Corrosion under insulation

The corrosion under insulation (CUI) war has been fought for many years in the petrochemical industry. The corrosion processes are well understood but yet CUI often goes undetected until the damage is significant which may lead to catastrophic failures, e.g. on equipment operating under high pressure. CUI May occur on:

• Carbon steel (general/localised corrosion)
• Stainless steel (localised corrosion/stress corrosion cracking)

CUI (Corrosion under insulation) cost studies have shown that:

• 40 to 60 % of pipe maintenance costs are caused by CUI
• NDE/inspection costs with a high confidence level for detecting CUI are equal to or exceed field painting costs
• Approximately 10 % of the total maintenance budget is spent repairing damage from CUl

CUI risk:

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 implemented when the outer steel temperature exceeds 50 °C (due to the risk of work-related injuries as well as heat loss).

Carbon steel:

Insulated carbon steel corrodes due to:

• Infiltration of water under insulation (rain, process liquids, fire water, etc.)
• Condensation water
• Ingress of external contaminants

The insulation material may also contribute to CUI:

• Creates a crevice for water retention
• May absorb water
• May leach contaminants that increase corrosion rate

Temperature and oxygen – corrosion rate:

Fig. 1: Curve Showing Corrosion rate of Steel with respect to temperature.

CUI of carbon steel:

Fig. 2: CUI of Carbon Steel

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 also contribute to CUI:

• Creates a crevice for water retention
• May absorb water
• May leach contaminants (e.g. chlorides) that cause ESCC

Fig. 3: Industry Accepted Chloride Stress Corrosion Cracking temperature Limits

How is CUI avoided?

Avoid that moisture enters the insulation material and the steel surface:

• Correct selection and design of the insulation material
• Good design of the item to be insulated – e.g. it is difficult to insulate around flanges, taps, flowmeters, supports, etc.
• Cover above the insulated item (against rainfall, etc).

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 ranges 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)

Methods of protection:

Fig. 4: Methods of protection

Thermal Sprayed Aluminium (TSA):

• TSA may provide long-term corrosion protection at significant life cycle cost savings, however at higher initial costs.
• TSA provides atmospheric corrosion protection for more than 40 years.
• Has been recorded to provide 25-30 years of maintenance and inspection-free CUI service.
• Application by electric arc or flame spray.
• Requires high quality surface preparation and strict application control.

Advantages:

• The coating is robust
• Minor pores/damages are protected by the surrounding coating
• No under-rusting
• Unlimited construction sizes
• No risk of deformations (”cold process”)

Disadvantages:

• No treatment in hollow spaces, inside pipes, etc.
• The treatment prescribes a certain structural design
• Sharp edges must be chamfered/rounded
• Often large variations in layer thickness
• Quality control
• Price

CUI Prevention Strategy:

Data from operating facilities shows that water-free insulation is not practical in ageing facilities. Thus a CUI prevention strategy is necessary in order to provide long-term and reliable prevention of CUI:

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 lowest total lifetime cost.