API 570 STUDY GUIDE
MODULE 2 : PART 11
SOIL CORROSION
API 571 : CLAUSE 4.3.9 Soil Corrosion
Description Of Damage
As the word soil comes, and the code deals with damage mechanisms, we can assume that soil corrosion is the corrosion of metals exposed to soils.
Affected Materials
CS, Cast Iron and Ductile Iron
Critical Factors
Soil Related factors
1) Soil resistivity (Soil Condition and Characteristics)
a. Soil resistivity is easy to measure so it is one of the properties that is measured to determine soil corrosivity.
b. Soil resistivity is related to soil moisture content and dissolved electrolytes in the soil water
c. Soils having high moisture content, high dissolved salt concentrations and high acidity are usually the more corrosive.
d. Soil-to-air interface areas are much more susceptible to corrosion than the rest of the structure because of moisture and oxygen availability. Decrease in soil resistivity is a sign of chances of increased soil corrosion.
2) Soil Type : Water drainage properties of soil
3) Soil homogeneity : Variation in soil type
Other Related factors
1) Operating temperature of the process fluid : Corrosion rates increases with increasing metal temperature
2) Galvanic Corrosion
3) Stray Current drainage
4) Differential Aeration Corrosion Cells
5) MIC
6) Cathodic Protection
7) Coating: Coating type, age and Condition. Poor condition of a protective coating is a tell tale sign of potential corrosion damage.
Inspection of Buried Pipe is a detailed topic in API 570. More on soil resistivity, its measurement and other details can be discussed along with API 570.
Affected Units or Equipment
1. Underground piping and equipment
2. Burried Tanks
3. Equipment that touch the soil: Bottom of above ground storage tanks.
4. Ground Supported metal structures
Appearance or Morphology of Damage
Soil corrosion appears as external thinning with localized losses due to pitting.
Prevention / Mitigation
1. Corrosion resistant coating and Cathodic Protection are the most effective means
2. Soil corrosion of CS pipes can be minimized by using special backfill, coatings and cathodic protection.
Inspection and Monitoring
1. Measure the soil potential of underground structures using dedicated reference electrodes near the structure.
2. Cathodic Protection accordance with NACE RP 0169
3. Piping can be inspected by inline inspection devices, guided ultrasonic thickness tools
4. Indirect methods used in piping are pressure testing and visual inspection of above ground soil conditions.
The details of the inspections are in API 570 under the topic Inspection of Buried Pipe. So we will go in detail about these inspection methods when API 570 is discussed.
Related Mechanisms
Galvanic Corrosion
QUESTIONS
Hope you have done your answers correctly...
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