Crevice corrosion elimination

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Bonding can eliminate crevices that often lead to crevice corrosion in riveted joints. 

Sealers play an important role in automobile corrosion protection by preventing water and contaminants from entering crevices and joints. They are usually rubber, vinyl, asphaltic, or hot melt or wax compounds [24,25]. The choice depends on the specific application such as at door hem flanges, brackets, fasteners, and body joints. Structural adhesives are also used to bond assemblies such as doors, deckUds, and hoods, thereby sealing crevices and eliminating welds. 

The chromium-nickel-molybdenum steels 1.4571 (SAE 316 Ti, X6CrNiMoTil7-12-2) and particularly 1.4435 (SAE 316 L, X2CrNiMol8-14-3) are resistant to crevice corrosion, which can occur under the operating conditions in a digester if steels with lower alloy contents are used. The importance of the correct constructive design as well as the elimination of welding tarnishes by pickling is discussed [58]. 

The material responsible for the crevice need not be metallic. Wood, plastics, rubber, glass, concrete, asbestos, wax, and living organisms have all been reported to cause crevice corrosion. Once the attack begins within the crevice, its progress is very rapid and it is frequently more intense in chloride environments. For this reason, the stainless steels containing molybdenum are often used to minimize the problem. However, the best solution to crevice corrosion is a design that eliminates crevices. 

A high resistance to pitting and crevice corrosion is also exhibited by Zeron 100. It has a PREN of 48.2. Intergranular corrosion is not a problem because the alloy is produced to a low carbon specification and water-quenched from solution annealing, which prevents the formation of any harmful precipitates and eliminates the risk of intergranular corrosion. 

Crevice corrosion occurs in restricted transport zones where the access of reactants and the elimination of corrosion products can be very slow. The detrimental effect of a crevice is related to a very small volume of solution in contact with very large metal surfaces. The restricted transport path between crevice and bulk solution is responsible not only for low diffusion exchanges but also for the buildup of a potential difference between the free surfeces and the crevice that becomes more anodic (ohmic drop effect). 

Eliminate or minimize crevice corrosion at the design stage. 

Change of design. Avoid sharp corners, as they act as stress raisers. Also eliminate sites for crevice corrosion. 

Crevices are gaps formed on joining two structural members. The formation of a crevice is followed by formation of a differential aeration cell. The oxygen inside the crevice becomes depleted after a length of time and the crevice becomes anodic, Al +- -3H20 3H+-t-Al(0H)3. The precipitation of AI(OH)3 leads to decreased pH and acidification (H" "). Crevice corrosion of aluminum is shovra in Fig. 9.16. If aluminum is coated, a cathodic reduction-reaction would not occur around the crevice mouth and the cathodic reduction reaction would then take place slowly inside the crevice, 2H+-f2e->-H21- Crevices must be eliminated by sealing or welding. 

Preveiilian of Crevice Corrosion. Many fiictors noted in Table 2 must be considefed if crevice corrosion is to be eliminated ortnuumized. Whoever possible, oevices should be diminated at the design stage-When unavoiflable, they should be kept as open and shallow as possible to allow continued entry of the bulk envitonmenL Cleanliness is an important factor, particularly when conditions promote deposition on metal surfaces. 

Stress-corrosion cracking of steel was first encountered in a practical way in riveted steam boilers. Stresses at rivets always exceed the elastic Unfit, and boiler waters are normally treated with alkalies to minimize corrosion. Crevices between rivets and boiler plate allow boiler water to concentrate, until the concentration of alkali suffices to induce S.C.C., sometimes accompanied by explosion of the boiler. Because alkalies were recognized as one of the causes, failures of this kind were first called caustic embrittlement. With the advent of welded boilers and with improved boiler-water treatment, S.C.C. of boilers has become less common. Its occurrence has not been eliminated entirely, however, because significant stresses, for example, may be established at welded sections of boilers or in tanks used for storing concentrated alkalies. 

The multiple crevice assembly (MCA) consists of two "serrated washers made of TFE-fluorocarbon bolted together through a hole drilled in the specimen, as illustrated in Fig. 2(b). The bolts, made of a highly corrosion resistant alloy, are electrically isolated from the specimen. The MCA eliminates the crevices created where the rubber band contacts the specimen and facilitates the measurement of the crevice area to boldly exposed area ratio. The advantage of the MCA is that the crevice geometry can be made very severe by... 

Design improvements have played a major role in the development of corrosion resistant vehicles. Areas of entrapment, where salt, poultice and other materials could build up and provide a corrosive environment, have been eliminated to enhance corrosion resistance in vehicles. For example, wheel wells are now designed to have a sloping configuration so that corrosives can be washed or will drain out. Drain holes have been incorporated in areas so that corrosive liquid cannot accumulate. Sealers are used to eliminate crevices and joints where corrosives can build up. And, attempts have been made to reduce galvanic corrosion by avoiding dissimikkr metal contact. 

A better design to resist localized corrosion and elimination of possible side of crevices. 

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