Stress Cracking Corrosion in Instrumentation Fittings

What is Stress Cracking Corrosion?

SCC is a type of corrosion that results from the combined effects of tensile stress and a corrosive environment. Unlike other forms of corrosion, SCC is particularly dangerous because it can cause sudden and catastrophic failures in materials that otherwise appear to be in good condition. This phenomenon is especially concerning in high-pressure instrumentation fittings used in harsh environments. Here, failures can lead to significant safety risks, unexpected downtime, and increased maintenance costs.

The Three Prerequisites for SCC

1. Surface Tensile Stresses: These stresses can arise from various sources, including applied loads, residual stresses from manufacturing processes like welding and machining, and stresses developed during installation or service. The interplay of these stresses is crucial in determining the likelihood of SCC.
2. Corrosive Environment: The nature of the corrosive environment significantly influences SCC. Factors such as the type and concentration of corrodents (e.g., chlorides or sulphides), moisture levels, temperature, and the presence of gases like hydrogen sulphide and carbon dioxide all contribute to the risk of SCC.
3. Susceptibility of the Material: The chemical composition and metallurgical properties of the material play a vital role in its susceptibility to SCC. Materials like austenitic stainless steels (e.g., 304/304L, 316/316L) are more prone to chloride-induced SCC due to their specific chemical makeup.

Types of SCC in Instrumentation Fittings

– Chloride SCC (ClSCC): Common in chloride-rich environments such as offshore locations, ClSCC affects austenitic stainless steels, particularly where there are variations in metal composition or where protective films are compromised. Design considerations to minimize high-stress concentration areas can help mitigate this type of SCC.

– Sulphide SCC (SSC): Occurring in environments with hydrogen sulphide, SSC is prevalent in offshore applications and involves hydrogen embrittlement, where hydrogen atoms penetrate the metal surface, increasing residual stresses and leading to cracking.

Mitigation Strategies

– Stress Reduction: Techniques such as increasing material thickness to distribute stresses more evenly, or applying mechanical and thermal treatments to relieve residual stresses.

– Environmental Control: Reducing exposure to corrosive agents, lowering temperatures, and using inhibitors to isolate materials from their environments can effectively extend the life of fittings.

– Material Selection: Choosing more resistant materials, like Super Duplex or Super Austenitic stainless steels, and applying protective coatings can significantly reduce SCC risks. Though these materials might increase initial costs, they offer better long-term performance and reliability.

Our Expertise

Selecting a fittings supplier with the right expertise is crucial for preventing SCC. At A&C Engineering, we are committed to providing solutions that meet your specific needs, backed by our technical knowledge and practical experience. Trust us to help you maintain safe, reliable, and efficient operations.

For more information or to discuss your specific requirements, please contact us.