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4.3.2.5 Appearance or Morphology of Damage

a) The attack will be general or localized, depending upon whether or not the

moisture is trapped.

b) If there is no coating, corrosion or loss in thickness can be general.

c) Localized coating failures will tend to promote corrosion.

d) Metal loss may not be visually evident, although normally a distinctive iron

oxide (red rust) scale forms as shown in Figure 4-72.

4.3.2.6 Prevention / Mitigation

Surface preparation and proper coating application are critical for long-termprotection in corrosive environments.

4.3.2.7 Inspection and Monitoring

VT and UT are techniques that can be used.

4.3.2.8 Related Mechanisms

Corrosion under insulation (see 4.3.3).

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Atmospheric corrosion of an LPG line at close proximity to an cooling tower

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Corrosion rates increase with temperature up to about 250oF (121oC). Above 250oF

(121oC), surfaces are usually too dry for corrosion to occur except under insulation

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4.3.3 Corrosion Under Insulation (CUI)

4.3.3.1 Description of Damage

Corrosion of piping, pressure vessels and structural components resulting

from water trapped under insulation or fireproofing.

4.3.3.2 Affected Materials

Carbon steel, low alloy steels, 300 Series SS, and duplex stainless steels.

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4.3.3.3 Critical Factors

a) a) It affects externally insulated piping and equipment and those that are in

intermittent service or operate between:

1. 10F (-12C) and 350F (175C) for carbon and low alloy steels,

2. 140F (60C) and 400F (205C) for austenitic stainless steels and

duplex stainless steels

b) Corrosion rates increase with increasing metal temperature up to the point

where the water evaporates quickly. For insulated components, corrosion

becomes more severe at metal temperatures between the boiling point212F (100C) and 350F (121C), where water is less likely to vaporize and

insulation stays wet longer.

c) Design of insulation system, insulation type, temperature and environment

are critical factors.

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d) Poor design and/or installations that allow water to become trapped will

increase CUI.

e) Insulating materials that hold moisture (wick) can be more of a problem.f) Cyclic thermal operation or intermittent service can increase corrosion.

g) Equipment that operates below the water dewpoint tends to condense

water on the metal surface thus providing a wet environment and

increasing the risk of corrosion.h) Damage is aggravated by contaminants that may be leached out of the

insulation, such as chlorides.

i) Plants located in areas with high annual rainfall or warmer, marine

locations are more prone to CUI than plants located in cooler, drier, mid-continent locations.

j) Environments that provide airborne contaminants such as chlorides

(marine environments, cooling tower drift) or SO2 (stack emissions) can

accelerate corrosion.

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4.3.3.4 Affected Units or Equipment/

a) All insulated piping and equipment are susceptible to CUI under conditions

noted above even on piping and equipment where the insulation system

appears to be in good condition and no visual signs of corrosion are present.

b) Examples of locations where CUI can occur are listed below:

1. CUI can be found on equipment with damaged insulation, vapor barriers,weatherproofing or mastic, or protrusions through the insulation or at

insulation termination points such as flanges.

2. Equipment designed with insulation support rings welded directly to the

vessel wall (no standoff); particularly around ladder and platform clips,and lifting lugs, nozzles and stiffener rings.

3. Piping or equipment with damaged/leaking steam tracing.

4. Localized damage at paint and/or coating systems.

5. Locations where moisture/water will naturally collect (gravity drainage)before evaporating (insulation support rings on vertical equipment) and

improperly terminated fireproofing.

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6. Vibrating piping systems that have a tendency to inflict damage to

insulation jacketing providing a path for water ingress.

7. Deadlegs (vents, drains, and other similar items).8. Pipe hangers and other supports.

9. Valves and fittings (irregular insulation surfaces).

10. Bolted-on pipe shoes.

11. Steam tracer tubing penetrations.12. Termination of insulation at flanges and other piping components.

13. Insulation jacketing seams located on the top of horizontal piping or

improperly lapped or sealed insulation jacketing.

14. Termination of insulation in a vertical pipe.15. Caulking that has hardened, has separated, or is missing.

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16. Bulges or staining of the insulation or jacketing system or missing bands.

(Bulges may indicate corrosion product buildup.)

17. Low points in piping systems that have a known breach in the insulationsystem, including low points in long unsupported piping runs.

18. Carbon or low-alloy steel flanges, bolting, and other components under

insulation in high-alloy piping systems.

19. Locations where insulation plugs have been removed to permit pipingthickness measurements on insulated piping and equipment should

receive particular attention. These plugs should be promptly replaced

and sealed. Several types of removable plugs are commercially

available that permit inspection and identification of inspection points forfuture reference.

20. The first few feet of a horizontal pipe run adjacent to the bottom of a

vertical run.

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Equipment designed with insulation support rings welded directly to the vessel wall (no

standoff); particularly around ladder and platform clips, and lifting lugs, nozzles and

stiffener rings.

Stand-off installed

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Equipment designed with insulation support rings welded directly to the vessel wall (no

standoff); particularly around ladder and platform clips, and lifting lugs, nozzles and

stiffener rings.

Stand-off

installed

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Valves and fittings (irregular insulation surfaces).The first few feet of a horizontal

pipe run adjacent to the bottom of

a vertical run