Pipe to Soil Potential
Pipe to Soil Potential measurements are important for assessing the performance of cathodic protection (CP) systems and detection of certain types of anomalous conditions along underground, coated pipelines. Impressed current cathodic protection uses an electrical current that is introduced into the pipeline to suppress nodic activity and arrest or minimize corrosion activity. Another method of protection connects the metal to be protected (pipeline) to a more easily corroded ‘sacrificial anode’ such as magnesium (mag bag) or zinc to act as the anode. The sacrificial anode then corrodes instead of the protected metal.
Applications for Pipe to Soil Potential:
Measures Cathodic Protection (CP) potential in an area
Detects any problems with CP so client can make alteration
Here are some advantages of Pipe to Soil Potential:
Pipe to Soil Potential is an established method for measuring cathodic protection.
Measurements of structure to soil are helpful in location of possible corrosive areas in a metallic structure such as the pipeline. The corrosive areas are indicators of current flowing from the structure to the soil.
Provides useful data helping client determine the corrosivity of the pipeline in that areas.
Soil resistivity is measuring the conductivity of the soil around the pipeline. The soil resistivity is a critical factor to pipe to soil potential. With cathodic protection, you are introducing an electrical current between the pipe and the soil so the soil resistivity measures how much the soil resists the flow of electricity. When designing an extensive grounding system, it is advisable to locate the area of lowest soil resistivity in order to achieve the most economical grounding installation. Typically, the lower the resistivity the higher the corrosivity.
Field soil resistivity measurements are taken with a specially designed probe that is inserted into the ground near the underground pipeline. The soil resistivity helps clients make any adjustments needed to protect the pipelines.
Moisture Content affects soil resistivity
The moisture content in the soil is a large factor in soil corrosivity along with other variables. Water is considered one of the major elements needed for the process of electrochemical corrosion, along with other components such as metal and oxygen. Essentially, if the soil is totally dry, corrosion won’t happen. According to various studies, higher moisture content can reduce the soil resistivity, which can in turn increase the potential for corrosion. However, when the soil has reached its saturation point, extra moisture will have little or no effect on the resistivity.
Applications for Soil Resistivity:
Measures the conductivity of the soil
Determines soil resistivity so cathodic protection can be adjusted as needed
Soil pH is the measure of the acidity or alkalinity of the soil. Soil pH can range from between 2.5 and 10. Corrosion potential, premature pitting in steel and metallic structures is increased when the pH level reaches 5 or below. A pH of 7, or neutral, is favorable because it can minimize the possibility of damage due to corrosion. Rainfall affects soil pH.
Like soil resistivity, soil pH measurements are taken with a specially designed probe that is inserted into the ground near the underground pipeline. Soil pH and resistivity affect cathodic protection.
Applications for Soil pH:
Measures the soil pH
By determining soil pH and soil resistivity, the client can alter cathodic protection if needed
Entrapped Water pH
Entrapped Water pH is the measure of alkalinity in water that has become entrapped beneath disbonded coating. Alkalinity is a measure of the capacity of the water to resist a change in pH that would tend to make the water more acidic. In order to determine the corrosiveness of water, one needs the measurement of both the alkalinity and pH.
In general, water with a pH < 6.5 could be acidic, soft, and corrosive. Acidic water could contain metal ions such as iron, manganese, copper, lead, and zinc. In other words, acidic water contains elevated levels of toxic metals. Acidic water can trigger early damage to metal piping.
Entrapped water pH is determined by inserting a litmus strip into the water to determine the pH. This information is one more piece of data to help pipeline companies take necessary precautions to keep pipelines safe.
A coating inspection is a basic visual inspection performed by highly trained technicians. This is an important inspection that should be performed every time the pipeline is exposed. During the visual inspection, the technician records the condition of the coating or wrap noting features like: disbonded areas, blisters, discoloration, wrinkled, missing, torn, etc. are paramount and recorded when assessing coating condition. Pipeline coatings work in concert with cathodic protection and are the first line of defense against external damage.