In filiform corrosion the head of the advancing filament becomes ____________ with a low pH and lack of oxygen.

• A. Cathodic
• B. Anodic
• C. Neutral
• D. Passive

Answer: (B)

The main idea is that the advancing head in filiform corrosion is the actively dissolving part of the metal under the coating. At the leading edge, a thin electrolyte film concentrates with low pH, which makes metal dissolution more favorable. The acidic environment destabilizes any protective film and accelerates the anodic reaction, where the metal loses electrons and goes into solution (M -> M^n+ + ne−).

Even though oxygen is scarce at the head, the anodic process can still dominate there because the cathodic reactions that would consume electrons (like oxygen reduction) are limited by the lack of oxygen, so the region at the front continues to oxidize while electrons are conducted away to areas where reduction can occur. This is why the head is described as anodic. The other possibilities would imply little or no active dissolution or reduction at the head, which isn’t the case in this scenario.