ADAPTATION OF THE METHOD OF POLARIZATION RESISTANCE TO THE EVALUATION OF CORROSION RATE IN THE FORMATION OF DEPOSIT OF DIFFICULTLY DISSOLVED IRON OXIDES

摘要

We study the possibilities of application of the method of polarization resistance to the evaluation of corrosion rate in tap water. Under the analyzed conditions, iron oxyhydroxides beta- and gamma-FeOOH are formed on the surface of the gauge. If the external polarization is applied, then oxyhydroxides participate in electrochemical transformations, which leads to the overestimation of the measured values of corrosion rate. The effects of the water hardness, temperature, and flow rate on the composition, structure, and electrochemical activity of corrosion products are analyzed. It is shown that, in cold tap water, the presence of corrosion products leads to a 3.5-times overestimation of the results. In hot water, the surface is mainly covered with iron-oxide compounds, which do not participate in the electrochemical transformations. Hence, the results are overestimated by a factor of at most 1.6 times. As the flow rate of hot water increases to 0.45 m/sec, the influence of corrosion products becomes weaker. To guarantee the agreement between the weight-loss and electrochemical techniques, it is proposed to decrease the recalculation coefficient B in Stern's equation down to 8 mV for cold tap water and to 14 mV for hot tap water if the flow rates do not exceed 0.3 m/sec.