Intergranular cracks

The behaviour of austenitic stainless steels in caustic solutions has received less attention than cracking in chloride environments. Transgranular cracking has been reported for low-carbon (< 0.05%) steels in caustic solutions, whereas higher carbon content alloys cracked intergranularly. Wilson and Aspen showed that resistance to cracking was not decreased by sensitisation heat treatments. Type 316 stainless steel has been shown to be more susceptible to cracking in caustic than type 304. ... 

L. Gas-oil hydrodesulfurization unit. Elbow cracked intergranularly and decarburized at fusion line between weld metal and heat-affected zone no postweld heat treatment. 

NaCl solution at 105 °C, which compares to about 35 MN/m3/2 for Kiscc for the less-severely sensitized alloy. The air-cooled alloy exhibited transgranular stress corrosion and the severely-sensitized material cracked intergranularly as shown in Fig. 7.108. 

Crevice corrosion Stress corrosion cracking Intergranular corrosion Selective leaching Erosion corrosion Flydrogen damage Dilon [12] grouped corrosion into two categories. 

Eddy currents can be used to detect pits, cracks, intergranular attacks and selective corrosion. 

Evaluation of corrosive effects arising during the exposure can be evaluated by visual inspection, metaUographic examination of cross section under microscope with respect to pits, cracks, intergranular attack, and dezincification (of brass) and determination of loss in heat transmission capacity, a method which has direct relation to the function of automotive radiators. 

Classification by the morphology of the attack The attack may be homogeneous but the attack may also be rather localized (pitting corrosion, stress corrosion cracking, intergranular corrosion, etc.), or the material may remain virtually intact but the interface to a coating may be destroyed (cathodic delamination). 

The usual failure mode of chloride SCC in austenitic stainless steels is transgrannlar, highly branched cracking. Intergranular cracking is sometimes associated with transgrannlar cracking, but this is not common. If it occurs, it is usually because of a sensitized micro-structure. 

The appearance of stress corrosion cracking may be either intergranular or transgranular in nature. 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

The orientation of the cracks reveals that cyclic bending stresses or cyclic axial stresses were active. The intensification of these stresses at pits and intergranular corrosion sites produced the cracks observed. 

Microstructural examinations revealed intergranular, sparsely branched cracks originating on the external surface. Some cracks initiated as transgranular fissures. 

Intergranular corrosion-fatigue cracks in copper may he difficult to differentiate from stress-corrosion cracking. The longitudinal orientation of the cracks revealed that the cyclic stresses were induced by fluctuations in internal pressure. 

Fig. 2-17 Relation between the time to failure by intergranular stress corrosion cracking and potential for tensile specimens of soft iron (a) boiling 55% Ca(N03)2 solution, 5 = 0.65 R a = 0.90 R (b) 33% NaOH, a = 300 N mm, at various temperatures.

Some manufacturers have experienced die above mentioned Ni3S2 scale formation phenomenon under certain gas conditions, which led to die failure of a rotating blade. One such experience involved a fracture dial was distinctly intergranular with evidence of secondary intergranular cracks or grain separation across die fracture. Intergranular facets of die fracture were sharp and distinct with little evidence of any ductile mode. The fracture appeared to have occurred in a brittle intergranular mode. 

Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

When an alloy fails by a distinct crack, you might suspect stress-conosion cracking as the cause. Cracking will occur when there is a combination of corrosion and stress (either externally applied or internally applied by residual stress). It m.ay be either intergranular or trans-granular, depending on the alloy and the type of corrosion. 

Some of the most obvious examples of problems with gas and materials are frequently found in refining or petrochemical applications. One is the presence of hydrogen sulfide. Austenitic stainless steel, normally a premium material, cannot be used if chlorides are present due to intergranular corrosion and subsequent cracking problems. The material choice is influenced by hardness limitations as well as operating stresses that may limit certain perfonnance parameters. 

The importance of occluded cells cannot be overemphasised, and Brown considers that pitting, crevice corrosion, intergranular attack, filiform corrosion and hydrogen cracking are characterised by local acidification due to hydrolysis of metal ions, and that this phenomenon is of major significance in the overall mechanism. 

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