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Development and Testing of a Linear Polarization Resistance Corrosion Rate Probe for Ductile Iron Pipe (Web Report 4361)
Flounders, Jr., E. AND D. Lindemuth. Development and Testing of a Linear Polarization Resistance Corrosion Rate Probe for Ductile Iron Pipe (Web Report 4361). Water Research Foundation, Denver, CO, 2015.
Linear Polarization Resistance (LPR) is an electrochemical method for measuring the corrosion rate of a material in a given environment. It is an indirect measurement of the corrosion rate of surrogate electrodes that are part of the testing apparatus. LPR has primarily been applied in aqueous environments, but it can also be used in soils. In soils, LPR measures the corrosivity of the soil to a specific metal. A fully developed field-based LPR corrosion rate probe for ductile iron pipe could be an important tool for the water industry. Such a tool would allow many measurements to be made/along the alignment of an existing pipeline. The sturdy construction of the probe developed in this project allows it to be pushed into the soil to obtain soil LPR and related data. By providing instantaneous direct readings of the corrosion rate at a specific site, the timeframe for complete corrosion penetration of a section of ductile iron water main could be projected based on those readings. Depending on the nature of the data, a more detailed examination of a pipeline could be better prioritized, or subsequent follow-up readings could be scheduled. A large quantity of LPR data along a pipeline would allow more accurate predictions of corrosion rate and better predictions of corrosion penetration of the pipeline. While this project has demonstrated a viable field probe, more research will be required to determine key probe reliability issues and fully develop the empirical model to relate these LPR field measurements to corrosion losses and also possibly corrosion pit depth of ductile iron pipelines. Future studies to support development and eventual commercialization of this probe would need to include detailed soils testing near existing pipelines and excavation of pipeline sections to allow direct readings of pit depths and overall corrosion loss. These data could then be related to the LPR field measurements.
The North American water and wastewater community has hundreds of millions of feet of ductile iron pipe in service. Only a portion of the inventory has any form of external corrosion control. Ductile iron pipe, in certain environments, is subject to external corrosion.Linear Polarization Resistance (LPR) is an electrochemical method for measuring the corrosion rate of a material in a specific environment. LPR measures the electrochemical resistance of the surface in its environment. The lower the measured polarization resistance, the higher the general corrosion rate. In addition, the Current Imbalance between the electrodes can also be measured. When the Current Imbalance is higher, there is a greater tendency for localized, or pitting, corrosion (Farrag 2010). As applied to pipelines, LPR provides an indirect method of assessing the instantaneous corrosion rate of a ferrous pipeline. The LPR technique does not require the pipe to be taken out of service. Nothing has to be inserted into the pipe, nor, theoretically, would the pipe need to be excavated for direct measurement of corrosion losses. The existing LPR technique, primarily used in Australia where it is available from a commercial vendor, involves the collection and removal of test soils to the laboratory, where the soils are carefully prepared and LPR measurements are made in a soil box. The resulting LPR data provides information on the corrosion rate of the pipelines, and these data are useful in prioritizing existing pipelines for further investigation. The LPR technique, as applied in Australia, has not been developed for application to ductile iron pipe, nor has it been developed for field generation of LPR data. Development of a field-based LPR probe for ductile iron pipe may provide more data that could be useful in preliminary assessment of buried ductile iron pipelines.APPROACHA prototype LPR field probe had already been developed by the researchers in previous work, but modifications were envisioned to improve the probe’s usefulness in the field. The probe also needed further adaptation for improved measurement of corrosion rates on ductile iron pipes. Thus, two identical prototype field LPR probes were created as part of this project. Improvementson the original prototype included a smaller diameter body, relocation of wiring, and use of heat treated ductile iron rings in the probe with an annealing surface oxide, similar to that found on ductile iron pipe.Utilities were asked to provide three soil samples from their service area to be characterized and tested with the new LPR probes in the laboratory. These soil samples were used to generate LPR data from the new probes, and the data were correlated to other soil characteristics associated with soil corrosivity. All work was done in the laboratory, with simulated LPR field data generatedby testing soils in buckets, and correlating these data with soil box and other testing done on the same soils.RESULTS/CONCLUSIONSThis project established confidence that measurements from the two identical prototype LPR probes were similar in the same soil, were reproducible, and are indicative of soil corrosivity. In soil, the probes reached equilibrium and stabilized within two minutes. Side-by-side testing of the probes in the same soils obtained comparable corrosion rate values. The LPR data also trended well with the data obtained using an established two-electrode soil box corrosivity measurement. Also, solution conductivity readings taken by the LPR probes and converted to soil resistivity data were in close agreement with standard laboratory-generated soil resistivity data. Further studies will be required to establish a direct correlation between field-based LPR measurements (corrosion rate), actual corrosion loss, and possible pit corrosion of in-service ductile iron water mains.