Moisture-induced delayed spallation and interfacial hydrogen embrittlement of alumina scales

While interfacial sulfuris 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 +2.0V, 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.