Hydrogen blistering cracking

Ultrasonic techniques. Wall thickness can be measured to monitor the progress of general corrosion, cracks can be detected and hydrogen blisters identified. Certain construction materials such as cast iron cannot be examined by ultrasound. Skilled operators and specialist equipment is required. Plant can be examined in situ except when it is above 80°C. 

Most refiners employ continuous water wash as the principal mei of controlling corrosion and hydrogen blistering. The best sourc water is either steam condensate or well-stripped water from a water stripper. A number of refiners use ammonium polysulfat neutralize hydrogen cyanide and to control hydrogen stress crack... 

Normally, a layer of FeS scale, produced in the first reaction, protects the interior of the pipe or vessel. However, in the second reaction, cyanide removes the FeS protective scale exposing more free iron to react with H2S and releasing more hydrogen. Without protection, this cycle continues until blistering, cracking, and eventual total corrosion of equipment occurs. 

Figure 6.64 Hydrogen-induced cracking (a) centreline cracks and (b) blister crack1...

Figure 1.21 Hydrogen-induced o acking (a) centerline cracks and (b) blister crack. (Figure originally published in Reference 104. Reproduced with permission of the Canadian Institute of Mining, MetaUurgy and Petroleum, www.cim.org.)...

Blisters are a form of HIC in which the buildup of hydrogen gas pressure at the initiated cracks or preexisting laminations results in localized deformation and bulging of the steel to the closest surface. High hydrogen pressures occur in blisters. Blisters occur when the hydrogen-induced crack is unable to propagate and is unable to link up with HIC on adjacent planes in the steel. 

Fig. 8.14 Hydrogen blister (a and b) imitatingfrom the wallofthe blister during charging at ( = 50 mA/ cm and (c) cracks [72].

Fig. 8.15 Hydrogen-induced cracking initiate along the wall of a hydrogen blister ABCD [72].

Fig. 9.52 Hydrogen blistering and microcracks in the specimen precharged with /=50mA/cm for 30 h (a) unbroken blisterings (b) broken blistering with cracking, AFD and BEC are the fracture surface of the blistering cracks and (c) arrest lines EF, GH, and IJ on the fracture surface of the blistering crack [188].

Blistering/cracking due to gas evolution (e.g., through hydrogen sulfide produced by SRB)... 

HIC is also referred to as stepwise cracking. The cracks propagate and interconnect hydrogen blisters to form a step pattern. Similar to sulfide stress corrosion, HIC does not involve substantial metal ranoval but can cause failure of the component through internal crack propagation. 

In mild steels, blisters, cracks, and terraced fractures caused by slag and sulfide lines can occur under sufficiently high hydrogen uptake and static tensile stress below the yield point at low temperature (room temperature). Residual stress is often sufficient te generate local plastic deformation and initiate cracking. 

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