Relationship between cathodic protection current and limit diffusion current as an additional criteria of cathodic protection
Keywords:steel X70 for pipelines, protective polarization potential, cathodic protection, residual corrosion rate, criteria of cathodic protection efficiency.
The influence of the temperature and oxygen concentration in NS4 solution on the electrochemical characteristics of X70 steel for pipelines and the relationship of cathodic protection current density to limiting diffusion current density, in the normalized DSTU 4219 range of protective polarization potentials from -0.75 V to -1.05 V (сh.s.е.) were investigated. It has been established that with increasing of the temperature from 20 to 80 °C, the corrosion activity of X70 steel increases, which is confirmed by more negative values of corrosion potential and an increasing of the corrosion rate, and indicates on the prevailing effect of temperature on the corrosion process compared to the effect of oxygen concentration. In the temperature range considered, the limiting diffusion current has a maximum value at 40 °C and decreases with increasing of temperature, correlating with a decreasing of the oxygen concentration. Under conditions of free oxygen access to the NS4 solution in the normalized DSTU 4219 range of protective polarization potentials, the ratio of cathodic protection to limiting diffusion current varies from 0.4 to 1.3 at temperatures (20-40) oС, at temperatures (60-80) oC the ratio is less than 1. Under conditions of limited oxygen access, the ratio increases from 1.1 to 4.7. Using calculated and experimental results, it was shown that, at various values of relationship , conditions for excessive hydrogen evolution can be created, which represents a threat to the safe operation of the pipeline over time. Data on the ratio values should be taken into account when analyzing the protection state of pipelines.
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