Understanding the Type II Corrosion Mechanism

Abstract

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 Al₂(SO₄)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 NiSO₄-Na₂SO₄ 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.