Stress corrosion cracking in oil refineries

Stress corrosion cracking occurs due to combined corrosion and straining of a metal as a result of residual or applied stress. It create a remarkable loss of mechanical strength with nominal metal loss that can cause mechanical fast fracture and catastrophic failure of components and structures.

SCC is not a predictable process and for many metals in diverse conditions it doesn’t occur.  Therefore engineers try to use materials more efficiently by increasing work stress and using economical materials. Commonly used alloys for SCC systems are- Carbon steel, low alloy steel, strong steel, austenitic stainless steel, duplex stainless steel, martensitic stainless steel and copper alloys such as Monel 400 wire excluding Copper-Nickel alloys.

Oil refineries

Aqueous hydrogen sulfide in oil and gas development applications can create various challenges. H2S is a harmful gas that can cause vigorous metal loss corrosion and catastrophic brittle fractures of pressurized equipment and pipes. These brittle cracks on metallic structures can occur fast with nominal to no signs or may take a long time to be visible.

Sulfide stress cracking is cracking of a metal due to combined action of tensile stress and corrosion in availability of water and H2S.

Monoethanolamine is an absorbent used to eradicate acid gases comprising of H2S and CO2 in oil refining units. Recent damaged in various refineries have described that cracks can be parallel or normal to welds based on the orientation of basic tensile stresses. Cracking has been found to be both transgranular and intergranular.

Postweld stress relief of carbon steel weldments in MEA units was performed only when the metal temperature of the equipment was assumed to exceed 65oC and the acid gas comprised of above 80% carbon dioxide or when the temperatures were assumed to exceed 95oC or 200oF in any acid gas content.

Presently any system comprising of MEA at any temperature and at any acid gas content is postweld stress relieved. It is the result of studies performed on various refineries to define the level of SCC problem in this media. These inspection programs showed that leaks were widespread and were discovered in vessels that had been postweld stress relieved. Additionally, it was discovered that the whole concentrations of MEA were included and the most concentrations of MEA were often at comparatively low temperatures. Systems found to experience cracking included tanks, absorbers, carbon treater drums, skimming drums and pipes.

Materials used in oil refinery units have to be competent enough to perform in the various conditions for production environments and designs. They should be strong yet ductile enough to prevent brittle fractures. Alloy’s composition, yield strength, hardness, heat processing, microstructure, fluid pH, partial pressure of H2S, total applied tensile stress, cold work, temperature and time are the main factors that have an influence on deciding whether or not SCC will occur to a specific metallic structure.

The recommended alloys to use in oil refineries are Hastelloy C276, Monel alloys and Inconel alloys. These alloys offer seamless and long term performance against vigorous corrosion of SCC. So contact us to order your alloy today.