Corrosion Prevention Test 2026 – Complete Exam Preparation Guide

Filiform corrosion is a coating-related form of corrosion that starts at tiny defects or breaks in a protective coating and grows as thread-like filaments beneath the coating. It needs a moisture-containing electrolyte at the coating–substrate interface and a metal surface that can locally dissolve to drive the process. Metals that are common in structural use and that readily form and interact with oxide films under organic coatings are the ones where this phenomenon shows up most readily, especially aluminum alloys and magnesium, and it can also occur on steel when coatings fail. The filaments advance in the direction of the electrolyte flow under the coating, creating those worm-like, branchy patterns you see on the surface.

Copper-based alloys (like bronze and copper-tin) and highly corrosion-resistant metals such as titanium, zirconium, and niobium are less prone to this specific under-coating filament corrosion, and plastics or composites aren’t metallic substrates for this phenomenon. That’s why the metals listed—steel, aluminum, and magnesium—are the ones most associated with filiform corrosion.