Influence of silver flux residue on corrosion behavior of Bundy tube

Objective The purpose was to study influence of silver flux residue on corrosion behavior of Bundy tube. Methods By means of XRD, stereo-microscope and scanning electron microscope, influence of 308S silver flux residue on corrosion behavior of Bundy tube before and after welding was systematically studied and analyzed. Results The results showed that solution of 308S flux with main components of KBF₄ and B₂O₃ was acidic, Zn plating layer on the surface reacted with H⁺, HF⁻ and F⁻ in the solution to form soluble complex salts, and the rate decreased with the extension of time. In the salt spray corrosion box, due to more H⁺, F⁺, Cl⁻ and other active ions, there was an electric potential difference between Zn and Fe elements in Bundy tube and Ag and Cu elements in brazing filler metal, and plating Zn layer on the surface could form a galvanic cell with other metals. Zn substance on the surface was dissolved, it was accompanied by puffy cotton-like ZnO, ZnF₂ and other substances. Then, active ions entered the interior along the cracks and aggravated internal corrosion. After Zn plating layer was corroded, the internal Fe and O combined to form FeO, Fe₂O₃ and other oxides, while it reacted with NaCl to form FeCl₃ and other substances to accelerate corrosion, and the overall corrosion joint showed iron red. Conclusion After brazing operation, flux residue was highly corrosive, which had an adverse effect on sealing and life of refrigeration and pressure vessel pipelines. Therefore, in order to improve service life of brazed joints, amount of flux should be reduced as much as possible in brazing, it was carefully cleaned and polished after brazing and anti-rust and anti-corrosion treatment should be taken.