Copper tubes used in air-conditioners and refrigerators often fail due to ant nest corrosion (ANC) in formic acid environment. In this paper, corrosion behavior and corrosion mechanisms of copper tubes in formic acid (HCOOH) were analyzed by vapor corrosion tests, optical microscopy (OM), scanning electron microscope (SEM), energy-dispersive X-ray spectrometry (EDS). The effects of the surface condition on ANC of copper tube were also investigated.Results showed that ANC of copper tube was a spontaneous process. The surface integrity of copper tube surface oxide layer is not the decisive factor of ANC. ANC originated from the dissolution of the surface oxide layer in HCOOH, which exposed fresh copper matrix. ANC is a special electrochemical corrosion, where the copper matrix acts as an anode and the undissolved surface oxide layer acts as a cathode due to the potential difference. The accumulation of corrosion products consisting of Cu(HCOO) 2 and Cu 2 O can produce a wedge effect and generate many microcracks until to penetrate the copper tube wall. These findings would provide a deep understanding of the corrosion behavior of copper and copper alloys.

Oxygen‐free copper (OFC) tubes are widely used in heat‐transfer units. Due to the thickness of the OFC tube wall becoming thinner and thinner, higher demands are put forward for its corrosion resistance properties. We investigated the effects of trace La addition on the corrosion properties of OFC tubes in a formic acid environment through experiments of vapor corrosion and electrochemical corrosion. Although a small number of La would exist on the grain boundaries of copper tubes, there was no apparent influence on the grain size of OFC tubes. In HCOOH vapor, La addition increased the compactness of corrosion product films, decreased the size of corrosion pits, and improved the corrosion properties of OFC tube. Furthermore, rare earth La purified copper matrix, decreased the cathode area of impurities and inhibited the cathode reaction. Electrochemical corrosion resistance properties of the OFC tube were improved by the addition of trace La.