Corrosion Under Insulation, Protective Coatings and Linings in Process Plants

Corrosion Under Insulation, Protective Coatings and Linings in Process Plants

Corrosion Under Insulation with Protective Coatings and Linings in Process Plants

Corrosion under insulation refers to any type of corrosion that occurs due to moisture buildup on the external surface of insulated steel, piping and vessels.

Introduction to Corrosion Under Insulation (CUI)
Corrosion under insulation refers to any type of corrosion that occurs due to moisture buildup on the external surface of insulated steel, piping and vessels. The most type of CUI is galvanic, chloride, acidic, or alkaline corrosion. According to API 570, for the carbon and steel piping system, the susceptible temperature ranges under which CUI may occur is 25 and 250°F where operating temperature cause frequent or continuous condensation of atmospheric moisture.   
 
Why does Corrosion Under Insulation (CUI) Occur:
As we all aware corrosion under insulation has become a serious concern and more critical for the Industry for the number of reasons. So many Industries like Chemical, petrochemical, oil & gas, and food processing facilities, require more smart instrumentation and sophisticated equipment.

According to Inspection techniques for detecting corrosion under insulation, the deterioration of material components is caused due by adverse interactions with their environments, and excessive/sudden changes in the amount of pH value in water.

The rate at which corrosion occurs on carbon steel when the relative humidity of the air is 70% to 80% and the air temperature is above 32F.  

One of the leading factors of corrosion is “Temperature”. For example, Pipe’s material normal operating temperature is elevated above `ambient temperature, condensation could occur during shutdown when the pipe’s temperature is reduced to the ambient temperature range.

Effect of Temperature on Steel Corrosion in Water: 

The effect of temperature on steel corrosion in high-temperature water was investigated by scanning electron microscope, electrochemical noise, and Raman spectrum, and X-ray photoelectron spectroscopy (XPS). In an open system, it is observed that the oxygen content of the water decreases with the increase in temperature. The corrosion rate of carbon steel in aerated water begins to decrease with increasing temperature when the temperature increases to 80 °C (176 °F). In a closed system, the corrosion rate of carbon steel in situated water continues to increase as the water temperature increases. Field measurements of the corrosion rate of carbon steel corroding under insulation confirm that the corrosion rate increases with temperature in a manner similar to that of a closed system. 

As per API RP 583, the CUI occurs for insulated piping/equipment when these operate between:
  1. -12 deg C to 175  deg C for CS or low alloy steel
  2. 60 deg C to 205 deg C for SS material (austenitic/duplex)

The NACE 0198 have slightly different ranges eg:

  1. -4 deg C to 175 deg C for CS or low alloy steel
  2. 50 deg C to 175 deg C for SS material (austenitic/duplex)

Today Industry uses steel in a huge amount. But at the same time industry is facing problem due to it's a disadvantage – its high corrosion rate.  Steel structure and components is a big concern for the maintenance of steel. 

Preventing Corrosion Under Insulation with Coating and Lining: 

Effective Protective coatings, lining, and weather barriers can help minimize the potential for CUI, But Effective maintenance practices play an important role to prevent material from corrosion.
 
Corrosion under insulation in piping systems in the petrochemical industries consumes a significant percentage of the maintenance budget. 
The typical insulation of carbon steel pipe is shown below:

The typical insulation of carbon steel pipe is shown below:

We find that there is an annular space between the metal cladding and carbon steel pipe surface. We find that this annular space is filled up with insulating material and through the pipe is painted we find that it is subject to corrosion!!!
 
Corrosion under insulation Cycle:
shows how water/moisture/air enters the annular space due to some breach in Insulation cladding and how the water ultimately gets trapped and then carbon steel corrodes, not because it is insulated, but because it is in contact with aerated water.
 
Protective Coatings and Linings in Process Plants
The figures below show the typical cross-sectional details of the thermal spray machine and actual machine in use on a field application
Other advantages of Thermal spray Aluminum coating are listed below:
  • Thermal spray coatings are used to improve friction to enhance the life of machine parts. It uses a wide range of coating materials starting from pure metals to alloy, ceramic, plastic, and polymer and can be used in the form of powder. In the petrochemical industry, users expect thermal spray coatings with 200-micron thickness to provide >30 years with no corrosion inspection and maintenance.
  • The thermal spray coating is formed in minimal heating of the substrate and to form a bond the coating does not need to fuse with the substrate.
  • The thermal spray coatings are also specified as a solution to stress corrosion cracking (SCC) of austenitic stainless steel.
Typical Protective Coating Systems for Carbon Steels Under Thermal Insulation 
System No  Temp Range Surface Preparation Surface Profile Prime Coat Finish Coat

CS1

- 45 to 60 deg C

NACE 2/

SSPC10
50 – 75 microns

HBE

130 microns

HBE

130 microns

CS2

- 45 to 60 deg C

NACE 2/

SSPC10
50 – 75 microns N/A Fusion-bonded epoxy (FBE), 300 microns
CS3 - 45 to 150 deg C

NACE 2/

SSPC10
50 – 75 microns Epoxy phenolic, 100–150 microns

Epoxy phenolic,

100–150 microns
CS4 - 45 to 205 deg C

NACE 2/

SSPC10
50 – 75 microns Epoxy novolac or silicone hybrid, 100–200 microns Epoxy novolac or silicone hybrid, 100–200 microns
CS5 - 45 to 595 deg C

NACE  1/

SSPC-SP 5
50 – 100 microns

TSA, 250–375 microns with minimum of 99%

aluminum

Optional: Sealer with either a thinned epoxy based or silicone coating (depending on maximum

service temperature) at approximately 40 microns

thickness.
CS6 - 45 to 650 deg C

NACE 2/

SSPC10
40 – 65 microns Inorganic copolymer or coatings with an inert multipolymeric matrix, 100–150microns

Inorganic copolymer or

coatings with an inert

multipolymeric matrix,

100–150 microns
CS7 60 deg C max

SSPC-SP 2/

SSPC-SP 3
N/A

Thin film of petrolatum or petroleum wax primer

Petrolatum or petroleum wax tape, 40–80 microns
CS8 - 45 to 400 deg C

Low-pressure

water cleaning to

3,000 psi

 if

necessary
N/A N/A

Epoxy novolac, epoxy phenolic, silicone, modified silicone, inorganic copolymer, or a coating with an inert multi polymeric matrix, is typically applied in

the field. Consult coating manufacturer for

thickness and service temperature limits. (E)

 

Maintenance and Mitigation for CUI:
 
CUI mitigation and maintenance is a process by which CUI can be slowed down or prevented. Most mitigation and maintenance strategies begin with a regular inspection of the damaged components. Due to an irregular inspection of the components, CUI can form. Once the CUI began to form, it is difficult to recover the damage.
It is hard to find CUI of pipe in petrochemical plants because the arrangement or connectivity of pipes in petrochemical plants is too complex. CUI can occur due to long operations of chemical plants equipment.
There are various techniques to locate CUI in a pipeline of chemicals plants environment:
  1. Pipeline metallurgy
  2. Operating temperature of CUI
  3. Operating temperature cycles
  4. Coating under insulation
Corrosion Under Insulation (CUI) | Corrosion Under Insulation Protective Coating and Lining | Thermal Sprayed Aluminum (TSA) coating

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