What Is SSC?
SSC stands for sulfide stress cracking.It is a specific type of spontaneous brittle fracture that occurs in high-strength steels and low-alloy steels.This mechanical failure happens when the material is simultaneously exposed to tensile stress and a corrosive environment containing moisture and hydrogen sulfide (H₂S).
The Three Required Factors
For SSC to occur,three components must be present at the same time:
- Susceptible Material:
High-strength,high-hardness steels (typically with a hardness above 22 HRC). - Tensile Stress:
Can be applied stress (operating loads) or residual stress (from welding or cold working). - Specific Environment:
Wet hydrogen sulfide (H₂S),often referred to as “sour service.”
Unique Characteristics
- Temperature Sensitivity:
Unlike most corrosion types that get worse as it gets hotter,it is most severe at room or ambient temperatures (typically between 20°C to 60°C).
At high temperatures (above 80°C–100°C), hydrogen diffuses out of the steel too quickly to cause cracking. - Sudden Failure:
It causes catastrophic,brittle failure at stress levels well below the steel’s actual yield strength.
How SSC Works
Unlike regular rust or general corrosion that eats away at the surface,it is a form of Hydrogen Embrittlement:
- Chemical Reaction:
The H₂S reacts with the steel surface in the presence of water, generating iron sulfides and atomic hydrogen (H). - Hydrogen Penetration:
The H₂S acts as a poison that prevents hydrogen atoms from forming safe hydrogen gas (H₂). Instead,single hydrogen atoms diffuse into the steel crystal lattice. - Brittle Cracking:
These atoms collect at grain boundaries and stress points inside the metal.Under tensile stress,they weaken the metal bonds,causing the steel to suddenly crack and fail without warning.
Stainless Tubings Supplier in china
What Is SCC?
SCC stands for stress corrosion cracking.It is a highly dangerous material failure mechanism where a normally ductile metal develops brittle cracks and fails suddenly when exposed to a specific corrosive environment while under tensile stress.Unlike general corrosion which thins the metal evenly over time,SCC can cause catastrophic failure with almost no visible loss of metal thickness.
The Three Pillars of SCC
SCC cannot happen unless all three of the following conditions are met simultaneously:
- Susceptible Material:
A specific metal or alloy that is vulnerable to a particular chemical environment (e.g., ordinary carbon steels, stainless steels, brass, aluminum, or titanium). - Tensile Stress:
This must be a pulling force. It can be applied stress (from pressure or structural loads) or residual stress (internal stress locked into the metal from welding, bending, or heat treatment). - Specific Corrosive Environment:
SCC is highly chemical-specific.A chemical that causes rapid cracking in one metal might be completely harmless to another.
How SCC Works
- Crack Initiation:
The specific chemical agent attacks and breaks down the metal’s microscopic protective oxide layer (passive film).Localized corrosion,like a tiny pit, forms at a point of high stress concentration. - Crack Propagation:
The pulling force (tensile stress) opens up the pit into a sharp crack tip.The corrosive fluid rushes into the tip,dissolving the highly stressed atoms at the very front of the crack,causing it to grow deeper. - Final Fracture:
The crack branches through the metal until the remaining solid material can no longer support the workload,leading to a sudden,brittle explosion or snap.
Why It is So Dangerous
- Zero Warning:
The metal appears perfectly shiny and normal on the outside.There is no visible wall-thinning or bulging before it fractures. - Low Stress Failure:
It occurs at stress levels well below the metal’s rated yield strength.Engineers cannot prevent it simply by making the metal walls thicker. - Branching Cracks:
Under a microscope,it cracks typically look like tree branches,either cutting straight through individual metal grains or traveling along the boundaries between grains.
How Engineers Prevent it
- Material Substitution:
Replacing standard stainless steel pipes with duplex stainless steel pipes or nickel pipes,which are highly resistant to chlorides. - Stress Relief:
Utilizing post-weld heat treatment or shot-peening to eliminate dangerous residual stresses left behind by fabrication. - Environmental Alteration:
Removing oxygen,lowering the operating temperature,filtering out chlorides,or adding corrosion inhibitors to the process fluid.
Common Metal-Environment Combinations
| Metal Alloy | Specific Environment causing SCC | Typical Industrial Example |
| Austenitic Stainless Steel | Chloride ions (Cl⁻) + Temp > 60°C | Coastal pipelines, heat exchangers |
| Carbon Steel & Low Alloy Steel | Nitrates, Hydroxides (Caustic), Carbonates | Boiler tubes, alkaline storage tanks |
| Copper Alloys | Ammonia (NH₃) and ammonium ions | “Season cracking” of brass cartridge cases |
| High-Strength Steels | Hydrogen or Hydrogen Sulfide (H₂S) | Sour gas wells |
What Is Chloride SCC?
Chloride SCC,also known as CLSCC,stands for chloride stress corrosion cracking.It is a highly destructive failure mechanism that specifically targets stainless steels,especially the common 300-series austenitic grades.
It causes normally tough,ductile metal to develop brittle,spiderweb-like cracks and fracture suddenly when exposed to chloride ions,moisture,oxygen,and tensile stress-usually at elevated temperatures.
The Four Required Factors
For Chloride SCC to trigger,four specific conditions must occur simultaneously:
- Susceptible Material:Austenitic stainless steels are highly vulnerable.This is due to their nickel content, which makes their crystal structure highly sensitive to chloride-driven cracking.
- Presence of Chlorides:
Even tiny,parts-per-million trace amounts of chloride in water,coastal air,or insulation can cause cracking if they concentrate on the hot metal surface. - Tensile Stress:
This includes applied stress or residual stress locked into the metal from welding,cold bending,or machining. - Elevated Temperature:
Chloride SCC typically becomes a major threat at temperatures above 60°C (140°F).Below this temperature,the cracking rate slows down significantly,though it can still happen in heavily stressed or highly concentrated environments.
The Four Required Factors
Stainless steel is normally corrosion-resistant because it forms a microscopic,protective chromium-oxide “passive layer” on its surface.It destroys this protection:
- Breakdown of the Passive Film:
Chloride ions migrate to the metal surface and aggressively attack weak points in the protective oxide layer. - Pitting and Stress Concentration:
The localized attack creates microscopic pits.
These tiny pits act as “stress risers,” concentrating the pulling forces at a single sharp point. - Crack Propagation (Anodic Dissolution):
The tensile stress pulls the pit open into a crack tip. The metal atoms at the very tip of the crack are highly stressed and dissolve rapidly in the chemical solution.The crack propagates deeply,splitting the metal. - Catastrophic Failure:
The crack grows until the remaining solid metal can no longer hold the load,causing a sudden snap or rupture with zero warning.
Unique Characteristics
- Spiderweb Morphology:
Under a microscope,It is famous for its highly branched,tree-like appearance. - Transgranular Path:
The cracks typically slice right through the individual metal grains rather than traveling along the grain boundaries. - Insulation Trap:
A classic industry problem occurs under wet thermal insulation (Corrosion Under Insulation, or CUI).
Water leaches trace chlorides out of the insulation,traps them against a hot stainless steel pipe,and causes rapid CLSCC from the outside in.
How to Prevent Chloride SCC
Because chloride SCC happens at stresses well below the metal’s yield strength,simply making the metal thicker will not stop it.Engineers use these strategies instead:
Are trace amounts (< 10 ppm) of H₂S or Cl⁻ safe?
No,they are highly dangerous.
Cl⁻:
Evaporates and concentrates on hot pipe surfaces,multiplying from 5 ppm to 10,000 ppm.
H₂S:
NACE standards state that any gas partial pressure ≥ 0.05 psi triggers severe sulfide stress cracking risk.
Why are 304 and 316 stainless steels the most vulnerable to CLSCC?
Their nickel content sits in the “death zone.”
The metallurgical copson curve proves that 8% to 12% nickel creates maximum cracking sensitivity.
304 (~8% Ni) and 316 (~10% Ni) sit precisely at the deadliest peak of this curve.
What happens if H₂S and Cl⁻ are both present?
The destruction speed doubles.
Cl⁻ aggressively breaks down the metal’s protective surface oxide layer.
H₂S enters freely,forcing massive amounts of hydrogen inside to cause instant embrittlement.
Can ordinary Carbon Steel suffer from Chloride SCC?
No,never.
It is an exclusive killer of stainless steels and non-ferrous alloys.
Carbon steel in saltwater will suffer heavy rust and deep pitting,but it will not crack from chlorides.
Can Cathodic Protection stop all three cracking types?
No,and using it wrong will cause catastrophic failure.
For CLSCC:It works perfectly by stopping surface metal dissolution.
For SSC:Strictly prohibited.CP generates atomic hydrogen,which directly accelerates hydrogen embrittlement.
How do plants detect these invisible cracks before failure?
Visual checks and standard X-rays will miss them.
For Surface Cracks:Engineers use eddy current or ACFM.
For Deep Internal Cracks:Plants rely on PAUT or TOFD.
write to us
Have a Question?
Contact Us!
Should you have any questions regarding stainless steel pipes, including manufacturing processes, chemical composition or other unfamiliar matters, our staff will be happy to assist you.
