Corrosion and Corrosion Control Flashcards
Sweet Corrosion
predominates at low levels of H2S <0.0035 bar
occurs where free water is present
general attack or localised pitting
Sour Corrosion
occurs at levels of H2S >0.0035 bar (threshold varies with pH)
Attack by hydrogen cracking mechanisms
Times to failure of days or hours
Internal Corrosion - Formation water
– Water naturally present in the rock formation that is mobilised with the oil or gas
– High HCO3 from limestone reservoir gives high pH and low corrosivity
– High chloride content increases corrosivity and risk of pitting
– High soluble salt content can produce protective surface scale …. But scaling can also be a problem
Internal Corrosion - Condensed water
– Water in the gas phase
– Condenses where steel surface temperature falls below dew point
– Pure, corrosivity depends on carbon dioxide
Top of Line (TOL) corrosion
Condensation of water in wet gas pipelines can cause top of line corrosion when the water condensation rate is high or acetic acid is present in the gas. When acetic acid is present in the gas in addition to CO2 the solubility of iron in the condensed water is increased, and this increases the top of line corrosion.
Sour Corrosion Mechanisms
> 0.0035bar partial pressure of H2S results in sour corrosion mechanisms:
Pitting
Mesa Corrosion
FILC (Flow Induced Localised Corrosion)
What are the 4 cracking mechanisms seen in sour corrosion?
- Sulphide Stress Corrosion Cracking (SSCC): occurs in high strength steels, results in hydrogen embrittlement of the steel, requires externally applied load to occur.
- Hydrogen Induced Cracking (HIC): associated with non-metallic inclusions, visible blistering occurs on the surface
• Stress Oriented Hydrogen Induced Cracking (SOHIC):
small ‘ladder-type’ cracks aligned perpendicular to principal stress, combination of external stress and local straining around HIC.
• Soft Zone Cracking: similar to SOHIC but occurs in softened weld HAZs
