A corrosion survey may be conducted to determine these factors:
1. How severely a pipeline or other structure is attacked in a proposed location.
2. How severely an existing structure is being attacked.
3. How much damage has already been done.
4. What steps can be taken to control the corrosive exposure.
A survey of the resistivity along an existing or proposed route will give good answers to items 1., 2., and 4., and will permit some fairly good estimations of item 3.
Except under what can only be described as research conditions, it is seldom profitable to conduct an extensive survey of resistivity along the route of a coated line, either existing or planned. There is really only one decision to make about a coated line, and that is whether to place it under cathodic protection (CP). If the answer is negative, then the resistivity data serve no useful purpose. If it is affirmative, then all that is needed is information about possible anode bed sites, and these should be taken at a greater depth than those for the pipeline corrosion survey.
On a bare line, however, a resistivity survey makes a great deal of sense. It assembles data concerning two important problems: it tells where the line is subject to attack (hot spots) and it furnishes data needed for the selection of galvanic anodes of the proper sizes or the amount of impressed current that is likely to be needed.
There are many adherents to the belief that any soil that is sufficiently corrosive to justify coating a line also justifies CP. In other words, no line should be coated unless it is also protected. Acceptance of this conclusion, of course, covers the first decision referred to previously, thereby eliminating any justification (except as a research study) for a detailed resistivity survey along a coated line. Instead, a much more limited survey is conducted in conjunction with current requirement tests to obtain the data needed for the design of the CP system.
Once the line has been placed under CP, potential surveys, with readings usually at wide spacing using permanent test leads, are needed to ensure complete and continued protection. If trouble occurs and some sections of the line are found to be less than fully protected, then close potential measurements—for example, 8-m spacing—may be useful in tracking down the difficulty.
A profile of the potential along the survey of the soil immediately above the pipeline can be run by any of several methods. Any sharp break in that profile must have an explanation:
• It may be an uninsulated lateral.
• It may be an accidentally shorted crossing line.
• It may be a shorted casing.
• It may be just a sharp change in soil resistivity.
• It may be something else.
However, it must have some reason for existence, and whatever is causing it may be what is robbing the line of protection.
This article is adapted from Corrosion Basics—An Introduction, Second Edition, Pierre R. Roberge, ed. (Houston, TX: NACE International, 2006), pp. 180-181.