Ant‐Nest Corrosion Morphology of Deoxidized High Phosphorous Copper and Oxygen‐Free Copper Tubes Exposed to Formic Acid Vapor

ABSTRACT In this study, the effect of phosphorus on ant‐nest corrosion (ANC), a form of pitting corrosion that occurs in carboxylic acid environments, was evaluated using deoxidized high phosphorous copper (DHP) and oxygen‐free copper (OFC) with different phosphorus contents. The samples were exposed to 1 vol% formic acid vapor for 30, 60, and 90 days. Surface and cross‐sectional observations, measurements of pitting area and depth, corrosion potential and pH monitoring, and X‐ray diffraction (XRD) analysis were conducted. As a result, it was observed that corrosion in DHP progressed locally and penetrated deeply in the thickness direction of the copper tube, while corrosion was suppressed in the surrounding areas. In contrast, OFC exhibited uniform corrosion across the entire surface, with circular pits deepening over time in the thickness direction. After 30 days of exposure, XRD analysis revealed that DHP exhibited stronger peaks corresponding to Cu(OH)₂·H₂O, suggesting the generation of OH⁻ ions and a localized increase in pH during the corrosion process. These findings indicate that phosphorus significantly affects corrosion morphology by altering the local pH environment.