Effect of phosphorus on the ant-nest corrosion mechanism

Abstract

Ant-nest corrosion is a unique premature failure of phosphorus-deoxidized copper tubes in the HVAC industry. This study aimed to investigate the effect of phosphorus on ant-nest corrosion by immersing oxygen-free copper tubes (P0%), phosphorus-deoxidized copper tubes (P0.02%), and copper tubes with 0.21 and 0.29% phosphorus in 103ppmHCOOH solutions for 80 days. Under microscopic examination, the surfaces of all the copper tubes changed from purple-orange to purple gray/brown after immersion for 60 days, indicating residual solid formation caused by redox reactions, identified as the Cu2O layer by XRD. Using FESEM/EDX, more branched tunnels were found in oxygen-free copper tubes than in the phosphorus-deoxidized copper tubes (P0.02%), where only Cu and O were present inside the copper wall. Copper tubes with 0.21 and 0.29% phosphorus suffered several micron-pitting corrosions. MP-AES results showed that P0% samples had a lower mean of Cu concentration than copper tubes (P0.02%), and acid-base reactions played a role in early-stage corrosion. The results showed that the addition of a phosphorus solid to the copper tube had no effect on the accelerated progression of ant-nest corrosion. Electrochemical analysis demonstrated that phosphorus was rapidly dissolved in the copper tubes upon immersion in the HCOOH solution during the early-stage of corrosion, leading to significant the consumption of oxygen during the anodic reaction of the ant-nest corrosion mechanism. Therefore, it can be concluded that ant-nest corrosion can occur on high-purity copper tubes, which require moisture, oxygen, and formic acid simultaneously.