Probability of Failure

Examples of Inspection Category are from API RP 581 Second Edition September 2008, created by the American Petroleum Institute.

A description of each damage factor used to calculate the probability of failure can be found below.

Thinning Damage Factor

Encompasses general and/or local thinning inside the equipment. All equipment and components are assumed to have or be capable of having damage caused by thinning.

Inspection Effectiveness Categories (General Thinning)

Inspection Effectiveness Categories (General Thinning)

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	50 to 100% examination of the surface (partial internals removed), and accompanied by thickness measurements.	50 to 100% ultrasonic scanning coverage (automated or manual) or profile radiography.
Usually Effective (CAT. B)	Nominally 20% examination (no internals removed), and spot external ultrasonic thickness measurements.	Nominally 20% ultrasonic scanning coverage (automated or manual), or profile radiography, or external spot thickness (statistically validated).
Fairly Effective (CAT. C)	Visual examination with thickness measurements.	2 to 3% examination, spot external ultrasonic thickness measurements, and little or no internal visual examination.
Poorly Effective (CAT. D)	Visual examination.	Several thickness measurements, and a documented inspection planning system.
Ineffective (CAT. E)	No inspection.	Several thickness measurements taken only externally, and a poorly documented inspection planning system.

Inspection Effectiveness Categories (Local Thinning)

Inspection Effectiveness Categories (Local Thinning)

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	100% visual examination (with removal of internal packing, trays, etc.) and thickness measurements.	50 to 100% coverage using automated ultrasonic scanning, or profile radiography in areas specified by a corrosion engineer or other knowledgeable specialist.
Usually Effective (CAT. B)	100% visual examination (with partial removal of the internals) including manways, nozzles, etc. and thickness measurements.	20% coverage using automated ultrasonic scanning, or 50% manual ultrasonic scanning, or 50% profile radiography in areas specified by a corrosion engineer or other knowledgeable specialist.
Fairly Effective (CAT. C)	Nominally 50% visual examination and spot ultrasonic thickness measurements.	Nominally 20% coverage using automated or manual ultrasonic scanning, or profile radiography, and spot thickness measurements at areas specified by a corrosion engineer or other knowledgeable specialist.
Poorly Effective (CAT. D)	Nominally 20% visual examination and spot ultrasonic thickness measurements.	Spot ultrasonic thickness measurements or profile radiography without areas being specified by a corrosion engineer or other knowledgeable specialist.
Ineffective (CAT. E)	No inspection.	Spot ultrasonic thickness measurements without areas being specified by a corrosion engineer or other knowledgeable specialist.

Inspection Effectiveness Categories (Thinning/Buried Components)

Inspection Effectiveness Categories (Thinning/Buried Components)

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	100% internal inspection via state-of-the-art pigging and in-line inspection technologies (UT, MFL, internal rotary UT, etc.) 100% external inspection of equipment that is only partially buried using an NDE crawler with circumferential inspection technology (MFL, lamb-wave UT).	Complete excavation, 100% external visual inspection, and 100% inspection with NDE technologies (UT thickness measurement such as handheld devices at close-interval grid locations, UT B-scan, automated ultrasonic scanning, guided-wave UT global search, crawler with circumferential inspection technology such as MFL or lambwave UT, digital radiography in more than one direction).	a) Cathodic Protection (CP) System maintained and managed by NACE certified personnel and complying with NACE SP0169 [14]– includes Stray current surveys on a regular basis. b) Close Interval Survey (at excavation sites) to assess the performance of the CP system locally. c) Sample soil and water resistivity and chemistry measurements along entire structure.
Usually Effective (CAT. B)	Internal inspection via pigging and in-line inspection technologies (UT, MFL, internal rotary UT, etc.) of selected areas/sections, combined with statistical analysis or extreme value analysis (EVA). External inspection of equipment that is only partially buried using an NDE crawler with circumferential inspection technology (MFL, lambwave UT) on selected areas/sections, combined with statistical analysis or extreme value analysis (EVA).	Excavation at "Selected" locations, 100% external visual, and 100% inspection with NDE technologies (UT thickness measurement such as handheld devices at close-interval grid locations, UT B-scan, automated ultrasonic scanning, guided-wave UT global search, crawler with circumferential inspection technology such as MFL or lambwave UT, digital radiography in more than one direction).	a) CP System maintained and managed by NACE certified personnel and complying with NACE SP0169 [14] – includes Stray current surveys on a regular basis. b) Close Interval Survey (at excavation sites) to assess the performance of the CP system locally. c) Sample soil and water resistivity and chemistry measurements along entire structure. d) DC Voltage Gradient (DCVG) to determine coating damage.
Fairly Effective (CAT. C)	Partly inspection by internal smart pig or specialized crawler device, including a representative portion of the buried pipe. (<25%).	Partial excavation guided-wave UT global search inspection in each direction of pipe. Corrosion Inspection and Maintenance managed by NACE certified and CP specialist, or equivalent.	-
Poorly Effective (CAT. D)	Hydrostatic testing.	Spot check with conventional NDE equipment of local areas exposed by excavation.	-
Ineffective (CAT. E)	No inspection.	-	-

Inspection Effectiveness Categories (Thinning/Tank Bottoms)

Inspection Effectiveness Categories (Thinning/Tank Bottoms)

Inspection Category	Soil Side	Product Side
Highly Effective (CAT. A)	a) Floor scan 90+% & UT follow-up b) Include welds if warranted from the results on the plate scanning c) Hand scan of the critical zone	a) Commercial blast b) Effective supplementary light c) Visual 100% (API 653) d) Pit depth gauge e) 100% vacuum box testing of suspect welded joints Coating or Liner: a) Sponge test 100% b) Adhesion test c) Scrape test
Usually Effective (CAT. B)	a) Floor scan 50+% & UT follow-up OR b) EVA or other statistical method with Floor scan follow-up if warranted by the result.	a) Brush blast b) Effective supplementary light c) Visual 100% (API 653) d) Pit depth gauge Coating or Liner: a) Sponge test >75% b) Adhesion test c) Scrape test.
Fairly Effective (CAT. C)	a) Floor scan 5-10+% plates; supplement with scanning near Shell & UT follow-up; Scan circle and X pattern b) Progressively increase if damage found during scanning c) Helium/Argon test d) Hammer test e) Cut coupons.	a) Broom swept b) Effective supplementary light c) Visual 100% d) Pit depth gauge Coating or Liner: a) Sponge test 50 – 75% b) Adhesion test c) Scrape test.
Poorly Effective (CAT. D)	a) Spot UT b) Flood test	a) Broom swept b) No effective supplementary lighting c) Visual 25-50% Coating or Liner: a) Sponge test < 50
Ineffective (CAT. E)	None.	None.

Inspection Effectiveness - Tank Shell Course Internal Corrosion

Inspection Effectiveness – Tank Shell Course Internal Corrosion

Inspection Category	Inspection
Highly Effective (CAT. A)	a) Intrusive inspection – good visual inspection with pit depth gauge measurements at suspect locations. b) UT scanning follow up on suspect location and as general confirmation of wall thickness.
Usually Effective (CAT. B)	a) External spot UT scanning based on visual information from previous internal inspection of this tank or similar service tanks. b) Internal video survey with external UT follow-up.
Fairly Effective (CAT. C)	External spot UT scanning based at suspect locations without benefit of any internal inspection information on tank type or service.
Poorly Effective (CAT. D)	External spot UT based at suspect locations without benefit of any internal inspection information on tank type or service.
Ineffective (CAT. E)	No inspection.

Inspection Effectiveness - Tank Shell Course External Corrosion

Inspection Effectiveness – Tank Shell Course External Corrosion

Inspection Category	Inspection
Highly Effective (CAT. A)	a) Insulated – >95% external visual inspection prior to removal of insulation b) Remove >90% of insulation at suspect locations, OR >90% pulse eddy current inspection. c) Visual inspection of the exposed surface area with follow-up by UT or pit gauge as required. d) Non-Insulated - >95% visual inspection of the exposed surface area with follow-up by UT or pit gauge as required.
Usually Effective (CAT. B)	a) Insulated – >95% external visual inspection prior to removal of insulation b) Remove >30% of insulation at suspect locations, OR >30% pulse eddy current inspection. c) Visual inspection of the exposed surface area with follow-up by UT or pit gauge as required. d) Non-Insulated - >50% visual inspection of the exposed surface area with follow-up by UT or pit gauge as required.
Fairly Effective (CAT. C)	a) Insulated – >95% external visual inspection prior to removal of insulation b) Remove >10% of insulation at suspect locations, OR >10% pulse eddy current inspection. c) Visual inspection of the exposed surface area with follow-up by UT or pit gauge as required. d) Non-Insulated - >25% visual inspection of the exposed surface area with follow-up by UT or pit gauge as required.
Poorly Effective (CAT. D)	a) Insulated – >95% external visual inspection prior to removal of insulation b) Remove >5% of insulation at suspect locations, OR >5% pulse eddy current inspection. c) Visual inspection of the exposed surface area with follow-up by UT or pit gauge as required. d) Non-Insulated - >10% visual inspection of the exposed surface area with follow-up by UT or pit gauge as required.
Ineffective (CAT. E)	a) Insulated – No visual inspection of insulation surface area or removal of insulation. b) Non-Insulated - <5% visual of the exposed surface area.

Component Lining Damage Factor

An equipment or component may have lining damage. Linings of inorganic and organic origins can be deteriorated, usually because of corrosion.

The Lining Damage Factor does not have Inspection Effectiveness Categories. It can either be inspected or not be inspected.

Stress Corrosion Cracking (SCC) Damage Factor- Caustic Cracking

Caustic Cracking is the combined action of tensile stress and corrosion in the presence of NaOH at elevated temperature.

Cracking is predominantly intergranular and typically occurs as a network of fine cracks in carbon steels. Low alloy steels have similar cracking susceptibility.

There are three key parameters:
- Caustic concentration,
- Metal temperature
- Level of tensile stress

Caustic cracking of carbon steel is not anticipated at metal temperatures less than about 46°C. Between 46°C and 82°C, it is a function of caustic concentration. Above 82°C it is highly likely for all concentrations above about 5 wt%.

Post Weld Heat Treatment (PWHT) is a proven method to prevent caustic cracking.

Inspection Effectiveness - Caustic Cracking

Inspection Effectiveness - Caustic Cracking

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Wet fluorescent Magnetic particle or dye penetrant testing of 25-100% of welds/cold bends; or Dye penetrant testing of 25-100% of welds/cold bends.	Shear wave ultrasonic testing of 25-100% of welds/cold bends; or Radiographic testing of 50-100% of welds/cold bends.
Usually Effective (CAT. B)	Wet fluorescent Magnetic particle or dye penetrant testing of 10-24% of welds/cold bends; or Dye penetrant testing of 10-24% of welds/cold bends.	Shear wave ultrasonic testing of 10-24% of welds/cold bends; or Radiographic testing of 25-49% of welds/cold bends.
Fairly Effective (CAT. C)	Magnetic particle or dye penetrant testing of less than 10% of welds/cold bends; or Dye penetrant testing of less than 10% of welds/cold bends.	Shear wave ultrasonic testing of less than 10% of welds/cold bends; or Radiographic testing of less than 25% of welds/cold bends.
Poorly Effective (CAT. D)	Visual inspection.	Visual inspection for leaks.
Ineffective (CAT. E)	No inspection.	No inspection.

Stress Corrosion Cracking (SCC) Damage Factor- Amine Cracking

Amine Cracking is the combined action of tensile stress and corrosion in the presence of an aqueous alkanolamine at elevated temperature.

Cracking is predominantly intergranular and typically occurs as a network of fine cracks in carbon steels. Low alloy steels have similar cracking susceptibility.

The four key parameters:
- Type of amine
- Amine solution composition
- Metal temperature
- Level of tensile stress.

Amine cracking is not anticipated in fresh amine solutions that have not been exposed to acid gases or in rich alkanolamine solutions which contain high levels of acid gases, but mostly in lean alkanolamine solutions which contain low levels of acid gases.

Post Weld Heat Treatment (PWHT) is a proven method to prevent amine cracking.

Inspection Effectiveness Categories (Amine Cracking)

Inspection Effectiveness Categories (Amine Cracking)

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Wet fluorescent magnetic particle testing of 100% of repair welds and 50-100% of other welds/cold bends.	None.
Usually Effective (CAT. B)	Wet fluorescent magnetic particle testing of 20-49% of welds/cold bends.	Shear wave ultrasonic testing of 50-100% of welds/cold bends; or Acoustic Emission testing with follow-up shear wave UT.
Fairly Effective (CAT. C)	Wet fluorescent magnetic particle testing of less than 20% of welds/cold bends; or Dry magnetic particle testing of 50-100% of welds/cold bends; or Dye penetrant testing of 50-100% of welds/cold bends.	Shear wave ultrasonic testing of 20-49% of welds/cold bends.
Poorly Effective (CAT. D)	Dry magnetic particle testing of less than 50% of welds/cold bends; or Dye penetrant testing of less than 50% of welds/cold bends.	Shear wave ultrasonic testing of less than 20% of welds/cold bends; or Radiographic testing; or Visual inspection for leaks.
Ineffective (CAT. E)	Visual inspection.	No inspection.

Stress Corrosion Cracking (SCC) Damage Factor- Sulfide Stress Cracking

Sulfide Stress Cracking is the combined action of tensile stress and corrosion in the presence of water and hydrogen sulfide.

There are four key parameters:
- Metal hardness
- Stress Level
- pH
- H₂S content of process fluid

Sulfide stress cracking is more prevalent in high hardness metals, with the hydrogen flux being lowest at neutral pH and increasing at both lower and higher pHs.

Cyanide presence at high pH can increase the hydrogen flux into the steel.

Source of hydrogen in the steel is the corrosion reaction with wet hydrogen sulfide in the presence of water.

Post Weld Heat Treatment (PWHT) is a proven method to prevent sulfide stress cracking.

Inspection Effectiveness - Sulfide Stress Cracking

Inspection Effectiveness –Sulfide Stress Cracking

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Wet fluorescent magnetic particle testing of 25-100% of weldments.	Shear wave ultrasonic testing of 25-100% of weldments, transverse and parallel to the weld with the weld cap removed; or Acoustic Emission testing with follow-up shear wave UT.
Usually Effective (CAT. B)	Wet fluorescent magnetic particle testing of 10-24% of weldments; or Dry magnetic particle testing of 25-100% of weldments; or Dye penetrant testing of 25-100% of weldments.	Shear wave ultrasonic testing of 10-24% of weldments; Radiographic testing of 50-100% of weldments.
Fairly Effective (CAT. C)	Wet fluorescent magnetic particle testing of less than 10% of weldments; or Dry magnetic particle testing of less than 25% of weldments; or Dye penetrant testing of less than 25% of weldments.	Shear wave ultrasonic testing of less than 10% of weldments; Radiographic testing of 20-49% of weldments.
Poorly Effective (CAT. D)	Visual inspection.	Radiographic testing of less than 20% of weldments.
Ineffective (CAT. E)	No inspection.	No inspection.

Stress Corrosion Cracking (SCC) Damage Factor- HIC/SOHIC-H₂S

H₂S components are subject to hydrogen-induced cracking and stress-oriented hydrogen induced cracking in H₂S services.

Internal cracks can connect adjacent hydrogen blisters on different planes on the metal surface.

External applied stress does not always exist.

Driving force is high stresses at the circumference of the hydrogen blisters that are caused by buildup of internal pressure.

Buildup of blister pressure is related to the hydrogen permeation flux in the steel.

Source of hydrogen in the steel is the corrosion reaction with wet hydrogen sulfide in the presence of water.

Corrosion at low pH values is caused by H₂S, whereas corrosion at high pH values is caused by high concentrations of bisulfide ion.

Cyanides at elevated pH can aggravate hydrogen penetration into the steel.

Presence of 50 ppm of H₂S in the water is sufficient to cause HIC.

Sulfur content of the steel is a key parameter for the susceptibility. Reducing the sulfur content of the steel reduces the susceptibility to blistering. Additions of calcium or REMS (rare-earth elements) which control sulfide inclusion are generally beneficial.

Cleanliness of the steel is measured by the sulfur content.

Blistering does pose a danger to mechanical integrity when it approaches a weld which contains sufficient residual stresses.

SOHIC is a stacked array of blisters joined by hydrogen-induced cracking aligned in the through-thickness direction of the steel as a result of high localized tensile stresses.

Reduction of residual stresses by PWHT can reduce the occurrance and severity of SOHIC.

Inspection Effectiveness - HIC/SOHIC-H₂S

Inspection Effectiveness – HIC/SOHIC-H₂S

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Wet fluorescent magnetic particle testing of 50-100% of weldments, plus additional shear wave UT for subsurface cracking.	None.
Usually Effective (CAT. B)	Wet fluorescent magnetic particle testing of 20-49% of weldments.	Automated shear wave ultrasonic testing of 20-100% of weldments; or Acoustic Emission testing with follow-up shear wave UT.
Fairly Effective (CAT. C)	Wet fluorescent magnetic particle testing of less than 20% of weldments; or Dry magnetic particle testing of 50-100% of weldments; or Dye penetrant testing of 50-100% of weldments.	Automated shear wave ultrasonic testing of less than 20% of weldments; or Manual shear wave ultrasonic testing of 20-100% of weldments.
Poorly Effective (CAT. D)	Dye penetrant testing of less than 50% of weldments; Visual inspection for hydrogen blisters.	Manual shear wave ultrasonic testing of less than 20% of weldments.
Ineffective (CAT. E)	No inspection.	Radiographic testing.

Stress Corrosion Cracking (SCC) Damage Factor- Carbonate Cracking

Combined action of tensile stress and corrosion in the presence of an alkaline sour water containing CO₃.

Cracking is predominantly intergranular and typically occurs as a network of fine cracks in carbon steels. Low alloy steels have similar cracking susceptibility.

Three key parameters:
- pH of the sour water
- Carbonate concentration
- Level of tensile stress

Carbonate cracking occurs in a narrow range of electrochemical potential, which is caused by moderate to high levels of carbonates in alkaline sour water.

Cyanide presence influences cracking susceptibility.

PWHT is a proven method to prevent amine cracking.

Inspection Effectiveness Categories- Carbonate Cracking

Inspection Effectiveness Categories– Carbonate Cracking

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Wet fluorescent magnetic particle testing of 100% of repair welds and 50-100% of other welds/cold bends.	None.
Usually Effective (CAT. B)	Wet fluorescent magnetic particle testing of 20-49% of welds/cold bends.	Shear wave ultrasonic testing of 50-100% of welds/cold bends; or Acoustic Emission testing with follow-up shear wave UT.
Fairly Effective (CAT. C)	Wet fluorescent magnetic particle testing of less than 20% of welds/cold bends; or Dry magnetic particle testing of 50- 100% of welds/cold bends; or Dye penetrant testing of 50-100% of welds/cold bends.	Shear wave ultrasonic testing of 20-49% of welds/cold bends.
Poorly Effective (CAT. D)	Dry magnetic particle testing of less than 50% of welds/cold bends; or Dye penetrant testing of less than 50% of welds/cold bends.	Shear wave ultrasonic testing of less than 20% of welds/cold bends; or Radiographic testing; or Visual inspection for leaks.
Ineffective (CAT. E)	Visual inspection.	No inspection.

Stress Corrosion Cracking (SCC) Damage Factor- PTA Cracking

Combined action of tensile stress with the presence of sulfide containing deposits during shutdown.
Cracking is always intergranular and requires low tensile stresses for initiation and propagation.
Four key parameters:
- Presence of sulfide
- Exposure to air and moisture during shutdown
- Carbonate concentration
- Level of tensile stress
PTA cracking is often found in as-welded stainless steels, particularly in the weld heat-affected zone.
Downtime protection according to NACE RP0170 reduces the risk of PTA cracking.

Inspection Effectiveness Categories- PTA Cracking

Inspection Effectiveness Categories – PTA Cracking

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Dye penetrant (25%+).	Radiography (25%+) and Shear wave ultrasonics (25%+).
Usually Effective (CAT. B)	Dye penetrant testing (10-24%).	Radiography approx. (5%) and Shear wave ultrasonics (25%+).
Fairly Effective (CAT. C)	Dye penetrant (<10%).	Spot Radiography and Spot shear wave ultrasonics.
Poorly Effective (CAT. D)	Visual.	Visual for leaks.
Ineffective (CAT. E)	No inspection.	No inspection.
Note: There is no highly effective inspection without a minimum of partial insulation removal and external VT and PT.

Stress Corrosion Cracking (SCC) Damage Factor- CLSCC

Occurs to austenitic stainless steel components in chloride containing aqueous environments.
Cracking is predominantly intergranular and highly branched.
Three key parameters:
- pH of the process fluid
- Chloride ion concentration
- Temperature.
CLSCC is most prevalent in austenitic SS with an 8% nickel content. Lower or higher nickel content SS shows greater resistance, with Duplex SS with a low nickel content or alloys with greater than 42% nickel content being generally immune to CLSCC.

Inspection Effectiveness Categories- SCC CLSCC

Inspection Effectiveness Categories – SCC CLSCC

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Dye penetrant testing of 50% to 100% of weldments.	Shear wave ultrasonic testing of 25% to 100% of weldments, transverse and parallel to the weld with the weld cap removed.
Usually Effective (CAT. B)	Dye penetrant testing of 25% to 50% of weldments.	Shear wave ultrasonic testing of 10% to 24% of weldments, and radiographic testing of 50% to 100% of weldments or AE test with partial insulation removal and PT.
Fairly Effective (CAT. C)	Dye penetrant testing of less than 25% of weldments.	Shear wave ultrasonic testing of less than 10% of weldments, and radiographic testing of 20% to 49% of weldments.
Poorly Effective (CAT. D)	Visual.	Visual for leaks.
Ineffective (CAT. E)	No inspection.	No inspection.

Stress Corrosion Cracking (SCC) Damage Factor- HSC-HF

Combined action of tensile stress and corrosion in the presence of hydrogen from hydrofluoric acid (HF).
Two key parameters: metal hardness and presence of HF.
HSC-HF is prevalent in high strength (hardness) steels or in hard heat-affected zones of lower strength steels.
The requirements of NACE RP 0472 should be followed to prevent HSC-HF. PWHT can lower the affect of HSC-HF; however, it is not preventative.

Inspection Effectiveness Categories- SCC HSC-HF

Inspection Effectiveness Categories – SCC HSC-HF

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Wet fluorescent magnetic particle testing of 25-100% of weldments.	Shear wave ultrasonic testing of 25-100% of weldments, transverse and parallel to the weld with the weld cap removed; or Acoustic Emission testing with follow-up shear wave UT.
Usually Effective (CAT. B)	Wet fluorescent magnetic particle testing of 10-24% of weldments; or Dry magnetic particle testing of 25-100% of weldments; or Dye penetrant testing of 25-100% of weldments.	Shear wave ultrasonic testing of 10-24% of weldments; and Radiographic testing of 50-100% of weldments.
Fairly Effective (CAT. C)	Wet fluorescent magnetic particle testing of less than 10% of weldments; or Dry magnetic particle testing of less than 25% of weldments; or Dye penetrant testing of less than 25% of weldments.	Shear wave ultrasonic testing of less than 10% of weldments; and Radiographic testing of 20-49% of weldments.
Poorly Effective (CAT. D)	Visual inspection.	Radiographic testing of less than 20% of weldments.
Ineffective (CAT. E)	No inspection.	No inspection.

Stress Corrosion Cracking (SCC) Damage Factor- HIC/SOHIC-HF

Components are subject to hydrogen-induced cracking and stress-oriented hydrogen induced cracking in HF services.
Source of hydrogen in the steel is the corrosion reaction with hydrofluoric acid.
Internal cracks can connect adjacent hydrogen blisters on different planes on the metal surface.
External applied stress does not always exist.
Driving force is high stresses at the circumference of the hydrogen blisters that are caused by buildup of internal pressure.
Buildup of blister pressure is related to the hydrogen permeation flux in the steel. Blistering does pose a danger to mechanical integrity when it approaches a weld which contains sufficient residual stresses.
Cleanliness of the steel is measured by the sulfur content. Reducing the sulfur content of the steel reduces the susceptibility to blistering. Additions of calcium or REMS (rare-earth elements) which control sulfide inclusion are generally beneficial.
SOHIC is a stacked array of blisters joined by hydrogen-induced cracking aligned in the through-thickness direction of the steel as a result of high localized tensile stresses.
Reduction of residual stresses by PWHT can reduce the occurrance and severity of SOHIC.

Inspection Effectiveness Categories- SCC HIC/SOHIC-HF

Inspection Effectiveness Categories – HIC/SOHIC-HF

Inspection Category	Intrusive Inspection Example	Non-Intrusive Inspection Example
Highly Effective (CAT. A)	Wet fluorescent magnetic particle testing of 50-100% of weldments, plus additional shear wave UT for subsurface cracking.	None.
Usually Effective (CAT. B)	Wet fluorescent magnetic particle testing of 20-49% of weldments.	Automated shear wave ultrasonic testing of 20-100% of weldments; or Acoustic Emission testing with follow-up shear wave UT.
Fairly Effective (CAT. C)	Wet fluorescent magnetic particle testing of less than 20% of weldments; or Dry magnetic particle testing of 50-100% of weldments; or Dye penetrant testing of 50-100% of weldments.	Automated shear wave ultrasonic testing of less than 20% of weldments; or Manual shear wave ultrasonic testing of 20-100% of weldments.
Poorly Effective (CAT. D)	Dye penetrant testing of less than 50% of weldments; or Visual inspection for hydrogen blisters.	Manual shear wave ultrasonic testing of less than 20% of weldments.
Ineffective (CAT. E)	No inspection.	Radiographic testing.

External Corrosion Damage Factor- Ferritic Component

Plants located in areas with high annual rainfall, warmer climate, or marine locations are more prone to external corrosion than those located in cooler, drier areas.
Units located near cooling towers and steam vents or those whose operating temperatures cycle through the dew point on a regular basis are highly susceptible to external corrosion regardless of climate.
Proper painting/coating of the component surface is an accomplished method of mitigation for external corrosion.

Inspection Effectiveness Categories- External -Ferritic Component

Inspection Effectiveness Categories – External – Ferritic Component

Inspection Category	Inspection
Highly Effective (CAT. A)	Visual inspection of >95% of the exposed surface area with follow-up by UT, RT or pit gauge as required.
Usually Effective (CAT. B)	Visual inspection of >60% of the exposed surface area with follow-up by UT, RT or pit gauge as required.
Fairly Effective (CAT. C)	Visual inspection of >30% of the exposed surface area with follow-up by UT, RT or pit gauge as required.
Poorly Effective (CAT. D)	Visual inspection of >5% of the exposed surface area with follow-up by UT, RT or pit gauge as required.
Ineffective (CAT. E)	Visual inspection of <5% of the exposed surface area with follow-up by UT, RT or pit gauge as required.

CUI Damage Factor- Ferritic Component

Plants located in areas with high annual rainfall, warmer climate or marine locations are more prone to external corrosion than those located in cooler, drier areas.
Units located near cooling towers and steam vents or those whose operating temperatures cycle through the dew point on a regular basis are highly susceptible to external corrosion regardless of climate.
CUI results from the collection of water in the annulus space between the insulation and the metal surface and causes wall loss by localized corrosion.
Proper painting/coating and insulation practices of the component surface is an accomplished method of mitigation for CUI. High quality immersion grade coating, such as those used in hot water tanks, is recommended.

Inspection Effectiveness Categories- CUI -Ferritic Component

Inspection Effectiveness Categories – CUI – Ferritic Component

Inspection Category	Insulation Removed	Insulation Not Removed
Highly Effective (CAT. A)	For the total surface area: 100% visual inspection prior to removal of insulation AND Remove >95% of the insulation including suspect areas AND 100% visual inspection of the exposed surface area with follow-up by UT, RT or pit gauge as required.	For the total surface area: 100% visual inspection AND 100% inspection with highly effective NDE technique (such as profile or real-time radiography or guided wave UT) AND strip 100% of areas where NDE technique is not effective (e.g., fittings) AND 100% visual inspection of the exposed surface area with follow-up by UT, RT or pit gauge as required or real-time radiography.
Usually Effective (CAT. B)	For the total surface area: >95% external visual inspection prior to removal of insulation AND remove >60% of total surface area of insulation including suspect areas AND visual inspection of the exposed surface area with follow-up by UT, RT or pit gauge as required.	For the total surface area: >95% visual inspection AND > 60% inspection with highly effective NDE technique (such as profile or real-time radiography or guided wave UT) including suspect areas AND strip > 60% of areas where NDE technique is not effective (e.g., fittings) AND 100% visual inspection of the exposed surface area with follow-up by UT, RT or pit gauge as required.
Fairly Effective (CAT. C)	For the total surface area: >95% external visual inspection prior to removal of insulation AND remove >30% of total surface area of insulation including suspect areas AND visual inspection of the exposed surface area with follow-up by UT, RT or pit gauge as required.	For the total surface area: >95% visual inspection AND > 24% inspection with highly effective NDE technique (such as profile or real-time radiography or guided wave UT) including suspect areas AND strip > 24% of areas where NDE technique is not effective (e.g., fittings) AND 100% visual inspection of the exposed surface area with follow-up by UT, RT or pit gauge as required.
Poorly Effective (CAT. D)	>95% external visual inspection prior to removal of insulation AND remove >5% of total surface area of insulation including suspect areas AND visual inspection of the exposed surface area with follow-up by UT, RT or pit gauge as required.	For the total surface area: >95% visual inspection AND > 5% inspection with highly effective NDE technique (such as profile or real-time radiography or guided wave UT) including suspect areas AND strip > 5% of areas where NDE technique is not effective (e.g., fittings) AND 100% visual inspection of the exposed surface area with follow-up by UT, RT or pit gauge as required.
Ineffective (CAT. E)	<5% insulation removal and inspection OR No inspection or ineffective inspection technique.	No inspection or ineffective inspection technique or <95% visual inspection.

External CLSCC Damage Factor- Austenitic Component

Plants located in areas where an accumulation of chloride can result from local conditions or units located in the mist areas of cooling towers or steam vents can be subject to external CLSCC.
Cracking is predominantly intergranular and highly branched.
Two key parameters:
- Chloride ion concentration
- Temperature.

CLSCC is most prevalent in austenitic SS with an 8% nickel content. Lower or higher nickel content SS shows greater resistance, with Duplex SS with a low nickel content or alloys with greater than 42% nickel content being generally immune to CLSCC.

Mitigation is accomplished by preventing chloride buildup on the SS surface.

Inspection Effectiveness Categories- CLSCC -Austenitic Component

Inspection Effectiveness Categories – CLSCC – Austenitic Component

Inspection Category	Inspection
Highly Effective (CAT. A)	For the total surface area: greater than 95% dye penetrant or eddy current test with UT follow-up of relevant indications.
Usually Effective (CAT. B)	For the total surface area: greater than 60% dye penetrant or eddy current testing with UT follow-up of all relevant indications.
Fairly Effective (CAT. C)	For the total surface area: greater than 30% dye penetrant or eddy current testing with UT follow-up of all relevant indications.
Poorly Effective (CAT. D)	For the total surface area: greater than 5% dye penetrant or eddy current testing with UT follow-up of all relevant indications.
Ineffective (CAT. E)	Less than “D” effectiveness or no inspection or ineffective inspection technique used.

External CUI CLSCC Damage Factor- Austenitic Component

Plants located in areas where an accumulation of chloride can result from local conditions or units located in the mist areas of cooling towers or steam vents can be subject to external CUI CLSCC. Even the insulation itself can be a source of chloride.

Cracking is predominantly intergranular and highly branched.
Two key parameters:
- Chloride ion concentration
- Temperature

CLSCC is most prevalent in austenitic SS with an 8% nickel content. Lower or higher nickel content SS shows greater resistance, with Duplex SS with a low nickel content or alloys with greater than 42% nickel content being generally immune to CLSCC.

Mitigation is accomplished by preventing chloride buildup on the SS surface under the insulation or with the use of coating or wrapping of the component in aluminum foil.

Inspection Effectiveness Categories- CUI CLSCC - Austenitic Component

Inspection Effectiveness Categories – CUI CLSCC – Austenitic Component

Inspection Category	Insulation Removed	Insulation Not Removed
Highly Effective (CAT. A)	For the total surface area: greater than 95% dye penetrant or eddy current test with UT follow-up of relevant indications.	No inspection techniques yet available meet requirements of "A.”
Usually Effective (CAT. B)	For the total surface area: greater than 60% dye penetrant or eddy current testing with UT follow-up of all relevant indications.	For the total surface area: Greater then 95% automated or manual ultrasonic scanning from the internal surface OR AE testing with 100% follow-up of relevant indications.
Fairly Effective (CAT. C)	For the total surface area: greater than 30% dye penetrant or eddy current testing with UT follow-up of all relevant indications.	For the total surface area: Greater than 67% automated or manual ultrasonic scanning from the internal surface.
Poorly Effective (CAT. D)	For the total surface area: greater than 5% dye penetrant or eddy current testing with UT follow-up of all relevant indications.	For the total surface area: Greater than 30% automated or manual ultrasonic scanning from the internal surface OR Greater than 60% radiographic testing.
Ineffective (CAT. E)	Less than “D” effectiveness or no inspection or ineffective inspection technique used.	Less than “D” effectiveness or no inspection or ineffective inspection technique used.

HTHA Damage Factor

Components exposed to high temperatures and high hydrogen partial pressures are susceptible to high temperature hydrogen attack (HTHA).
Damage is either internal decarburization and fissuring due to accumulated methane gas or just surface decarburization when methane gas can escape.
Two key parameters:
- Partial pressure of hydrogen
- Temperature.
HTHA is prevalent in carbon steels, C-0.5Mo, and Cr-Mo low alloy steels. Carbon steel that only contains Fe₃C carbides has less resistance to HTHA than those that contain Cr-Mo carbides.
Mitigation is accomplished by increasing the alloy content of the steel and therefore increasing the stability of the carbides.

Inspection Effectiveness Categories- HTHA

Inspection Effectiveness Categories – HTHA

Inspection Category	Inspection
Highly Effective (CAT. A)	Inspection techniques for HTHA are not available to qualify for a category A inspection.
Usually Effective (CAT. B)	Extensive Advanced Ultrasonic Backscatter Technique (AUBT), spot AUBT based on stress analysis or extensive in-situ metallography.
Fairly Effective (CAT. C)	Spot AUBT or spot in-situ metallographyc.
Poorly Effective (CAT. D)	Ultrasonic backscatter plus attenuation.
Ineffective (CAT. E)	Attenuation only.