Page 99 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

Scoring the corrosion potential 4/77
therefore seems to be one of the best and most commonly used areas within the larger area (perhaps near the creeks and
general measures of soil corrosivity. Soil resistivity is a func- ravines) that have relatively high moisture contents most of the
tion of interdependent variables such as moisture content, year. This might be significant information for pipelines tra-
porosity. temperature, ion concentrations, and soil type. Some versing such areas. Therefore, very coarse resolution data are
of these are seasonal variables. corresponding to rainfall or sometimes used only as a default or as a factor to consider in
atmospheric temperatures. Some researchers report that abrupt addition to other, more location-specific information.
changes in soil resistivity are even more important to assessing
corrosivity than the resistivity value itself. In other words, Scoring soil corrosivih.
strong correlations are reported between corrosion rates and
amount of change in soil resistivity along a pipeline [41]. A simple soil corrosivity assessment scale might use only soil
A schedule can be developed to assess the average or worst resistivity as an indicator. An example is shown in Table 4.3.
case (either could be appropriate-the choice, however, must A more detailed evaluation might involve several additional
he consistent across all sections evaluated) soil resistivity. variables as discussed above. Each variable is assessed on its
This is a broad-brush measure of the electrolytic characteristic own scale, either using actual measurements or in relative
of the soil. terms (such as high, medium. or !OM'). They would then be
combined using some relative weighting scheme in order to
MIC Microorganism activity can promote corrosion. This is arrive at a final soil corrosivity score. An example is shown in
often termed microbially induced corrosion or MZC. A family Table 4.4.
of anaerobic bacteria (no oxygen needed for the bacteria to The soil corrosivity score could be the result of summing the
reproduce), called sulfate-reducing bacteria, can cause the subvariable scores:
depletion of the hydrogen layer adjacent to the outside pipe
wall. This hydrogen layer normally provides a degree ofprotec- Soil corrosivity score = [soil resistivity] + [pH] + [soil moisture] +
tion from corrosion. As it is removed corrosion reactions can [MIC] + [STATSGO steel corrosion].
actually be accelerated. Soils with sulfates or soluble salts are
favorable environments for anaerobic sulfate-reducing bacteria Weightings are established based on the corrosion expert's
~91. judgments or empirical data showing which factors are more
Although it does not actually attack the metal, the microor- critical in determining soil corrosivity.
ganism activity tends to produce conditions that accelerate cor- Different pipe materials have differing susceptibilities to
rosion. The sulfate-reducing bacteria are commonly found in damage by various soil conditions. Sulfates and acids in the soil
areas where stagnant water or water-logged soil is in contact can deteriorate cement-containing materials such as concrete
with the steel. or asbestos-cement pipe. Polyethylene pipe may be vulnerable
Previous discovery of MIC or at least microorganism pres- to damage hy hydrocarbons. Any and all special knowledge of
ence is often the best indicator of such damage potential. Some pipe material susceptibility to soil characteristics should be
operators train employees to look for signs during any and all incorporated into this section of the corrosion index.
pipe excavation and exposure. On excavation, evidence of bac- Chapter 11 shows an approach where soil corrosivity is
terial activity is sometimes seen as a layer of black iron-sulfide being assessed against various different pipe materials.
on the pipe wall. An oxidation-reduction probe can be used to
test for conditions favorable for bacteria activity. (It does not Mechanical corrosion effects (Weighting: 5% of
determine if corrosion is taking place, however.) A normal cure corrosion threat)
for microorganism-promoted corrosion is increased levels of
cathodic protection currents. This risk variable involves the potential for damaging phenom-
ena that consist of both a corrosion component and a mechani-
pH The ion concentration in the soil, as measured by pH, cal component. This includes hydrogen stress corrosion
can have a dramatic effect on corrosion potential. A pH
lower than 3 or higher than 9 (either side of the neutral 48
range) can promote corrosion. For metals, more acidic (lower Table 4.3 Example soil corrosivity assessment scale using only
pH) soils promote corrosion more than the more alkaline resistivity
(higher pH) soils. The soil pH may affect other pipe materials in
other ways.

Data sources Some publicly available databases have rela- < 1,000 ohm-cm High I? 0
tive soil corrosivity evaluations for steel and concrete. These Medium 1,000-15.000 Medium 6 50
correspond to specific geographical regions of the world. They ohm-cm or moderately
also show pH, moisture content, sulfates, chlorides. water table active corrosion indicated
depths, and many other soil characteristics. As of this writing, High resistivity Low 3 100
these data sets tend to be very coarse-averaging many factors (low corrosion potential);
so that the resolution is not fine enough to distinguish local hot >15.000 ohm-cm and
no active corrosion
spots of differing characteristics. In fact. the generalized infor- indicated
mation might exactly contradict more local information. An Do not know High 0
example would be where a large-area evaluation shows a very
low soil moisture content, but in reality, there are several small a From ASMEIANSI 831 8