A 6”, A106 Gr B, flanged line carrying caustic wash water at 200º F has signs of atmospheric corrosion. Which of the following may have help accelerate the corrosion?

Atmospheric corrosion affects carbon steel piping and equipment exposed to moist atmospheric air containing oxygen and contaminants. Airborne particulates such as fly ash significantly accelerate atmospheric corrosion rates. Fly ash deposited on uninsulated exterior surfaces acts as a hygroscopic medium, drawing moisture out of the air and retaining it against the metal surface for prolonged durations. Furthermore, fly ash often contains acidic compounds, salts, or carbonaceous species that enhance the conductivity of the electrolyte film. This localized wet environment creates aggressive corrosion cells, accelerating the oxidation of A106 Grade B carbon steel, especially in warm humid outdoor environments or operating temperatures below water condensation points.

A. Sulfides are internal process contaminants affecting internal fluid contact surfaces rather than external atmospheric exposure corrosion mechanisms on bare piping.
C. Caustic is an internal process chemical (caustic wash water) and does not drive external atmospheric corrosion unless severe external leakage and evaporation occur specifically causing caustic cracking.
D. Option B is correct because fly ash is a recognized particulate contaminant that holds moisture and accelerates atmospheric corrosion.

- American Petroleum Institute. API RP 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 3rd Edition, 2020, Section 3.11 (Atmospheric Corrosion), pp. 3-41–3-44.