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-95
2.B.12 Soil-Side Corrosion
2.B.12.1
2.B.12.1.1
Description of Damage
Overview
The objective for this supplement is to give a conservative approach for assessing the potential for soil
corrosion and determining appropriate mitigation measures, while taking the most significant factors for soil
corrosion into account.
This supplement pertains to any carbon steel equipment or structure having surface metal exposed to soil,
with the exception of the soil side of aboveground storage tanks, which are covered in Section 2.B.14. The
most typical equipment exposed to soil corrosion is buried or partly buried carbon steel vessels/drums and
piping, with some type of coating.
The method described in this paragraph may be used to establish an estimate of the corrosion rate that
would be expected in a given environment. If actual corrosion rates are known for particular pieces of
equipment or other similar equipment in similar service, that data may be used in lieu of this method.
2.B.12.1.2
Soil Corrosivity
The damage to the exterior of metals exposed to soils is usually referred to as soil corrosion and is often
attributed to soil characteristics. Soils having high moisture content, high dissolved salt concentrations, and
high acidity are expected to be the most corrosive. However, soil composition alone has been found to have
little correlation with soil corrosivity.
There is no single easily measured soil parameter that can be used to determine soil corrosivity. Instead, a
number of characteristics must be combined to estimate the corrosion that may be expected on a steel
structure from a particular soil. According to ASTM STP 741, soil corrosivity classes can be characterized by
total acidity, resistivity, conductivity, drainage (soil texture), and aeration (water–air permeability). The most
significant causes for soil corrosion are described in Section 2.B.12.2.
Soils frequently have characteristics of which some indicate that the soil is corrosive, and others indicate just
the opposite. By virtue of water and related water-soluble salts being present, soil becomes an effective
electrolyte for completing the corrosion circuit between anode and cathode. This can be true even if the soil
is fairly dry and nonconductive (high resistivity). The water content in the soils relates to drainage, which is
defined as the ability to allow water percolation. In the long term, the residence time for water or moisture on
the metal surface will control the degree of corrosion in soil. Measuring this residence time is difficult or
impossible in practice. Therefore, it becomes necessary to use more easily measured soil characteristics,
which have a less certain correlation with soil corrosivity. The parameters usually considered include soil
resistivity, pH, chloride content, redox potential, and type of soil.
Soil resistivity is frequently used to estimate soil corrosivity, mainly because it is easy to measure (commonly
measured by the 4-pin Werner technique as described in ASME G57, or electromagnetic non-contacting
methods, Geonics). In practice, the conditions around the equipment surface are likely to be different than in
the surrounding native soil, due to different compaction and possibly also different soil type and texture
(especially where sand is used for backfill). Furthermore, the conditions probably vary along the equipment
surface as well. These variations will cause local effects that are not easily predicted by bulk resistivity
measurements, and these local effects again make a direct correlation solely between soil resistivity and soil
corrosivity of questionable value.