Corrosion Under Insolation (CUI)

Corrosion under insulation (CUI) occurs when water enters external insulation through holes or gaps in the insulation covering, or when moisture in the air condenses on the metal surface below the insulation (known as sweating). If any good quality protective coating is not present, significant wall loss (corrosion) for carbon- and low alloy ferritic steel can occur. The often random and localized nature of the attack, and the fact that it is hidden under the insulation, makes CUI difficult to detect.

Corrosion under insulation for Austenitic and Duplex Stainless Steels takes the form of external Chloride Stress Corrosion Cracking (Cl-SCC) or Chloride Pitting. Cl-SCC occurs above a metal wall temperature threshold (Austenitic and Duplex Stainless Steel have different temperature thresholds), while pitting may occur at lower temperatures. See the Parameters tables (in Step 2 below).

CUI S-RBI Approach

The approach varies based on the assigned Degradation Mechanism (DM). Either CUI-CS (for Carbon and Low Alloy Steel) or CUI-SS (for Austenitic and Duplex Stainless Steels) can be selected. For CUI-CS a RL is estimated based on the CR data. For CUI-SS, a MII is determined based on the identified Criticality. The CR data (for CUI-CS) or MII (for CUI-SS), together with the Coating life is then used to estimate a NID. The estimated Criticality also determines an IS.

Take note that Duplex Stainless Steel corrodes below the Cl-SCC threshold temperature. If this is applicable, the CUI-CS DM should be assigned to the Duplex Steel Component to do the appropriate RBI Assessment. Above the threshold, the CUI-SS DM is applicable.

The RBI approach for CUI DMs is as follows:

• RBI is used for CCAM priority 2. A CCAM assessment is done on the Equipment Details Page.
• A questionnaire (based on the Component CUI Material) is used to determine the Susceptibility to Failure (StF).
• For CUI-SS the identified Criticality is used to determine a MII.
• For CUI-CS a Corrosion Rate (ECR / MCR) and a Corrosion Allowance (DCA / RCA) are used to predict when the wall reaches renewal, i.e., the Remnant Life (RL). A calculator can be used to determine external CUI CR.
• RBI allows credit for Coating life (both CUI-SS and CUI-CS).
• NID is determined as follows:
  • For CUI-CS the NID is the estimated date at which the Equipment or piping will reach renewal (IF=1) after the coating life has been used up and the corrosion allowance has been consumed.
  • For CUI-SS the NID is the date after the coating life has been used up with a MII added.
• The identified Criticality is used to determine an IS.
• The IS specifies the size of the area to delag for inspection.
• There are no Confidence questions.

For Tanks the approach is similar, except that there is no questionnaire for StF, and the Criticality matrix starts at “L”.

CCAM (Define Asset) - Step 1 & Step 8 

See Step 1 and Step 8.

The CUI Consequence Assessment Model (CCAM) is implemented at Equipment level and should be performed to assess CUI maintenance requirements. CCAM assesses the Consequence of Failure and the priority (priority 1 / priority 2). This is a dedicated Consequence Assessment (see Step 10 below). Consequential Business Loss (CBL) should not be included in CCAM Assessments, which should focus primarily on mitigating process safety hazards.

It is advised to manage CUI using CCAM for CCAM priority 1 and S-RBI for priority 2.

To perform a CCAM:

• Go to the Equipment Details page.
• Scroll down to Equipment Strategy section.
• Click the CCAM tab.
• Fill in the assessment to determine the Priority.

Note: For process steams not listed, use “other” and manually enter data for release rate, flammability, and toxicity.

A CCAM assessment is done on the Equipment Details Page.

To see all Equipment with CCAM CUI Priorities:

• Go to the Equipment List (Main screen).
• Click on the filter button.
• Select the CCAM CUI Priority Filter to filter for:
  • None
  • Priority 1
  • Priority 2

Filter for CCAM CUI Priorities.

Important

For all Priority 2 Components (i.e., S-RBI), you must check if the “Component CUI material” has been applied yet. This will determine the StF questionnaire (see Step 7 below). For Duplex, select the SS CUI material.

Parameters Affecting Severity and IOW – Step 2

See Step 2.

See below the relevant parameters affecting CUI severity. Only the Operating Temperature is useful to include in the IOW.

CUI-CS Parameters affecting severity & IOW:

CUI-SS Parameters affecting severity & IOW:

RL and NID – Step 6 & Step 11

See Step 6 and Step 11.

Corrosion Rate for CUI-CS

Model the two thinnest Components for CUI on each piece of Equipment or Piping System. Examples:

• Vessel - one Component for the Shell and one for the Nozzles.
• Piping - one Component for the main piping and one for the Deadlegs.

Rationale: Nozzles and Deadlegs typically have thinner walls. Deadleg temperatures may different significantly from the main piping, resulting in different external CRs. Modelling with two Components gives flexibility, i.e., work can be Scheduled to complete the CUI inspection of Nozzles or Deadlegs on a separate Schedule from the main Piping System or Vessel. If a single Component is used, the thinnest Component should be used.

The CUI Corrosion Rates can be:

• Calculated: Using the RBI CUI calculator.

CUI CR calculator. 

• Estimated: From other CUI inspections completed at the site for Equipment and piping in the same temperature range, and with the same insulation and coating types.
• Measured: Actual results from CUI inspections on the same piece of Equipment or Piping System provided that the same coating and insulation type are used again.

Corrosion Allowance for CUI-CS

The Design Corrosion Allowance (DCA) is used in the IMS NID calculation. It is assumed that any internal corrosion does not line up with the external corrosion.

Rationale: Small bore piping and nozzles could have both internal and external corrosion occurring at the same location. If current inspection practice is to use RT for the internal corrosion, CUI can be inspected for at the same time as thinning.

CUI-SS Corrosion Data is not Required

For CUI-SS, the CR and CA are not required for the NID calculation. In the case of Duplex Steel that are corroding (below the Cl-SCC threshold temperature), the CUI-CS DM should be assigned to the Component, so that the CR can be incorporated into the assessment.

For CUI-SS, the CR and CA are not required.

Expected Coating Life

Estimated coating lives used for CUI are:

• 10 years: All conventional shop coatings.
• 40 years: Shop coating Thermal Sprayed Aluminum (TSA).
• 5 years: Conventional coating (unknown surface preparation or local repairs).
• 10 years: Conventional shop coating (QA/QC system for surface preparation).

NID calculation for CUI-CS

IMS determines the CUI-CS NID as follows:

CUI-CS NID = In-service date + Expected Coating Life + (DCA/ECR)

Or

CUI-CS NID = Date of last coating assessment + Expected Coating Life + (RCA/MCR)

Where:

DCA = Design Corrosion Allowance

ECR = Estimated External CR

RCA = Remaining Corrosion Allowance at the time of the coating assessment

MCR = Measured External CR determined at the time of coating assessment or from inspection of other Equipment / piping at the site, in same temperature range, and with same insulation and coating types.

SCR = CR selected, source ECR or MCR.

In-service date = Coating Install Date

Expected Coating Life = Effective coating years

Date of last coating assessment = (Coating) Inspection Date

Note: For a CUI DM, you can enter the corrosion data on the Remnant Life tab. The coating information comes from the Lining/Coating tab.

The DCA and the ECR are entered on the Remnant Life tab.

 

NID calculation for CUI-SS

For CUI-SS, the CR and CA are not required for the NID calculation. In this case a MII is set based on the Criticality. This is based on the Slow Acting SCC MII Lookup table.

CUI-SS NID = Coating Install Date + Effective coating years + MII

Or

CUI-SS NID = (Coating) Inspection Date + Effective coating years + MII

Maximum Inspection Interval lookup table for CUI SS:

StF – Step 7

See Step 7.

The parameters determining the CUI Susceptibility (see the Parameter tables above in Step 2) are assessed by answering the StF questionnaire in IMS. The questionnaire will depend on the Component CUI Material, e.g., CS or SS. 

You can also choose to do a direct StF assessment.

Choose either RBI StF (CUI Material type) or StF Direct.

Notes on CUI-CS:

• This probability table applies to Equipment operating outdoors and in the temperature range of -5°C to 175°C [25°F to 350°F].
• Deadlegs on a higher temperature system may likely have a lower metal temperature within the CUI range at some location. Therefore, Deadlegs on Equipment or piping operating outside of the CUI range must also be considered. For example, a long Deadleg on a 230°C [450°F] line could easily be in the 50–110°C [120-225°F] metal temperature range for high probability of CUI.
• In case of cyclic service (or regular temperature changes), the range corresponding to the most susceptible temperature reached, should be taken.
• Equipment and piping operating part-time (idle part of the year) should be risk assessed under the most susceptible conditions.
• For duplex steel below the Cl-SCC threshold temperature:
  • The difference between 22 and 25 wt.% Cr duplex chloride stress corrosion cracking threshold temperatures has not been taken into account here, but 80°C (176°F) can be applied for 22 wt.% Cr and 110°C (230°F) for 25 wt.% Cr Duplex steels.
  • The temperature ranges for external chloride stress corrosion cracking assessment (CUI-SS) and corrosion assessment overlap in the 80 – 130°C (176 – 280°F) range.

Notes on CUI-SS:

• Deadlegs should be treated the same as main pipes, except that temperature should be re-estimated, since the deadleg will be much cooler, especially if it is a long section. For example, a deadleg on a 230°C (450°F) line could easily be in the 50-175°C (120-350°F) metal temperature range for high probability.
• In case of cyclic service, the range corresponding to the most critical temperature should be used.
• If the system is aluminium wrapped, default to zero points when aluminium wrap is less than 20 years old.
• Duplex steels can be susceptible to (pitting) corrosion below the Cl-SCC threshold temperature. In this case the CUI-CS DM should rather be assigned.
• The difference between 22 and 25 wt.% Cr duplex Cl-SCC threshold temperatures has not been considered here, but 80°C (176°F) can be applied for 22 wt.% Cr and 110°C (230°F) for 25 wt.% Cr Duplex steels.

Consequence – Step 8

See Step 8.

Use the existing S-RBI Consequence Assessment for the Component or piping circuit assuming a 12.5 mm [½”] hole size for the release calculation.

The Consequence Assessment should preferably include a Consequential Business Loss (CBL) assessment, although initial CUI assessments are typically performed without a CBL to first determine the priority. (CBL is the Production loss, deferment, demurrage, etc. added to the asset replacement value/costs.)

Confidence – Step 10

See Step 10.

The Confidence Assessment is not used for CUI. This is because it is very difficult to predict the Confidence elements in case of CUI. (For the same reason, the StF is replaced with a set of questionnaires which reflects the probability of a failure.)

IS – Step 11 & Step 12

See Step 11 and Step 12.

Several Inspection Strategies (IS) were developed for CUI based on Criticality, which IMS determines from a lookup. The IS determines the scope/coverage of inspection.

The inspection sample size should be larger for the higher IS (Criticality) Equipment and piping to provide assurance that the degradation is assessed. Inspection plan execution should be completed by the NID.

The IS differs depending on the CUI Material Type. See the two tables below. Since the ISs are based on risk, IS-1 ranked items should generally be planned to be completed before IS-2 ranked items, IS-2 before IS-3, etc.

CUI-CS

Inspection Strategy lookup table for CUI CS:

For CUI-CS estimating, the following initial inspection strategies should be used at MII. Note: The IF is 1, so MII is at the RL.:

• IS-1: Delag 100% of susceptible areas followed by visual inspection & restoration
• IS-2: Delag 50% of susceptible areas followed by visual inspection & restoration
• IS-3: Delag 50% of susceptible areas followed by a visual inspection & restoration and when find significant CUI 100% delag for consequence classes High and Extensive and 50% for Medium and Low items
• IS-4: No initial inspection. Reassessment interval to be assigned.

Note: In the planning stages for CUI IS-1 inspection strategies, due to the number of susceptible areas on a typical piece of Equipment, the scaffolding necessary to access all these areas, the potential consequences of a leak, the overall condition of the installation, the insidious nature of CUI and the probability of finding deterioration that will cause the inspection to be expanded, plan to entirely strip the Equipment or piping for this strategy.

CUI-SS

Inspection Strategy lookup table for CUI SS:

For CUI-SS estimating, the following initial inspection strategies should be used at MII. Note: MII is based on the MII lookup table for CUI SS MII as explained above.:

• IS-1: Risk mitigating measures. Immediately delag 100% of susceptible areas followed by paint inspection and restoration of coating system; Dye Penetrant Testing or other crack detection and sizing technique must follow up at locations where paint system has failed.
• IS-2: Delag 100% of susceptible areas followed by paint inspection and restoration of coating system; Dye Penetrant Testing or other crack detection and sizing technique must follow up at locations where paint system has failed.
• IS-3: Delag 50% of susceptible areas followed by paint inspection and restoration of coating system; Dye Penetrant Testing or other crack detection and sizing technique, as follow up at locations where paint system has failed. Increase delag to 100% if paint system has failed.
• IS-4: Delag 50% of susceptible areas followed by paint inspection and local repair of coating where required (consider restoration of coating system if it is over design life); Dye Penetrant Testing as follow up where paint system has failed.
• IS-5: Optional, consider delag & inspection based on damage found at other Equipment and piping, including damage to coating system. Restore coating system at its expected design life.

Required Inspection Effectiveness – Step 11 & Step 12

See Step 11 and Step 12.

The Required Inspection Effectiveness is designed to complement the Inspection (planning) Strategies (IS). Use the Required Inspection Effectiveness, determined in Step 11, to determine the Inspection technique and coverage for CUI-CS - see first table below. For CUI-SS a required Inspection Effectiveness cannot be determined, since there is no RL in this case. However, the second table below can still be used to determine what is required in terms of inspection technique and coverage to ensure a specific Inspection Effectiveness.

The inspection technique and coverage, for CUI-CS, to achieve a required Inspection Effectiveness:

Note: Despite a thorough understanding of potentially susceptible areas, CUI remains difficult to predict. Therefore, the inspection approach should include removal of an ample area (proportional to the consequence of leak of Equipment being inspected) of insulation around the susceptible area up to 100% to enable a thorough assessment to be performed.

The inspection technique and coverage, for CUI-SS, to achieve an Inspection Effectiveness:

For a Component or piping Circuit that includes nozzles, the Susceptibility and resulting IS may differ for the nozzles as compared to the associated mainline Equipment or piping (e.g., hot/cold ends branching from the mainline Equipment – a Deadleg). In this case, either the most conservative assumption should be made for the whole, or the parts should be treated separately.