Journal of the Southern African Institute of Mining and Metallurgy
On-line version ISSN 2411-9717
VAN DER MERWE, J.W. and PALAZZO, A.. Comparison of linear polarization resistance corrosion monitoring probe readings and immersion test results for typical cooling water conditions. J. S. Afr. Inst. Min. Metall. [online]. 2015, vol.115, n.3, pp. 173-178. ISSN 2411-9717.
Owing to the corrosiveness of the untreated brackish cooling water typically used for steel mills (and other environments), it is important to treat the water and monitor corrosion in such systems. Generally, corrosion rates are monitored with corrosion probes inserted into a pipeline or vessel. This has been standard practice for many years, and is widely accepted in industry. Typically, two kinds of probes are used - electrical resistance and linear polarization resistance (LPR) probes. In this study, the effectiveness and accuracy of the LPR technique was evaluated by comparing the electrochemical measurements with the results of mass loss tests using corrosion coupons. The corrosivity of the environment, a synthetic brackish water, was varied by varying the calcium hardness and alkalinity, and to simulate actual plant conditions temperatures of 35°C and 45°C were used. In addition to the corrosion rate measurements, the iron concentration was measured, as well as the imbalance from the probe. The corrosion rates obtained by LPR were from 57% lower to 385% higher than those from the immersion tests. Most probe measurements were higher than the immersion results, and 50% of the probe results were 50% or more higher than the immersion results. The best correspondence between the two methods was obtained at low calcium levels, except for one measurement that was 93% higher than the coupon results. There was no clear correlation between parameters such as temperature and total alkalinity and the difference between the results. It would therefore appear that LPR measurements can differ significantly from immersion results, and LPR results should therefore be used with caution in industrial applications.
Keywords : linear polarization resistance; probe; corrosion rate; corrosion monitoring; cooling water.