How is corrosion below aluminum typically indicated?

Corrosion below aluminum is typically indicated through the presence of a visible white powdery residue. This residue forms as aluminum oxidizes, leading to the development of aluminum oxide. The white powder is a clear sign that corrosion is occurring and can often be observed on surfaces that have not been adequately protected.

The other choices may relate to different types of surface degradation or material issues but do not directly indicate corrosion occurring specifically beneath aluminum surfaces. For instance, bubbling and peeling paint may suggest moisture issues or poor surface preparation rather than highlighting aluminum corrosion itself. A significant change in aircraft weight is not a direct indication of corrosion but rather could result from various factors like fuel load or structural issues. Small dark grey lumps can arise from several conditions unrelated to the aluminum's corroding state. Therefore, the presence of powdery white residue is the most accurate indicator of corrosion occurring below aluminum.