Final answer:
Corrosion on aluminum alloy parts might not be easily detectable through visual inspection due to the formation of a protective aluminum oxide layer that masks underlying corrosion, which can be a galvanic process happening underneath the surface layer.
Step-by-step explanation:
The question seeks to identify which fault might not be easily detected by a visual inspection of aluminum alloy parts or structures. Among corrosion, cracks, weld defects, and surface contamination, the first closely related to being undetectable is corrosion. Aluminum is known for its corrosion resistance because it forms a protective layer of aluminum oxide on its surface when it reacts with oxygen. This oxide layer is generally imperceptible to the eye and can effectively mask underlying corrosion, which could be spreading beneath this protective barrier. Furthermore, corrosion is a galvanic process that can occur under protective coatings. Small gaps in coatings, like paint, may lead to rapid corrosion beneath the surface, which is not visible until significant damage has occurred.