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Corrosion technicians conduct routine operational status checks on Cathodic Protection (CP) Systems to ensure that the system is providing adequate levels of protection. Many common systems can include multiple 2-wire, magnesium anode test stations (1 wire for the anode and one wire for measuring Pipe-to-Soil (P/S) potentials) with simple 2-wire P/S test stations between the anode test stations. Normal potential readings should be between -0.950 and -1.150 volts with respect to a CuSO4 reference electrode along the pipeline.

Metals

Many materials have been developed to mitigate corrosion in buried and submerged metals (pipelines, storage tanks, etc.) corrosion.  The development of corrosion resistant materials i.e., plastic composites, epoxies, urethanes and the like have significantly contributed to the industry. For many applications, the “non-metallic” products are used to simply replace the metal or are applied to the metal as protective coatings. Despite this, metals and their unique properties are still major players in the corrosion control industry. 

Interrupters
An “interrupter” is a switch installed on Cathodic Protection (CP) rectifiers to enable the rectifier to be switched OFF and ON in a timed cycle. The interrupter switch can be a conventional mechanical (metallic contact) switch or an electronic, solid-state switch. The purpose of the Interrupter is to enable the CP technician to measure pipe-to-soil potentials in both an ON (CP current applied) & OFF (CP current interrupted) condition.  Referred to as an “interrupted survey”, the results of the survey reveal the effectiveness of the CP system within the influence range of the CP rectifier.

Induced AC (Alternating Current) is a common problem that occurs when buried pipelines are installed in a common corridor or near energized high-power transmission lines. 

There are two main concerns with induced AC on metallic pipelines:

1. Induced voltages can present a shock hazard to technicians who physically touch the pipeline or metallic devices connected to the pipeline.
2. Induced AC is known to be the direct cause of soil side corrosion on buried pipelines.

Many cathodic protection (CP) technicians struggle with the proper way to read and interpret shunt measurements or readings. 

 The Basics

A shunt is a calibrated resistor of a known value, which is connected in series within an electrical circuit.  By measuring the voltage across the shunt (mathematical formula used to calculate the relationship between current, voltage, and resistance: Ohms Law), the magnitude of current flowing within the circuit can be calculated.

Because we cannot physically see volts, amperes or resistance, a comparison to something familiar like a water/hydraulic circuit, is being used to demonstrate the relationship.