API 570 EXAM STUDY GUIDE [MODULE 2 : PART 14] CHLORIDE STRESS CORROSION CRACKING

API 570 STUDY GUIDE
MODULE 2 : PART 14
CHLORIDE STRESS CORROSION CRACKING

API 571 : CLAUSE 4.5.1 Chloride Stress Corrosion Cracking

Description of Damage

1. Surface initiated cracking
2. cracking of 300 Series SS and some nickel base alloys
3. Combined action
4. Tensile stress, temperature, an aqueous chloride environment and the presence of dissolved oxygen

Affected Materials

1. All 300 Series SS are highly susceptible.
2. Duplex stainless steels are more resistant.
3. Nickel base alloys are highly resistant, but not immune.

Critical Factors

Read in this manner,

1. Six factors affect Cl-SCC
1. Cl Content [Read Clause ‘c’ & ‘d’ under the heading critical factors]
2. pH [Read Clause ‘f’ under the heading critical factors]
3. Temperature [Read Clause ‘b’, ‘e’ & ‘g’ under the heading critical factors]
4. Stress [Read Clause ‘h’ under the heading critical factors]
5. Presence of O2 [Read Clause ‘i’ under the heading critical factors]
6. Material alloy composition [Read Clause ‘j’, ‘k’ & ‘l’ under the heading critical factors]
2. No Lower limit for Cl content
3. For pH value >2, usually Cl-SCC occurs
4. In Lower pH, Uniform corrosion occurs and not Cl-SCC
5. SCC decreases towards alkaline pH region
6. Stress can be applied or residual.
7. Cold worked components (eg: Expansion bellows) are susceptible
8. Nickel content is a major factor.
9. 8% to 12% Ni materials are more more to Cl-SCC
10. Ni>35% are highly resistant and Ni>45% are immune
11. 400 Series SS are not susceptible
12. CS and Low alloy steels are susceptible
13. Dissolved O2 accelerates Cl-SCC. But limit of dissolved O2 is not identified.
14. Temperature >60 Deg.C are more prone to Cl-SCC
15. Wet-dry conditons of steam and water are susceptible to Cl-SCC
16. If level of Cl and Temperature increases chances for cracking increases.

Affected Units or Equipment

Piping

1. 300 Series SS process lines
2. Drains in hydro processing units are susceptible to cracking during startup/shutdown if not properly purged
3. boiler drain lines
4. External Cl–SCC on insulated surfaces when insulation gets wet
5. Bellows and instrument tubing, particularly those associated with hydrogen recycle streams contaminated with chlorides

 Pressure Vessel / Equipment

1. 300 Series SS process equipment
2. water-cooled condensers
3. process side of crude tower overhead condensers
4. External Cl–SCC on insulated surfaces when insulation gets wet

Appearance or Morphology of Damage

The material usually shows no visible signs of corrosion.

Surface breaking cracks can occur from the process side or externally under insulation

Metallography of cracked samples typically shows branched transgranular cracks

Fracture surfaces often have a brittle appearance.

Characteristic stress corrosion cracks have many branches and may be visually detectable by a craze-cracked appearance of the surface

Corrosion-resistant nickel-based alloys are also susceptible to cracking under severe conditions

Sometimes intergranular cracking of sensitized 300 Series SS may also be seen

Welds in 300 Series SS usually contain some ferrite, producing a duplex structure that is usually more resistant to Cl–SCC

Prevention / Mitigation

Avoid or reduce the chloride content.

During construction phase,

1. Use Cl-SCC resistant materials.
2. By design avoid regions that may cause chlorides to concentrate or deposit.
3. During hydrotest, use low chloride water and dry out the water as quickly and thoroughly.
4. Stress relief of 300 series SS after fabrication to reduce residual stress.

 Insulation related: Apply proper coatings under insulation and another way is to use low chloride coatings.

Stress relief of 300 series of SS may cause, 

1. increasing susceptibility to polythionic SCC 
2. possible distortion problems 
3. potential reheat cracking

Inspection and Monitoring

Cl-SCC is surface initiated cracking. So Surface NDE methods are used for detection.

Visual Inspection

In some cases, visual inspection reveals cracks.

Penetrant Testing (PT)

PT is one of the preferred methods for Cl-SCC. But extremely fine cracks cannot be found by PT. Special surface preparation methods, including polishing or high-pressure water blast, may be required in some cases, especially in high pressure services are done before PT.

Eddy Current Inspection

condenser tubes as well as piping and pressure vessels eddy current inspections are used.

Phase Analysis EC technique is one of the other preferred method for crack detection other than PT

UT

UT can also be done for detection

RT

As these are surface cracks, RT will not be sufficiently sensitive.

Related Mechanisms

Caustic SCC

Polythionic acid SCC

FROM API 570

Piping fabricated of or having components of austenitic stainless steel should be hydrotested with 

1. a solution made up of potable water : Maximum 250 ppm

2. de-ionized/de-mineralized water or steam condensate having a total chloride

concentration (not free chlorine concentration) of less than 50 ppm.

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