Pitting corrosion and mechanical resistance of 441 stainless steel pipes used in the production of condensing boilers

In this work, pitting corrosion resistance and mechanical tensile strength were evaluated for type 441 stainless steel pipes used to produce "spiral"-shaped heat exchangers for condensing boilers. Samples were obtained from untreated pipes and in-line annealing treated pipes at temperatures of 700, 800 and 900 °C. Pitting corrosion resistance was evaluated by cyclic potentiodynamic anodic curves in a three-electrodes electrochemical cell containing an aqueous solution of NaCl at room temperature and having a chloride concentration of 0.05 wt%. Mechanical tensile strength was evaluated by means of tests carried out on straight tubes. Electrochemical experimental results show that localized corrosion resistance is higher in annealed stainless steel as the treatment temperature increases. The tensile tests confirmed that an increase of 441 ductility has been observed, especially at high temperatures.