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.

在以前的论文中，使用高分辨率透射电子显微镜技术检查了在现场部件上观察到的典型II型腐蚀坑的腐蚀前沿。在凹坑中观察到纳米级硫化物和氧化物的混合物，这表明固相腐蚀传播过程。在当前工作中，对原始合金表面顶部的腐蚀产物进行了表征，目的是了解腐蚀引发过程。揭示了典型的层状氧化物结构，其包含交替的富NiO和富氧化铝层。重要的是，硫酸铝Al ₂（SO ₄）₃通过化学和衍射分析发现在富含氧化铝的层中存在Al 2 O 3。硫酸铝与氧化铝的共存表明，在独特的航空发动机运行环境下，原始的富氧化铝保护层很可能受到固相反应的侵蚀。在腐蚀坑或原始金属/气体表面未发现熔融NiSO ₄ -Na ₂ SO ₄混合物的迹象。讨论了缺乏熔融硫酸盐形成的潜在原因。