Predicting corrosion rate in chilled HVAC pipe network: Coupon vs linear polarisation resistance method

Research has been undertaken to predict corrosion rate of conveyance pipes used in a Heating Ventilation and Air Conditioning (HVAC) system. This work focuses on comparing the performance of a conventional corrosion predicting technique known as the “coupon method” versus a relatively modern technique called Linear Polarisation Resistance (LPR). An experimental study has been conducted by designing a relatively modern rig that was representative of a typical HVAC system. It was found that both the coupon method and the LPR probe predicted similar corrosion rates with a difference of 8.3% which is insignificant in practical applications. Four highly important in-situ scenarios were simulated for a water temperature range of 12–32 °C, and it was found that an LPR probe with corroded electrodes over-predicted corrosion rate by 20% compared to a clean LPR probe. Also, a probe in use for 2 months over-predicted corrosion by 44% which is a significant error. A pipe network dosed with inhibitor was observed to reduce the rate of corrosion by 50% and 25% for water temperature of 12 °C and 32 °C respectively. It was also determined that one degree (oC) increase in water temperature will increase the corrosion rate by 3.5% on average. It has been identified that both coupon and LPR probes give similar results though both have limitations and continuous monitoring of corrosion is the way to insure a low maintenance HVAC pipe network.