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

RISK-BASED INSPECTION METHODOLOGY, PART 2, ANNEX 2.B—DETERMINATION OF CORROSION RATES 2.B-97
structure and then flow along it to some other area(s) where they leave to reenter the earth causing
localized corrosion. Where stray current corrosion is a factor, CP may not be the best method of
controlling corrosion. The majority of stray current corrosion problems result from the interacting CP
systems where nearby equipment provides the low-resistance paths for the current from the impressedcurrent
CP system to pass before returning to the protected equipment. This disturbance in current
distribution makes the CP system incomplete and causes localized corrosion on the interfering
equipment. One solution to stray current problems is electrical bonding of nearby equipment. However,
the appropriate solution needs to be evaluated on a case by case basis.
e) Differential Aeration Corrosion Cells—Local differences in the packing of the soil and in its moisture
content may develop oxygen concentration cells where the area with the least oxygen is anodic to the
area in which oxygen is more readily available. Aeration corrosion cells are similar to the corrosion cells
that produce pitting in waters, when one area of the metal has more ready access to oxygen than other
areas. The areas with less access to oxygen will corrode preferentially. This sometimes occurs when a
pipe passes beneath a roadbed or crosses deep underneath a river.
f) MIC—Microbacterial action can also promote local corrosion. Corrosion enhanced by SRB is well
documented. These microbes reduce the commonly available sulfate ions in the soil to produce
hydrogen sulfide. This, in turn, results in increased acidity and acceleration of anodic dissolution.
Coating types, age, and condition are significant factors in preventing microbiologically influenced
corrosion.
2.B.12.3
Basic Data
The data listed in Table 2.B.12.1 are required to determine the estimated corrosion rate for soil-side
corrosion. If precise data have not been measured, a knowledgeable process specialist should be consulted.
2.B.12.4
2.B.12.4.1
Determination of Corrosion Rate
Corrosion Rate Equation
The steps required to determine the corrosion rate are shown in Figure 2.B.12.1. The corrosion rate may be
determined using the basic data in Table 2.B.12.1 in conjunction with Equation. (2.B.21):
CR = CRB ⋅FSR ⋅FT ⋅FCP ⋅ FCE
(2.B.21)
In this equation, the base corrosion rate, CR B , is adjusted for soil resistivity, F SR , temperature, F T , cathodic
protection, F CP , and coating effectiveness, F CE . Determination of the base corrosion rate and each of these
factors is discussed in the following paragraphs.
2.B.12.4.2
Establishing the Base Corrosion Rate
The base corrosion rate is the expected or observed corrosion rate for the buried equipment or structure.
Estimating a corrosion rate based on soil properties is difficult without extensive physical and chemical
analyses. Corrosion rates will be estimated based on rather easily established general characteristics, as
shown in Table 2.B.12.2. There are numerous possible combinations of these variables, with possible
offsetting or compounding effects on the expected corrosion rate. The user should use a corrosion rate that
is based on the actual conditions of the soil in contact with the structure. Corrosion rates should be
extrapolated between those values shown in the table to account for the expected effects of the various
factors. For example, if a soil had all the characteristics of those in row 2 of the table, then a corrosion rate of
0.13 mm/y (5 mpy) would be estimated. If, however, the soil was primarily clay, in an industrial setting
(moderate chemical contaminants), with a mixture of some other soil types and sizes and the soil is normally
saturated, a corrosion rate between 0.13 and 0.25 mm/y (5 and 10 mpy), or as an average 0.178 mm/y
(7 mpy), would be estimated. If actual corrosion rates are known for the particular piece of equipment or
other similar equipment in similar service, that data should be used.