On a steel tank without cathodic protection, a flaw in the coating causes what change at the flaw area?

On a steel tank without cathodic protection, a flaw in the coating causes what change at the flaw area?

• A. Cathodic surface is increased
• B. Current flow is increased from this area
• C. Current flow is increased to this area
• D. There is no change in current flow

Answer: (B)

Expose steel to an electrolyte through a coating flaw creates an anodic site compared with the surrounding coated metal. The oxidation reaction at this flaw pushes current out into the surrounding environment, so the corrosion current density increases specifically at the flaw and flows away from that area through the electrolyte and the metal to any cathodic sites. Without cathodic protection, there’s no external influence to drive current toward the flaw, so the increased current is outward from the flaw rather than into it. This is why the correct description is that current flow is increased from the flaw area. The other options imply the flaw becomes more cathodic, or that current would enter the flaw, or that nothing changes, which isn’t the case for a coating defect in unprotected steel.

Expose steel to an electrolyte through a coating flaw creates an anodic site compared with the surrounding coated metal. The oxidation reaction at this flaw pushes current out into the surrounding environment, so the corrosion current density increases specifically at the flaw and flows away from that area through the electrolyte and the metal to any cathodic sites. Without cathodic protection, there’s no external influence to drive current toward the flaw, so the increased current is outward from the flaw rather than into it. This is why the correct description is that current flow is increased from the flaw area. The other options imply the flaw becomes more cathodic, or that current would enter the flaw, or that nothing changes, which isn’t the case for a coating defect in unprotected steel.