ADVANCED MATERIALS & PROCESSES | OCTOBER 2024 27 Many corrosion engineers will have encountered stress corrosion cracking (SCC) during their plant or field work, or while performing failure analyses. SCC is an insidious form of corrosion that often goes undetected by visual inspection. Without much warning, a network of fine cracks penetrates through the thickness of a susceptible metal, causing the component to fracture or leak (in the case of process equipment). SCC involves the conjoint action of a tensile stress and a specific environ- ment on a susceptible metal (or alloy). Not all metals will stress crack in a specific environment. A partial listing DAMAGE CAUSED BY STRESS CORROSION CRACKING Stress corrosion cracking is an insidious form of damage that can occur when a susceptible metal is subjected to a tensile stress in a specific environment. Frank N. Smith, Kingston, Ontario, Canada features. In addition, the chemical conditions at the base of pits and crevices can be much more corrosive than the surrounding environment. Following are two examples of SCC. STAINLESS STEEL: STRESS CHLORIDE CRACKING A flanged joint in a piping system was held together with threaded stud bolts and nuts. The studs were made of Type 304 (UNS S30400) stainless steel, conforming to ASTM A193, Grade B8. of some metal–environment combinations that can lead to SCC is given in Table 1[1]. The tensile stress can be applied (in-service) or residual (“locked-in”) from prior manufacturing steps. The magnitude of the stress can be considerably less than the yield strength of the metal/alloy. The stress level can be enhanced by “stress raisers” on the surface of the object. For example, if the metal is susceptible to attack by pitting or crevice corrosion, these surface features can enhance the localized stresses. Hence, stress cracks are frequently observed to originate from the base of pits and similar surface Fig. 1 — Failed stud bolts from a pipe flange on an offshore platform. The studs were made of Type 304 stainless steel and conformed to ASTM A193, Grade B8. TABLE 1 — EXAMPLES OF METAL-ENVIRONMENT COMBINATIONS THAT MAY RESULT IN SCC Metals / alloys Environments (partial listing) Carbon steels Hot nitrate, hydroxide, and carbonate/ bicarbonate solutions; anhydrous ammonia; aqueous amines; sulfides (acidic) High strength steels Solutions containing sulfides (H2S) Austenitic stainless steels Chloride solutions; chloride-contaminated steam Copper and copper alloys Ammoniacal solutions; amines Titanium Aqueous chloride, bromide, and iodide solutions Aluminum alloys Aqueous chloride, bromide, and iodide solutions Magnesium alloys Aqueous chloride solutions High nickel alloys High-purity steam Fig. 2 — A cross-section through a failed stud bolt showing stress chloride cracks that originated from the thread roots. The cracking was branched and transgranular.
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