May 2010 Vol. 237 No. 5

Features

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.

The presence of bacteria in a soil and the identification of key types of bacteria can give rise to the issue of whether microbial influenced corrosion could be a risk in the soil environment.

Corrosion Product Testing
In addition to controlled soil testing in the laboratory, testing the corrosion products for a known or found corrosion can help to determine how to mitigate current corrosion issues by identifying the corrosion causing mechanism in a system. Elemental Testing of a corrosion product could identify elemental residues of a soil and therefore give information about the soil environment. Analyzing the corrosion product for microbiological influence can help to identify the presence or absence of bacteria. If the sample tests positive for bacteria, analysis can be carried out to determine the type of bacteria present which could give rise to the corrosion mechanism.

Testing the corrosion product in the lab either from an active corrosion area or from an area of corrosion in part of a failure investigation allows for the corrosion product to be tested in a controlled environment with standardized testing parameters and equipment that allow for quality and repeatable results.

Pipeline Environment
Laboratory testing can also look at the internal environment of a pipeline. The same testing parameters discussed above can also be used to test the corrosion product related to internal corrosion as well as internal product samples such as sludge. Chemical testing of these samples can give operators insight into possible corrosion related risks active in the internal pipeline system.

Overall Benefits
There are quite a few benefits to testing soils and corrosion related products in the laboratory. Testing samples in a controlled environment with standardized equipment allows for an in depth analysis of a compound that can identify various risks related to pipeline integrity and also can offer insight to various corrosion mechanisms and pipeline environment characteristics that normally would not be seen from a field investigation.

This can help to reduce costs by taking a preventive approach to pipeline integrity by analyzing the various internal and/or external pipeline environments. Lab testing is a simple procedure in which an an onsite pipeline operator takes a sample and sends it to a lab for testing. Results are normally produced in a short period of time.

Most lab tests are not very costly and if tests are carried out in concert with a particular assessment such as ECDA, digs, ICDA, or any other type where specimens are sent for analysis, the cost can decrease because many samples can be tested at one time and therefore have a direct impact on the cost of the various tests.

Having a sample tested by a laboratory as a preventative maintenance measure or before a third party consultant is hired by a pipeline company can give the operator an idea of what is happening in the system. This information allows the operator to better understand the environment and to be able to rule out costly assessments that might be offered.

Having existing personnel trained by a laboratory on how to gather quality samples and send them to laboratories for testing is much more cost effective than having a third party come on site to carry out an analysis, take various samples, and then have the samples sub contracted for analysis.

Regulatory laboratory testing of products and soils could be implemented into an existing management and/or integrity plan to meet regulatory requirements and maintain a healthy pipeline system at a fraction of a cost when compared to having a consultant fully control and maintain an integrity management plan for a pipeline company.

Acknowledgment
This article is based on a presentation at AGA’s 2009 Operations Conference and Biennial Exhibition in Pittsburgh, PA.

Author
Phil La Susa is environmental chemist and pipeline specialist at Environmental Investments & Solutions (EIS). He previously was with DNV, Dublin, OH. He has experience in both internal and external corrosion and has worked on many ICDA and ECDA projects, providing him with hands on experience of field work as well as the data processing and problem solving techniques used in the DA process. He also has experience in QA/QC development and chemical testing. He earned a B.S. degree in environmental sciences at Otterbein College, Westerville, OH. He can be reached at 419-571-6886, questions.eis@gmail.com or www.eistc.com.