Quality control in district heating systems to keep uniform corrosion rates low and localized corrosion minimal is based on water quality control. Side-stream units equipped with carbon steel probes for online monitoring were mounted in district heating plants to investigate which techniques would be applicable, and if on-line monitoring could improve the quality control. Water quality monitoring was applied as well as corrosion rate monitoring with linear polarization resistance (LPR), electrochemical impedance spectroscopy (EIS), electrical resistance (ER) technique, mass loss and a crevice corrosion cell for localized corrosion risk estimation. Important variations in corrosion rate due to changes in make-up water quality were detected with the continuous monitoring provided by ER and crevice cell, while LPR gave unreliable corrosion rates. The acquisition time of two-three days for EIS measurements was too long for real time data, and reliable mass loss data could only be obtained after 6 months exposure. It was furthermore found that localized corrosion events detected by the carbon steel crevice corrosion cell correlated with oxygen peaks of even a few hours duration.
The water in the Danish district heating systems is generally of high quality with low oxygen concentration and conductivity, and high pH resulting in low corrosion rates for both uniform and
localised corrosion of carbon steel. However, the corrosion resistance of carbon steel is highly affected by environmental changes such as pH decrease, increasing conductivity, increasing oxygen content, formation of deposits and microbial growth. In case of corrosion failures only periodical water analyses are available, hence it is difficult to correlate corrosion attacks with operation of the plant and the subsequent changes in the water quality. A three-year Nordic research project supported by Nordic Innovation Fund ~ has focused on on-line monitoring of corrosion rate and water parameters in district heating systems in order to provide better quality control. If the risk of corrosion is detected at an early stage, it might be possible to correct the problem and avoid a failure. In specially designed side-stream units on-line monitoring of oxygen, pH, conductivity and temperature was applied concurrently with monitoring of corrosion rate with electrochemical techniques (linear polarization resistance LPR, electrochemical impedance spectroscopy EIS) electrical resistance (ER) technique, mass loss and a crevice corrosion cell for localized corrosion risk estimation. Units were installed at seven different plants.
