Engineering Group Performs Long-term Corrosion Monitoring

Stray current is defined as the existence of electrical potential that can be found between objects which should not be subjected to voltage.

One of ICG’s projects involve working to prevent stray current corrosion caused by light rail transit systems. In 2010 a local gas utility company noticed during daily operations the light rail system was leaking stray current from the rails into the ground.  This is problematic as current discharged into the ground flows along paths of lesser resistance. These paths of lesser resistance can be highly conductive (low resistivity) soils, or metallic conductors (such as pipelines and other metallic utilities). Should the stray current hop onto one of these utility pipelines acting as a conductor, it can continue traveling and eventually discharge from the conductor to the traction power substation (see figure below). Leading to metal loss and potentially pipe failure.

Once a stray current occurrence was identified the local gas utility and the light rail operator quickly formed a cooperative with the goal of creating a Long-term Data Logging Program. The programs’ goal is to identify and mitigate these stray currents. Ultimately, reducing corrosion on any pipelines within the vicinity of the rail lines.

Who better to bring in on a long-term corrosion control effort than Infinity Corrosion Group? In April, 2015 ICG joined this effort and since then has been monitoring dozens of at-risk utility sites due to their proximity to the light rail system. The sites are monitored by data loggers which continuously record pipe-to-soil potentials. By bringing this continuous data stream into Grapher, Zach can document patterns and changes in key variables that provide insight to the need and type of mitigation or remediation required at these sites along the light rail system.

One way Zach uses Grapher to document these changes is by plotting the pipe-to-soil potentials for each utilities test point. These plots are called Stray Current Plots and are used to simultaneously keep a historical record and monitor new stray current events. Grapher’s customization options provide Zach with the ability to utilize multiple axes and color schemes; ensuring his team can take one look at the plot and quickly determine when a new stray current event has occurred.

The below Stray Current Plot represents data collected in November of 2018.  The Y-axis on the left denotes the pipe-to-soil potential voltage readings, and the X-axis represents the time in days. The four different background colors of blue, green, yellow, and red represent the different stray current corrosion risks. The two Y-axes on the right side of the plot correspond to the temperature and precipitation denoted by the darker blue line and light blue bars respectively.

This long-term data logging with Grapher also allows ICG to define baseline “quiet times,” when the trains are not operating. These periods of “quiet time” give an indication of the cathodic protection level on the protected steel pipelines when unaffected by stray currents. In the plot below we can notice the pipe-to-soil potentials remain fairly unchanged throughout the “quiet time” period, but a few small unknown events can occur. When these unknown events pop up ICG can contact the light rail operator so they can conduct their own investigation.

A crucial aspect of any long-term monitoring effort is the ability to efficiently update your existing plots with newly collected data. Zach creates the plots above for 30 locations each month. A painstaking data sorting/loading process without utilizing Grapher’s automation features. By leveraging Scripter, he can quickly call the latest dataset from any of the monitoring locations, and the data will autofill in a Grapher Template File (GRT) of his required plot type. The script can even be written to include historical information such as a running average of the site voltage for comparison, generated by Grapher’s fit curve feature.

ICG utilizes several other plot types in their monitoring efforts outside of the light rail system; one being Pipe-to-Soil potential plots. Pipe-to-soil potential plots are generated by data collected from Cathodic Protection Surveys. Cathodic Protection Surveys determine the level of corrosion protection a pipe has based on the change in pipe potential while the cathodic protection system is energized. By performing these surveys and collecting the pipe-to-soil potentials ICG is able to determine if pipelines are receiving adequate cathodic protection, and accordingly at a lower risk of corrosion.

Once the surveys are finished, Zach brings the data into Grapher to create overview plots that portray the pipe-to-soil potentials along specific pipelines. Satellite imagery is added above the graph to ensure clients understand exactly where the test stations and other associated pipeline facilities are located. Providing ICG with a graphic that has the context their clients need to fully understand reports.

In the graphs pictured the dark purple line represents the “ON” pipe-to-soil potential (volts), the dotted blue line represents the instant “OFF” or “Polarized” pipe-to-soil potential (volts), and the dashed red line denotes the -0.850 volt criterion the readings should be above to be considered protected by the lighter purple and dotted blue lines are historical data.