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Stress Corrosion Cracking (SCC)

09 Aug 2024
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Stress Corrosion Cracking (SCC)

Updated on 09 Aug 2024
5 Minutes to read
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Take Note

There are some specific cases where the S-RBI assessment approach in IMS PEI changes, depending on the Degradation Mechanism.
This section gives guidance for Stress Corrosion Cracking (SCC) in general.

Confidence – Step 10

See Step 10.

First consult the table below for guidance. Inspection results can then also be incorporated (as explained below).

Confidence Assessment for SCC guidance:

Key Questions	Guideline (i) Damage not found	Guideline (ii) Damage found	Guideline (iii) Damage found & StF to be reviewed
	SCC damage was not found during an inspection, with predefined Inspection Effectiveness.	SCC damage was found during an inspection. In this case credit should not be given for inspection, even if it highly effective with the proper coverage and techniques. For SCC, inspections are only to confirm absence of cracking, so if cracking is found, taking credit for reliable inspections would only increase the next inspection interval. If environmental cracking indications are found, it should be confirmed that they are not fabrication defects. This usually requires further investigation, e.g., expanding the susceptible area that is inspected, and using field metallography.	SCC damage was found during an inspection and the StF must be reviewed to determine if it needs to be changed: (a) If a process upset was the cause: - Susceptibility can stay the same if corrective actions have been taken on control of the process upset. - Confidence rating for control of the process should be lowered until able to proof confidence in corrective action. (b) If a material condition was the cause: - Susceptibility can stay the same. - Confidence rating for the control of the DM should be lowered. (c) If not able to confirm the cause: - Susceptibility should change to High. - Confidence should be lowered on both control of DM and of process.
DMs can be properly controlled.	"Yes"	"No" Likelihood of SCC damage could also be affected by material susceptibility (e.g., as welded instead of PWHT material).	lower in case of (b) and (c) (material condition was the cause or cause not confirmed)
Relevant process parameters are reliably monitored.	"Yes" or "No"	"Yes" or "No" SCC damage could also be affected by material susceptibility (e.g., as welded instead of PWHT material).	lower in case of (a) and (c) (process upset was the cause or cause not confirmed)
Reliable inspections have been carried out.	"Yes" Refer to Inspection Effectiveness tables (technique vs required effectiveness)	"Intermediate" or "Not Applicable" (no points)	"Intermediate" or "Not Applicable" (no points)

Key Questions

Guideline (i)

Damage not found

Guideline (ii)

Damage found

Guideline (iii)

Damage found & StF to be reviewed

SCC damage was not found during an inspection, with predefined Inspection Effectiveness.

SCC damage was found during an inspection.

In this case credit should not be given for inspection, even if it highly effective with the proper coverage and techniques. For SCC, inspections are only to confirm absence of cracking, so if cracking is found, taking credit for reliable inspections would only increase the next inspection interval.

If environmental cracking indications are found, it should be confirmed that they are not fabrication defects. This usually requires further investigation, e.g., expanding the susceptible area that is inspected, and using field metallography.

SCC damage was found during an inspection and the StF must be reviewed to determine if it needs to be changed:

(a) If a process upset was the cause:

- Susceptibility can stay the same if corrective actions have been taken on control of the process upset.

- Confidence rating for control of the process should be lowered until able to proof confidence in corrective action.

(b) If a material condition was the cause:

- Susceptibility can stay the same.

- Confidence rating for the control of the DM should be lowered.

- Susceptibility should change to High.

- Confidence should be lowered on both control of DM and of process.

DMs can be properly controlled.

"Yes"

"No"

Likelihood of SCC damage could also be affected by material susceptibility (e.g., as welded instead of PWHT material).

lower in case of (b) and (c)

(material condition was the cause or cause not confirmed)

Relevant process parameters are reliably monitored.

"Yes" or "No"

SCC damage could also be affected by material susceptibility (e.g., as welded instead of PWHT material).

lower in case of (a) and (c)

(process upset was the cause or cause not confirmed)

Reliable inspections have been carried out.

"Yes"

Refer to Inspection Effectiveness tables (technique vs required effectiveness)

"Intermediate" or "Not Applicable" (no points)

Incorporate Inspection Results

The Confidence assessment will also be modified to incorporate inspection results, including the number of past inspections. In addition to Guideline (ii) above, The table below can be used to incorporate the Inspection Effectiveness (see Risk Based Inspection (RBI) - Theory) for prior inspections into the Confidence assessment. Furthermore, the table can be used for future inspection planning.

SCC Confidence Score adjustment based on the StF and Inspection Effectiveness match:

Inspection Effectiveness		Confidence Score adjustment
Inspection Effectiveness		+0.1	0	-0.1
Susceptibility to Failure (StF)	H	(Highly) A	(Usually) B	(Fairly) C
	M	(Usually) B	(Fairly) C	(Not) D/E
	L	(Fairly) C	(Not) D/E	(Not) D/E
	N	(Not) D/E	(Not) D/E	(Not) D/E

Notes on the table above:

As per API 581 prior inspections can be taken credit for in the Confidence assessment by matching the StF and the Inspection Effectiveness of the last inspection per the table. For example, for a piece of Equipment with a “M” StF, the last inspection would have had to have been “B” effective for the SCC DM to get a “+0.1” confidence score.
Multiple prior inspections can be counted as follows, if operating conditions and the IOW have not changed:
- 2 times B = A
- 2 times C = B
If the last inspection found environmental SCC, the Confidence score should be treated as outlined in Guideline (ii) above.
The required Inspection Effectiveness for the next inspection can be determined using the table by planning the inspections to provide a Confidence score of +0.1 where possible.
If environmental SCC was found during the last inspection, the next inspection should be an “A” effective (regardless of repairs made). For example, a piece of Equipment with a “M” StF and a confidence score of “0” or “-0.1” for the last inspection would need to have a “B” effective inspection at the next inspection to increase the reliability of the inspection to “+0.1”. If this is not feasible and a less effective inspection was done, then credit should only be taken for either a “0” or “-0.1” as shown in the table.
Where a lining is used as a Barrier against corrosion or SCC, the same level of Inspection Effectiveness should be completed on the lining as specified in the Inspection Effectiveness Tables. Credit for the inspection is handled the same as inspection of Equipment with linings.

MII – Step 11 & Step 12

See Step 11 and Step 12.

There are three MII tables for cracking, based upon the expected rate of cracking:

Slow Acting: HIC/SOHIC, HIC/SOHIC-HF
Moderate Acting: Alcohol, Amine, CO, CO2, Carbonate, HSC-HF, Sulfide SSC
Fast Acting: Ammonia (anhydrous & aqueous), Caustic, Cl, PTA

IMS implements the following tables to determine the MII.

Slow acting SCC - MII Lookup table:

Inspection Intervals (yrs)		Confidence Rating
Inspection Intervals (yrs)		VL	L	M	H	VH
Criticality	E	0	1	2	4	8
	H	1	2	4	8	16
	MH	2	4	8	16	20
	M	4	8	16	No inspection	No inspection
	L	8	16	No inspection	No inspection	No inspection
	N	No inspection	No inspection	No inspection	No inspection	No inspection

Moderate acting SCC - MII Lookup table:

Inspection Intervals (yrs)		Confidence Rating
Inspection Intervals (yrs)		VL	L	M	H	VH
Criticality	E	0	0	0	2	4
	H	1	2	4	6	9
	MH	2	4	6	9	12
	M	4	6	12	No inspection	No inspection
	L	9	18	No inspection	No inspection	No inspection
	N	No inspection	No inspection	No inspection	No inspection	No inspection

Fast acting SCC - MII Lookup table:

Inspection Intervals (yrs)		Confidence Rating
Inspection Intervals (yrs)		VL	L	M	H	VH
Criticality	E	0	0	0	0	0
	H	0	0.5	1	2.5	4
	MH	1	2	4	6.5	9
	M	2	4.5	9	13.5	18
	L	6.5	13.5	18	No inspection	No inspection
	N	No inspection	No inspection	No inspection	No inspection	No inspection

Notes on the tables:

“No inspection “requires review during RBI revalidation / updates.
Inspection intervals less than 4 years or low or very low confidence ratings must be followed by an engineering evaluation of materials of construction, establishment of IOWs to control exposure to cracking conditions, and specific Inspection Effectiveness to improve the confidence rating.

IS – Step 11 & Step 12

See Step 11 and Step 12.

IMS uses the following tables to determine IS for SCC.

Slow (and Moderate) acting SCC (SCC Process Monitoring Regimes NAR) – IS lookup table:

Inspection Strategy		Confidence Rating
Inspection Strategy		VL	L	M	H	VH
Criticality	E	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management
	H	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Extensive Monitoring and Opportunity Inspection
	MH	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Extensive Monitoring and Opportunity Inspection	Extensive Monitoring and Opportunity Inspection
	M	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Extensive Monitoring and Opportunity Inspection	No inspection	No inspection
	L	Improve Monitoring	Improve Monitoring	No inspection	No inspection	No inspection
	N	No inspection	No inspection	No inspection	No inspection	No inspection

Fast acting SCC (SCC Fast Process Monitoring Regimes NAR) – IS lookup table:

Inspection Strategy		Confidence Rating
Inspection Strategy		VL	L	M	H	VH
Criticality	E	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management
	H	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Extensive Monitoring and Opportunity Inspection
	MH	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Extensive Monitoring and Opportunity Inspection	Extensive Monitoring and Opportunity Inspection
	M	Detailed Analysis and Specific Risk Management	Detailed Analysis and Specific Risk Management	Extensive Monitoring and Opportunity Inspection	Extensive Monitoring and Opportunity Inspection	Extensive Monitoring and Opportunity Inspection
	L	Improve Monitoring	Improve Monitoring	No Additional Inspection or Process Monitoring	No inspection	No inspection
	N	No inspection	No inspection	No inspection	No inspection	No inspection

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What's Next

Amine Stress Corrosion Cracking

Table of contents

Confidence – Step 10
MII – Step 11 & Step 12
IS – Step 11 & Step 12