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

2.B-96 API RECOMMENDED PRACTICE 581
2.B.12.1.3
Preventing Soil Corrosion
The common prevention methods for soil corrosion of carbon steel equipment are special backfill, coating,
and CP. The most effective corrosion protection is achieved by a combination of a corrosion-resistant coating
and an effective CP system. With an effective CP system in place, the corrosion rate can be maintained at a
level close to zero. However, maintaining and managing an effective CP system can be complex and should
involve personnel competent in this field. An effective CP system will normally be maintained in compliance
with a recognized standard, such as NACE RP0169.
Partially buried equipment must be handled in three ways. The portion under the soil will be treated as a
piece of buried equipment. The portion exposed to air will be treated as any other outdoor equipment and
often requires little attention. The soil-to-air interface is unique for the location and alloy, and soil-to-air
interface corrosion may present a higher corrosion concern than underground corrosion.
Equipment that is fully encased in concrete is not normally in need of additional corrosion protection provided
that a chloride-free concrete mix is used, and moisture content is stable, and it is not subject to chloride
intrusion.
2.B.12.2
Description of Damage
The soil corrosion damage morphology is generally expected to be localized external corrosion, i.e. pitting at
the anode. The severity of corrosion depends on the local soil conditions and changes in the immediate
environment along the equipment metal surface. The following are the main theoretical causes of soil and
underground corrosion.
a) Equipment Temperature—For a moist environment containing dissolved oxygen, an increase in the
equipment temperature (operating temperature for piping or pipelines) can significantly increase the
external corrosion rate. Theoretically corrosion by oxygen (oxidation) ceases when all the dissolved
oxygen is consumed. Oxygen can be replenished by drain water or from the air (especially at the soil-toair
interface). The corrosion reaction is primarily controlled by diffusion of oxygen to the corroding
surface. Any process that slows oxygen diffusion slows the reaction, and ultimately reduce the corrosion
rate. As corrosion products accumulate on the corroding surface, oxygen diffusion is slowed. Corrosion
due to oxidation of steel doubles for every 20 °C to 30 °C (35 °F to 55 °F) rise in temperature, beginning
at room temperature. Corrosion is nearly proportional to temperature up to about 80 °C (180 °F) when
oxygen is replenished unrestricted to the corroding surface. With the increase in temperature, dissolved
oxygen is driven from the water solution, resulting in a decrease in the rate of corrosion by oxygen.
b) Galvanic Corrosion (Dissimilar Metal Corrosion)—This occurs when two different metals are joined in
the soil, such as steel and copper. Electrical current will flow from the steel into the soil and back into
the copper resulting in corrosion of the steel. A less recognized but similar phenomenon occurs when
new steel is connected to old steel in the soil, such as when replacing a section of corroded pipe. The
new steel that is not cathodically protected will frequently experience a higher corrosion rate.
c) Corrosion Resulting from Dissimilar Soils—In much the same manner as dissimilar metals, a structure
that contacts two or more different types of soil will have different electrical potentials between the metal
and each respective soil. Hence, variations in soil density and porosity can be a common cause of
corrosion in buried equipment, with more dense soil areas promoting an anodic reaction and lighter soil
cover promoting a cathodic reaction. The resultant pitting at the anode can lead to swift penetration of
the wall. The phenomena can occur even over long distances. For example, on a buried pipeline, the
anodic areas and cathodic areas may be considerable distances apart, e.g. where a pipeline crosses a
marshy area near a river and then runs through much drier sandy soil. The differences in the native
pipe-to-soil potential can be sufficient to set up a corrosion cell with anode and cathode many hundreds
of yards apart.
d) Corrosion by Stray Current Drainage—This corrosion differs from other corrosion damage types in that
the current, which causes the corrosion, has a source external to the affected structure. The stray
current source can be AC power lines, telephone lines, adjacent CP systems, or any electrically driven
equipment, most notably rail systems. Stray currents flow from an external source onto a pipeline or