API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

2.B-98 API RECOMMENDED PRACTICE 581
2.B.12.4.3
Adjustment Factor for Soil Resistivity (Optional)
The soil resistivity factor might be considered as input information when estimating the base corrosion and
as such discarded as an adjustment factor. However, for a couple of cases, the soil resistivity factor should
be included as an adjustment factor for the base corrosion rate, i.e. where:
a) estimation of the base corrosion rate is determined by intuitive settings for the sub-factors in Table
2.B.12.2, and
b) regular measurements of soil resistivity are part of the inspection program and variation in soil resistivity
might be the only indicative variable for monitoring changes in the soil characteristics (an example
would be monitoring soil resistivity around a cooling tower basin with adjacent chemical treating
facilities).
Soil resistivity gives a composite measure of moisture content of soil and dissolved electrolytes in the soil
water, i.e. an indication for soil condition. Soil resistivity has often been used as a broad indicator of soil
corrosivity. Because ionic current flow is associated with soil corrosion reactions, high resistivity will arguably
slow down corrosion reactions although a high soil resistivity alone will not guarantee absence of serious
corrosion. Soil resistivity generally decreases with increasing water content and concentration of ionic
species. Variations in soil resistivity along the length of the structure are highly undesirable, as this will lead
to the formation of macro corrosion cells. Thus, the merit of a corrosion risk classification based on an
absolute value of soil resistivity is limited.
Resistivity of native undisturbed earth and the soil adjacent to the pipe (looser) may be very different.
However, over time the less compacted and possibly higher resistivity soil near the structure will assume the
characteristics of the native soil, i.e. the high resistivity soil might become contaminated by capillary action.
While the soil resistivity against the pipe or structure cannot be measured accurately, bulk measurements
can be taken for soil resistivity in the vicinity of the buried equipment. Given the above theory and assuming
several measurements will be taken, these resistivity measurements can be considered representative for
the soil adjacent to the structure.
Normal soil resistivity is 3,000 to 5,000 Ω-cm, although 20,000 Ω-cm is not uncommon. Corrosion rate
adjustment factors for soil resistivities are provided in Table 2.B.12.3. The ranges in this table are consistent
with API 651 (general classification of resistivity, based on NACE 51011). API 570 (1997) only stipulates 3
ranges (<2,000; 2,000 to 10,000; >10,000) for determining the recommended inspection frequencies of 5, 10,
and 15 years, respectively, for buried piping without CP.
2.B.12.4.4
Adjustment Factor for Temperature
The base corrosion rate is adjusted for the operating temperature in accordance with Table 2.B.12.4. See
ASM Handbook 13 (Corrosion) for temperature effect on external corrosion.
2.B.12.4.5
Adjustment Factor for CP and Stray Current Drainage
CP is the primary method used to avoid corrosion of buried structures from the soil corrosion. However, the
system must be installed and maintained properly. In Table 2.B.12.5 corrosion rate adjustment factors are
given for CP system coverage and expected efficiency of protection. “Hot spot” protection is the practice of
installing sacrificial anodes (aluminum, zinc, or magnesium) at locations of suspected anodic activity, as
determined by surveys of structure-to-soil potential. Complete protection is achieved by installation of
sacrificial anodes or impressed current protection systems sufficient to cover the entire surface of the buried
equipment. Anodic protection is not an applicable method for protection of buried equipment. NACE RP0169
establishes three criteria for protection. One common reference level is a structure-to-soil potential
measurement of −0.85 volts with reference to a copper/copper sulfate reference electrode. This criterion is
considered less effective than the other two criteria commonly known as 100 mV polarization and −0.85 volt
polarized structure-to-soil potential.