![PART 14] CHLORIDE STRESS CORROSION CRACKING](https://i3.wp.com/blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjmtAhEj-dhytYFigrpBFJEByvlBD0I-hlITPYhk8MOBby0d5SwAm88KiJ3iWSUaVPfrV5SwzD25Tchj0XkvZ1RKZ5vtSmogEpG8LjKOEc2O4iGlnZI7J981Q6UJ4Bubwy7TYs0McxWk5ji/d/SCC.jpg?w=1068&resize=1068,0&ssl=1 "PART 14] CHLORIDE STRESS CORROSION CRACKING")
![PART 14] CHLORIDE STRESS CORROSION CRACKING](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjmtAhEj-dhytYFigrpBFJEByvlBD0I-hlITPYhk8MOBby0d5SwAm88KiJ3iWSUaVPfrV5SwzD25Tchj0XkvZ1RKZ5vtSmogEpG8LjKOEc2O4iGlnZI7J981Q6UJ4Bubwy7TYs0McxWk5ji/d/SCC.jpg "PART 14] CHLORIDE STRESS CORROSION CRACKING")
API 571 : CLAUSE 4.5.1 Chloride Stress Corrosion Cracking
Description of Damage
- Surface initiated cracking
- cracking of 300 Series SS and
some nickel base alloys - Combined action
- Tensile stress, temperature, an aqueous
chloride environment and the presence of dissolved oxygen
Affected Materials
- All 300 Series SS are highly susceptible.
- Duplex stainless steels are more resistant.
- Nickel base alloys are
highly resistant, but not immune.
Critical Factors
Read in this manner,
- Six factors affect Cl-SCC
- Cl Content [Read Clause ‘c’ & ‘d’ under
the heading critical factors] - pH [Read Clause ‘f’ under the heading
critical factors] - Temperature [Read Clause ‘b’, ‘e’ & ‘g’
under the heading critical factors] - Stress [Read Clause ‘h’ under the heading
critical factors] - Presence of O2 [Read Clause ‘i’ under the
heading critical factors] - Material alloy composition [Read Clause ‘j’,
‘k’ & ‘l’ under the heading critical factors] - No Lower limit for Cl content
- For pH value >2, usually Cl-SCC occurs
- In Lower pH, Uniform corrosion occurs and
not Cl-SCC - SCC decreases towards alkaline pH region
- Stress can be applied or residual.
- Cold worked components (eg: Expansion
bellows) are susceptible - Nickel content is a major factor.
- 8% to 12% Ni materials are more more to
Cl-SCC - Ni>35% are highly resistant and
Ni>45% are immune - 400 Series SS are not susceptible
- CS and Low alloy steels are susceptible
- Dissolved O2 accelerates Cl-SCC. But limit
of dissolved O2 is not identified. - Temperature >60 Deg.C are more prone to
Cl-SCC - Wet-dry conditons of steam and water are
susceptible to Cl-SCC - If level of Cl and Temperature increases
chances for cracking increases.
Affected Units or Equipment
Piping
- 300 Series SS process lines
- Drains in hydro processing units are susceptible to cracking during startup/shutdown
if not properly purged - boiler drain lines
- External Cl–SCC on insulated surfaces when insulation gets wet
- Bellows and instrument tubing, particularly those associated with
hydrogen recycle streams contaminated with chlorides
Pressure Vessel / Equipment
- 300 Series SS process equipment
- water-cooled condensers
- process side of crude tower overhead condensers
- 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,
- Use Cl-SCC
resistant materials. - By
design avoid regions that may cause chlorides to concentrate or deposit. - During
hydrotest, use low chloride water and dry out the water as quickly and
thoroughly. - 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,
- increasing susceptibility to polythionic SCC
- possible distortion problems
- 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.
QUESTIONS
Hope you have done your answers correctly…
This series will continue…
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