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-99
As discussed earlier, stray current can originate from various external sources such as power lines,
electrically driven equipment, and impressed current CP systems in the environs. Except for the few cases
where AC stray current is present from inductively coupled situations such as paralleling high tension lines,
stray current would not occur if electrical systems were entirely insulated from earth. At the same time,
proper grounding of electrical power circuits is necessary to reduce electric shock hazards. Hence the stray
current corrosion problem will probably never be eliminated.
A low-level, steady state current may be controlled with CP systems. But a larger stray current that may be
dynamic would require special analysis and corrective measures. The corrosion potential from these larger
stray current problems is of a much higher magnitude than the other corrosion causes discussed. Therefore,
they should be addressed first before considering any other effects on the corrosion potential of the
equipment or structure being addressed. An effective CP system will include testing and mitigating the effect
of stray currents on a routine basis.
Adjustment factors for CP systems, based on the effectiveness of the system, are provided in Table
2.B.12.5. It shall be noted that the effectiveness of the CP system depends on the continuity of operation of
impressed current sources, the system complies to NACE RP0169 and managed by NACE certified
personal.
For structures which are only partly protected by a CP system, the unprotected areas will have corrosion
rates that are determined by the prevailing conditions.
2.B.12.4.6
Adjustment Factor for Coating Effectiveness
The primary effect that a coating has on the corrosion rate is related to the potential for the coating to shield
the CP current in the event that the coating becomes disbonded from the structure. This is a complicated
relationship between many factors but is primarily related to how well coating adheres to the pipe and how
age, temperature extremes, and maintenance practices affect the dielectric properties of the coating. Each
factor is considered to be independent of each other. All of multiplying factors that apply to the coating in
question should be used to determine the total coating effectiveness factor, F CE .
Table 2.B.12.6 is used for calculating the adjustment factor for a coating. When the multiplying factors criterion
does not apply, substitute factor with 1.0. For example, for a mill applied polyethelene (PE) tape that is 30 years
old, has been occasionally subjected to temperatures over the maximum, and there is never any coating
inspection or maintenance, the total coating effectiveness factor would be:
F CE = 15 . ⋅12 . ⋅30 . ⋅ 15 . = 135 .
(2.B.22)
For a bare pipe or structure, F CE = 1.0. For a pipe that does not have CP, the coating effectiveness factors
should still be used since holidays in the coating may allow concentrated corrosion to occur in the damaged
area.
2.B.12.5
CR
CR B
F CE
F CP
F SR
F T
Nomenclature
is the corrosion rate
is the base corrosion rate
is the corrosion rate correction factor for coating effectiveness
is the corrosion rate correction factor for CP
is the corrosion rate correction factor for soil resistivity
is the corrosion rate correction factor for temperature