Mitigating the Risk Factors for Stress Corrosion Cracking

Decoding the requirements for minimizing the risk of this catastrophic form of corrosion.

Many forms of corrosion progress in an easy-to-detect manner, causing visual distortion, discoloration, cracking, and more. 

This means that detection often occurs well before the point of failure. 

While resolving the solution might be costly and inconvenient, it’s typically something for which a company can make plans and correct before large-scale failures occur.

While rare, Stress Corrosion Cracking (SCC) offers very little in terms of visual hallmarks, material deformation, or other common detection methods. 

This means that metal which otherwise looks shiny and in excellent condition can suddenly catastrophically fail with little to no warning.

Fortunately, while detection often requires complex professional analysis, prevention is much more approachable. 

If you don’t create the situations in which SCC thrives, you have much less to worry about in the bigger picture.

In this guide, we’ll discuss the common causes of stress corrosion cracking, typical scenarios and industries where SCC occurs, and high-level prevention methods to help minimize risk.

What is Stress Corrosion Cracking?

Stress corrosion cracking is a form of galvanic corrosion in which mechanical and chemical forces that might otherwise pose no threat to a given material result in crack propagation -- often at a granular, microscopic level.

Left to progress undetected, SCC can quickly lead to unexpected sudden failure of normally ductile metals.

What Causes Stress Corrosion Cracking?

Stress corrosion cracking requires three variables to occur:

1. Applied or residual stresses

2. An aqueous corrosive media (Chlorides and Hydrogen Sulphide are common)

3. Elevated temperatures

Without all three factors, SCC cannot proceed. When they’re present, however, experts propose a range of models and mechanisms to explain stress corrosion cracking. 

Commonly accepted models include

• Adsorption model: Specific chemical species adsorbs on the crack surface and lowers the fracture stress.

• Film rupture model: Stress ruptures the passive film locally and sets up an active-passive cell. The newly formed passive film is ruptured again under stress and the cycle continues until failure.

• Pre-existing active path model: Pre-existing paths, such as grain boundaries where intermetallics and compounds are formed, are expanded and exaggerated leaving metal weaker.

• Embrittlement model: Hydrogen embrittlement is a major mechanism of SCC for steels and other alloys such as titanium. Hydrogen atoms diffuse to the crack tip and embrittle the metal.

On top of all these variables, the environmental conditions required to encourage SCC will also vary by the metal or alloy in question. 

For example, carbon steels are most susceptible to hot nitrate, hydroxide, and carbonate or bicarbonate solutions. 

High-strength steels can fall victim to hydrogen sulphides. 

Austenitic stainless steels are particularly susceptible to hot, concentrated chloride solutions and chlorine-contaminated steam.

However, duplex steels -- with their blend of austenitic and ferritic metallurgic composition -- can typically withstand higher temperatures before succumbing to SCC attack. This makes them an excellent choice for use in high-temperature processes with a risk of SCC.

Consult the table below for additional environmental factors:

It’s also important to consider that the applied stress of a process or installation might not be the only stress in play when determining the overall risk for SCC in a given design. 

Residual stress often plays a significant role in the development of SCC scenarios.

Examples include stress remaining after:

• Cold deformation or forming

• Heat treatment

• Welding

• Machining

• Grinding

Common Examples of Stress Cracking Corrosion

SCC risks are highest in high-temperature and high chloride or sulphide operations. As such, that means that industrial examples tend to be specific.

While broader examples such as maritime oil and petroleum extraction or processing show obvious risks due to the environment, more subtle situations arise in unexpected places as well.

For example, stainless steel is often used in food processing for its hygienic, easy-to-clean properties. 

But how pipework or pressure vessels respond -- and whether SCC occurs -- will vary from product to product. 

Something like chocolate with low acidity is likely lower risk than something involving pickles, ketchup, or other high-acidity food products.

It’s also of critical concern in structural engineering with carbon steel. The outdoor exposure of many structures and the often large loads in terms of weight, size, and support means that the entire design is subjected to a range of risk factors.

Minimizing and/or Eliminating SCC Risks

Due to the complexities involved, the best opportunities to reduce SCC risk aren’t in the operational phases of a piping process, containment system, or other stainless steel implementation. 

Ideally, considerations surrounding SCC should start at the design phase -- including material choice, environmental controls, and stress reduction and elimination.

When the likelihood for SCC is high, engineers must carefully design piping systems to minimize stress concentrations. They must also consider the impact of any corrosive solutions used. 

Pitting Resistance Equivalent Numbers (PRENs) or Pitting Resistance Equivalent Values (PRE-values) can serve as an excellent starting point to assess a given metal’s susceptibility to stress corrosion cracking. While the two forms of corrosion are not identical, the attacks are often initiated in similar ways. This allows PRENs to also serve as an indicator of SCC resistance.

More critically, total concentration levels -- particularly in processes involving chlorides or sulphides -- during the intended operation should be analyzed and estimated to ensure safety. 

As highlighted in our guide to crevice corrosion, the concentration of a specific region or closed area of a piping process or industrial environment can often far exceed the concentration levels of individual solutions used in processes.

This means that while a system might be rated and designed to resist SCC based on known concentration levels, that failure occurs over time as the solution collects in various areas and concentration levels fluctuate outside of initially considered tolerances.

Once a system is already in operation, managing risks gets trickier. Various metal coatings may provide additional protection but will also add another layer of maintenance complexity and concerns.

Heat treatments may also reduce residual stresses to further improve resilience and prevent SCC. 

However, it’s critical to ensure that heat treatments are performed correctly as temperatures are an essential variable in the formation of stress crack corrosion. 

Failure to properly apply the treatment could cause the very condition the treatment is meant to protect against.

Key Takeaways

• Stress Corrosion Cracking (SCC) requires three conditions to occur: applied or residual stresses, an aqueous corrosive media (Chlorides and Hydrogen Sulphide are common), and elevated temperatures.

• Unlike many forms of corrosion, SCC may not appear visible to the naked eye. This means component failure may come without warning should risk factors remain unchecked.

• Both applied stress and residual stresses must be considered when ascertaining risks. Common residual stress factors include cold deformation or forming, heat treatment, welding, machining, and grinding.

• Industries often affected by SCC include oil and petroleum extraction and processing, food processing, medical and pharmaceutical industries, chemical refinement and processing, and nuclear power creation. 

• Common components subject to SCC include pipe, containment vessels, pressure vessels, and structural elements -- however any component exposed to the three required conditions might be susceptible to corrosion.

• SCC avoidance is best implemented in the design phase, considering the ideal materials and designs to avoid concerns. However, when properly applied, metal coatings and heat treatments can help to manage SCC risks in operation as well.

Unified Alloys has the experience and infrastructure to handle the needs of any industry looking for high-quality stainless steel supplies. From piping and tubing to valves and gates, our selection of parts simplifies sourcing the various components needed to implement your next design or maintain existing piping processes. As a leading provider in North American for more than four decades, we’ve helped industries big and small throughout Canada and the US. Call today to speak with an expert sales analyst and discuss your needs and how we can help you.

References

1. ASM International: Mechanisms of Stress-Corrosion Cracking

2. IntechOpen: Stress Corrosion Cracking Damages

3. UNSW Sydney School of Materials Science and Engineering: Stress Corrosion

4. CD Corrosion: Stress Corrosion Cracking (SCC)

5. GovInfo: Stress Corrosion Cracking Control Measures

6. Penflex: How to Avoid Stress Corrosion Cracking

7. A Marine Blog: Why Stainless Steel Rust / Corrosion? – Part 2 – Other Corrosion

8. NACE International: Stress Corrosion Cracking (SCC)

9. Wikipedia: Stress Corrosion Cracking

10. ScienceDirect: Stress Corrosion Cracking

11. Summary Planet: Stress Corrosion Cracking (SCC)

12. WebbCorr: Different Types of Corrosion - Recognition, Mechanisms & Prevention - Stress Corrosion Cracking (SCC)

13. Corrosion Doctors: Controlling Stress Corrosion Cracking (SCC)

14. CorrosionPedia: What Causes Stress Corrosion Cracking In Pipelines?

15. Sandvik: Stress Corrosion Cracking (SCC)