Is this the first time you’ve heard of corrosion under insulation (CUI) and stress corrosion cracking caused by external chloride (ECSSS)? If not, how much do you understand about them? What are the contributing factors for both? Let’s read this brief discussion of CUI and ECSCC.
The generally recognized definition for corrosion under insulation for ferritic alloys is defined as thinning due to localized aqueous corrosion under insulation/lagging.
For austenitic stainless steels and other susceptible materials, the concern is with external chloride-induced stress corrosion cracking.
Contributing Factors for CUI and ECSCC
When we talk about contributing factors, there are few things that we need to be aware of:
- Susceptible Material
- Water
- Chlorides For ECSCC
- External Environment
- Insulation Type And Condition
- Process Temperature
- Coating Condition
- Heat Tracing Condition
- Susceptible Material
a. CUI can occur in carbon steel, low nickel steels and low alloys.
-primary consideration is temperature.
b. External CSCC can occur in austenitic stainless steels (304 [L], 316 [L], 321, 347 and associated weldments).
c. Alloys containing > 32% Ni are generally nor susceptible to external CSCC (E.G. Alloy 825).
2. Water
a. Corrosion of carbon steel and other low alloy steels under insulation occurs when water or water vapour under the insulation remains in the presence of the steel without a protective coating or on steel with a failed protective coating in the susceptible temperature range.
b. Excluding water from reaching the equipment is therefore key to preventing CUI.
c. Can also occur if equipment sweats.
d. Area with a high exposure to wet conditions have a higher likelihood of water intrusion and subsequent accumulation and corrosion:
– Mist from cooling water towers, steam vents and process vents.
– Unattended steam and/or condensate leaks.
– Periodically tested deluge systems.
– Coastal/marine areas.
3. Chlorides
a. External SCC (ECSCC) of stainless steel under insulation occurs when water or water vapour and chlorides under the insulation remain in the presence of the surface without a protective coating (or Al-foil wrapping) or on the surface with a failed protecting coating (or Al-foil wrapping) in the susceptible temperature range.
b. Origin of chlorides is typically water, including rain.
c. Additional source can be insulation materials (older plants), PVC insulation of heat tracing and neighbouring plants.
3.1 Purpose of Thermal Insulation
- Heat conservation
- Personal protection
- Process control
- Acoustic control
- Condensation control
- Fire protection
4. Insulation Type
a. Insulation that absorb and retains water has a higher likelihood of creating an environment that promotes CUI or ECSSC.
b. Mineral wool, fibre glass and calcium silicate tend to have the highest susceptibility for CUI and ECSCC due to their tendency to absorb water (and chlorides).
c. Materials with a closed cell structure like expanded perlite and foam glass tend to be better.
d. Contract free insulation systems prevent accumulation of water and chloride build-uup on the steel surface and thereby reduce the possibility of developing both CUI and ECSCC significantly.
5. Temperature
a. Temperature susceptibility ranges are based on the actual process temperature of the piping or equipment.
b. Temperature cycles that include all or a portion of the CUI/ECSCC range increase the likelihood of CUI/ECSCC.
– Breaches and damage points in insulation, dead-legs.
c. Surface temperatures in some areas may be different:
– The reason for extending the CUI range to 175 C (350 F).
6. Coating
a. CUI/ECSCC under insulation will not occur if the protective coating has not failed or been damaged.
b. However, ECSCC has been known to occur under visually intact (yet porous) coatings.
c. Organics coatings have a finite life and must be rewarded if the piping and equipment are to be adequately protected from CUI and ECSCC.
6.1 Coating System
– Red lead + topcoat [8 years coastal/marine; 10 years temperate]
– Inorganic zinc + top coat [8 years coastal/marine; 10 years temperate]
– Inorganic zinc only [< 5 years coastal/marine; 8 years temperate]
– Immersion service coating [10 years coastal/marine; 15 years temperate]
6.2 Coating System
a. For carbon/low alloy steel, a thermally sprayed aluminium coating offers superior protection.
b. For stainless steel, aluminium foil wrapping is effective in protecting the surface from ECSCC:
– Effective of aluminium wrapping can be affected by mechanical damage.
– Organic coatings are sometimes appropriate provided good quality assurance and control systems are in place. life cycle coasts should be well evaluated since the use of aluminium wrapping is almost always more cost effective and provides better protection.
7. Heat Tracing
a. Steam tracing failure defeats all CUI barriers:
– Introduces moisture, strips away coatings, provides worst case CUI/ECSCC temperatures.
b. CUI risk is significantly affected based on the level of integrity of the steam tracing system.
8. External Environment
a. External environment considerations are based upon rainfall, weather patterns (prevailing winds) and wet industrial conditions.
b. Areas with a higher exposure to wet conditions have a higher likelihood of water intrusion and subsequent accumulation and corrosion.
9. Susceptible Areas
a. Protrusions extending through the insulation sheathing will provide a means of water intrusion:
– Vacuum stiffener rings; insulation support rings; lifting lugs; brackets; dead legs; pipe hangers, supports and shoes; valves, fittings.
b. Other areas of water intrusion:
– Termination of insulation; inspection holes; insulated flanges; damaged or missing jacketing; improperly installed jacketing; cracked or improper termination of fireproofing on skirts.
c. After intrusion, water will saturate the insulation and eventually accumulate at low points and other natural collection area.
d. Accumulation of water can occur long distances from the point of intrusion, especially in services where the surface temperature does not cause the water to evaporate.
-Leads to the unpredictable nature and difficulty of locally inspecting for CUI and ECSCC.
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Credit information: Tuan Norzafry Norhalim.
