Electrochemical noise (EN), linear polarization resistance (LPR), and harmonic distortion analysis (HDA) were used with three-electrode probes to monitor the corrosion occurring in soil near a gas pipeline under cathodic protection. The test site was a cathodic protection (CP) test station where impressed current CP was applied to a 2 in. (5.1 cm) diameter coated steel pipe using an 84 in. (0.2 m) TA-2 high-silicon cast iron anode. Electrochemical measurements were made at two locations inside the CP field and one outside the CP field. One set of measurements was made with the CP system off to obtain base line data and two with the CP system on. Results indicate that CP does not interfere with the measurement of corrosion rate and pitting factor using EN, LPR, and HDA techniques.
An analysis1 of 5872 pipeline incidents in the U.S. from 1970 to 1984 reported that 54% of the service failures of gas pipelines were attributable to outside forces such as earth movement, weather, and equipment operation by outside parties, 17% to corrosion, and 17% to material failures. A later analysis2 of 12,137 failures in Canada from 1980 to 1997 concluded that 63% of pipeline failures were caused by corrosion, with 50% due to internal corrosion and 13% due to external corrosion. Corrosion failures represent a significant proportion of the total number of failures. Probes that can detect internal and external corrosion in real time before failure occurs will enhance gas transmission pipeline reliability.
External corrosion of gas transmission pipelines is usually controlled by the application of various polymeric coatings augmented with cathodic protection (CP). When corrosion does occur on the outside of the pipeline, the combination of general and localized corrosion with the high stresses in the pressurized pipelines can sometimes lead to stress corrosion cracking (SCC). In cases where CP is inadequate or non-existent, pipelines exposed to ground waters can experience transgranular SCC due to low pH (6.5) carbon dioxide containing water.3 Even when CP is adequate, gas pipelines may be susceptible to intergranular SCC due to the higher pH external environment generated by CP3. One method to address this type of corrosion problem is to place corrosion rate probes to monitor the corrosivity of the soil in areas that have a higher risk for corrosion.
Laboratory studies have shown that electrochemistry-based corrosion rate probes can be used to monitor the corrosion of steels in soils.4 Corrosion rates were shown to have a good agreement with gravimetric weight loss measurements and were also sensitive to changes in O2, CO2, and moisture content. Others have shown that electrical resistivity probes can be used to monitor the effectiveness of cathodic protection (CP) of pipelines.5
While there are a variety of electrochemical techniques that can be used to measure the corrosion rates of metals in soils, the effect CP will have on the corrosion rate measurements is unknown. The purpose of the research reported here is to verify that corrosion rate and pitting factor measurements can be made in the soil adjacent to a natural gas transmission pipeline under applied CP. This would allow a continuous direct assessment of the corrosivity of the soil and ground water around natural gas transmission pipelines.
EXPERIMENTAL DESIGN
All of the electrochemical corrosion tests were performed using three-electrode electrochemical probes with electrodes made of the same material and surface area. The first electrode was used as the working electrode, the second as the counter electrode, and the third as the reference electrode.
