Lab Testing Reveals Relationships Among Soil Quality, Corrosion And The Pipeline Environment

While most operators understand the importance of acquiring data to efficiently manage the integrity of their pipeline systems, many are unaware of laboratory testing procedures that are available to help guide risk and integrity management. This article will look at lab testing capabilities related to corrosion and integrity that can benefit the pipeline and gas distribution industry.

In the early 1900s it was believed that pipeline corrosion was related to stray current from rail traction systems. In 1910, the National Bureau of Standards (NBS) began a stray current electrolysis study. It was observed that corrosion was also occurring where there was no stray current present. In 1920, the NBS concluded that there was a relationship between soil quality and the presence of corrosion.

Corrosion And Soil Quality
Based on the research carried out by the NBS as well as other soil and corrosion scientists, it was determined that the corrosivity of a particular soil can be related to the interaction of (1) soil resistivity, (2) presence of dissolved salts in the soil, (3) soil moisture content, (4) soil pH, (5) types of bacteria in the soil, (6) soil oxygen content, and (7) elemental composition of the soil (soil chemistry).

Soil resistivity can be observed and measured both in the lab as well as in the field. Field soil resistivity measurements are most often conducted using the Wenner four pin method and a soil resistance meter. The Wenner method requires the use of four metal probes or electrodes, driven into the ground along a straight line, equidistant from each other. Soil resistivity is calculated from the voltage drop between the center pair of pins, with current flowing between the two outside pins.

In the lab, a soil sample can be tested in a controlled environment using the M.C Miller soil boxes. Testing in the field can lead to varied results because environmental factors can vary on a day to day basis because of weather conditions, etc. Testing the soil in a controlled environment allows for a better resistivity measurement that can be repeated with the same result consecutively. M.C. Miller soil boxes satisfy both the ASTM (G57) and AASHTO (T-288) standard methodologies for soils testing and analysis of soil resistivity. It is generally concluded that the lower the resistivity, the higher the corrosivity of a tested soil.

The M.C. Miller soil boxes can also be used to determine the linear polarization resistance (LPR) of a soil and therefore calculate a direct corrosion rate of a pipe based on soil composition characteristics.

Moisture content is determined by taking a soil sample and carrying out mass calculations of the sample as received and after the sample has been dried in a laboratory oven to total dryness. A calculation determines the overall moisture content of the soil. In general, as moisture content of a soil sample increases, the resistivity of the sample decreases.

The soil chemistry is determined by various techniques in the laboratory that analyze the soil for the presence and concentrations of various elements and compounds (dissolved salts, pH, oxygen content, chlorides, among others) that can directly give rise to the risk of corrosion based on what elements and compounds are found to be present in various concentrations in the soil.