factor affecting Al₂O₃ scale adhesion, moisture-induced delayed spallation appears as a secondary, but impressive, mechanistic detail. Similarities with bulk metallic phenomena suggest that hydrogen embrittlement from ambient humidity, resulting from the reaction Al_alloy +3(H₂O)_air = Al(OH)–3 +3H⁺ may be the operative mechanism. This proposal was tested on pre-oxidized René N5 by standard cathodic hydrogen charging in 1N H₂SO₄, as monitored by weight change, induced current, and microstructure. Cathodic polarization at –2.0 V abruptly stripped mature Al₂O₃ scales at the oxide-metal interface. Anodic polarization at +2.0 V, however, produced alloy dissolution. Finally, with no applied voltage, the acid electrolyte produced neither scale spallation nor alloy dissolution. Thus, hydrogen charging was detrimental to alumina scale adhesion. Moisture-induced interfacial hydrogen embrittlement is concluded to be the cause of delayed scale spallation and desktop thermal barrier coating failures.