Understanding the Type II Corrosion Mechanism

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Vol. 52; no. 4; pp. 1492 - 1502
• Main Authors: Zhang, Wei-Jun, Sharghi-Moshtaghin, Reza
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.04.2021; Springer Nature B.V
• Subjects: Aluminum; Aluminum oxide; Aluminum sulfate; Characterization and Evaluation of Materials; Chemistry and Materials Science; Corrosion; Corrosion mechanisms; Corrosion products; High resolution electron microscopy; Materials Science; Metallic Materials; Nanotechnology; Original Research Article; Pitting (corrosion); Sodium sulfate; Solid phases; Structural Materials; Surfaces and Interfaces; Thin Films
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-021-06168-x

In a previous paper, the corrosion fronts of typical Type II corrosion pits observed on field parts were examined using the high-resolution transmission electron microscopy technique. A mixture of nanosized sulfides and oxides was observed in the pits, which suggests a solid-phase corrosion propagation process. In the present work, the corrosion products atop of the original alloy surface were characterized with an aim to understand the corrosion initiation process. A typical layered oxide structure was revealed comprising alternated NiO-rich and alumina-rich layers. Importantly, aluminum sulfate Al 2 (SO 4 ) 3 was found to be present in the alumina-rich layers by chemistry and diffraction analyses. The co-existence of aluminum sulfate with alumina suggests that the original protective alumina-rich layer was likely attacked by a solid reaction under the unique aeroengine operation environment. No indication of molten NiSO 4 -Na 2 SO 4 mixture was identified in the corrosion pits or at the original metal/gas surface. The potential causes of lack of molten sulfate formation were discussed.