Corrosion under insulation, CUI is one of the most common damage mechanisms in the Oil and Gas and chemical processing industries. CUI is any type of corrosion damage that occurs due to moisture present on the external surface of insulated equipment. The damage/attack can be caused by one of multiple factors, and can occur in equipment operating at ambient, low, and heated services, depending upon conditions. Moreover, CUI can occur in equipment that is in service, out of service, or in cyclic service. The corrosion itself is most commonly galvanic, chloride, acidic, or alkaline corrosion. CUI often goes undetected until the damage lead to catastrophic failure and lead to shutdown of a process unit or entire facility. The impact of CUI is most prevalent in the refineries, chemical industries, petrochemical industries, offshore industries and maritime industries.
CUI May occur on:
· Carbon steel (general / localized corrosion) :
· Stainless steel (localized corrosion / stress corrosion cracking)
General temperature ranges in which risk of CUI is present:
· Carbon steel: -4 °C to + 175 °C: Risk of CUI (highest risk area: +60 °C to +120 °C)
· Stainless steel: +50 °C to +175 °C
· Insulation of process equipment is normally applied when the outer steel temperature exceeds 50 °C (due to the risk of work-related injuries as well as heat loss).
Steps in CUI phenomenon
1. Ingress of water , either in liquid form (e.g. rain) or vapour,
2. Water accumulation under the insulation, more technically in the space between the insulation and coating,
3. Dissolution of corrosive species, either from the insulation material or from the surrounding
Mechanism of Corrosion Under Insulation
CUI includes the necessity of the following elements:
Water
Contaminants
Temperature
Insulation
Corrosion of Carbon Steel
Insulated carbon steel corrodes due to:
Infiltration of water under insulation (rain, process liquids, firewater, etc.)
Condensation water
Entry of external contaminants
Insulation material creates crevice for water retention
Insulation material may absorb water
Insulation material may leach contaminants that increase corrosion rate
Corrosion of Stainless Steel
Stainless steel (austenitic or duplex) corrodes due to:
1. Chlorides (or other halides) in presence of water are transported to the hot surface
2. The chlorides are concentrated by evaporation of the water
3. The chloride concentration reaches a critical level causing stress corrosion cracking (SCC)
4. (The critical chloride concentration depends on Temperature and alloy type) The insulation material may create crevice for water retention, leach contaminants that cause ESCC
Factors preventing CUI
Insulation selection
Equipment design
Protective paints and coatings
Weather barriers
Maintenance practices.
Application of corrosion protection
On average, 60% of all insulation in service for more than 10 years will contain corrosion-inducing moisture.
Carbon steel:
· Organic coatings
· Thermal-sprayed aluminum (TSA)
· (Personnel protecting cages)
Stainless steel (austenitic or duplex)
· Organic coatings
· Thermal-sprayed aluminum (TSA)
· Al-foil wrapping (prevents ESCC and pitting)
· (Personnel protecting cages)
Organic coatings on carbon steel
· Experience shows that organic protective coatings on piping in CUI service range from 5-13 years
· Once the protective life is reached, field re-painting is necessary (or periodic NDE in order to monitor CUI).
· Coatings may be damaged during handling and installation – leaving unprotected areas.
· Important parameters for coatings used for CUI prevention: – High permeability resistance (barrier protection) – High flexibility (for cyclic temperature service)
Prevention methods
Barrier coatings
Hot-dip galvanization
Alloyed(Stainless) Steel
Cathodic protection
Methods of protection
Thermal Sprayed Aluminum (TSA)
· TSA application is using electric arc or flame spray.
· TSA is cost effective in long run.
· TSA requires high quality surface preparation and strict application control.
· TSA provides atmospheric corrosion protection for more than 40 years.
· TSA provides maintenance and inspection free CUI service for more than 25 years.
Advantages:
Minimum need for maintenance.
Superior adhesion.
Excellent service life.
Resistant to mechanical damage.
Unlimited construction sizes.
No under rusting.
No health hazards from solvents.
No drying/curing time.
Minor pores are protected by surrounding coating.
Disadvantages:
Sharp edges must be chamfered/rounded
Hollow pipes and inside are ignored.
Often large variations in layer thickness
The treatment prescribes a certain structural design
Quality control
Price is high
CUI Prevention Strategy
Data from operating facilities shows that water-free insulation is not practical in aging facilities. Thus a CUI prevention strategy is necessary in order to provide long-term and reliable prevention of CUI:
A choice between strategies:
1. Organic coatings on carbon steel; Ongoing re-painting; NDE (does not prevent CUI but predicts remaining life).
2. TSA on Carbon Steel
Stainless steel (+ TSA or Al foil wrapping) Initial, maintenance, and inspection costs need to be assessed for each choice in order to give the lowest total lifetime cost.
Codes and standards
ASTM STP 880, Corrosion of Metals Under Thermal Insulation
NACE SP0198-2010, Control of Corrosion Under Thermal Insulation and Fireproofing Materials – A Systems Approach
**The content of this article is taken from web open source. The blogs are intended only to give technical knowledge to young engineers. Any engineering calculators, technical equations and write ups are only for reference and educational purpose.