Joints bimetallic

Galvanic corrosion of dissimilar metals can be minimized by controlling humidity near such bimetallic connections. In general, continuously dry bimetallic joints do not corrode. 

Use sealing (encapsulating or enveloping type with shrinkable plastic) on bimetallic joints if geometrical arrangements prohibit access to such joints for replacement. 

Bimetallic corrosion in atmospheres is confined to the area of the less noble metal in the vicinity of the bimetallic joint, owing to the high electrolytic resistance of the condensed electrolyte film. Electrolytic resistance considerations limit the effective anodic and cathodic areas to approximately equal size and therefore prevent alleviation of atmospheric galvanic corrosion through strict application of the catchment area principle. 

Additional corrosion may occur where the supply of oxygen at interfaces between joined components is limited. This is not bimetallic corrosion, but its effect at bimetallic joints can be as detrimental as true bimetallic corrosion. However, no crevice corrosion occurred in couples of zinc-aluminum alloys and polyethylene in 10-year atmosphere tests (Noranda, personal communication) nor around nylon bolt heads. Jointing compounds are useful in preventing crevice corrosion as well as bimetallic corrosion. Also, stressed parts of the surface tend to be anodic to unstressed parts, but this effect is not usually of practical significance with zinc and zinc-coated steel. 

Figure 9.19 Exclusion of the environment from bimetallic joints using sealing encapsulating, or enveloping with shrinkable plastic. (Reproduced with permission from Wesfarmers Chemicals, Energy Fertilisers.)...

The hot leg piping joins the reactor outlet to the turbine inlet. Both of these component interfaces would have required significant development, including potentially the development of bimetallic joints and methods for Insulation containment. At the reactor outlet there was the possibility of a bimetallic joint between an inner refractory metal alloy reactor vessel and the Ni-base superalloy piping liner, if a Ni-base superalloy was chosen as the liner material. Another dissimilar joint would have occurred between the cast Ni-base superalloy turbine inlet and the wrought Ni-base superalloy outer piping. Finally, should a refractory metal alloy liner have been selected, the end caps for insulation containment would have required bimetallic Joints. 

Advice on the reduction of bimetallic corrosion at welded and brazed joints can be found in Reference. The use of replaceable wastage pieces to take up the bimetallic corrosion in various systems is proposed in References... 

Transition joints are used to join dissimilar metals where flanged, screwed, or threaded connections are not practical. They are used when fusion welding of two dissimilar metals forms interfaces that are deficient in mechanical strength and the ability to keep the system leak-tight. Transition joints consist of a bimetallic composite, a stainless steel, and a particular kind of aluminum bonded together by some proprietary process. Some of the types in use throughout the cryogenic industry are friction- or inertia-welded bond, roll-bonded joint, explosion-bonded joint, and braze-bonded joint. 

While all of these challenges point to the need for new methodological developments, it is also important to remember that quantitative modeling of complex materials such as bimetallic catalysts must continually make contact with experimental reality. Innovative ways to combine theoretical and experimental observations that can jointly speed up the search for new catalysts are in the long run likely to make the largest impacts in the field. 

Parallel to the development of the chemistry of dinuclear transition metal complexes with metal-metal bonds, studies related to bimetallic complexes, hetero- and homonuclear, without metal-metal bonds are appearing in the literature at ever increasing frequency (10-13). In these complexes, two or more metals are separated by a common ligand which acts as a bridge between them. Two metal centers acting in a joint fashion could enhance the activation of an organic substrate considerably and in... 

BIMETALLIC COUPLE - A joint or union of two dissimilar metals. 

Silver like gold was used in coins but the cost of silver compared to that of gold was gradually decreasing. In 1874 the cost of one pound of gold was equal to that of 15.5 pounds of silver but after the discovery of silver deposits in Australia this ratio fell to 1 46. In England bimetallism, i.e. the use of gold and silver jointly as a monetary standard, was discontinued in 1816. Later other countries followed this example. 

Internal stress can also be produced with Hot melt adhesives where the adhesive shrinks more on cooling than the adherend. In this case, there may also be a difference in thermal expansion coefficient between the two adherends, and this difference causes additional internal stress in the joint. Such internal stress is seen most dramatically in bimetallic strips used in thermostats, where the internal stress causes large bending of the joined materials. Solvent-based adhesives also produce internal stress when shrinkage occurs during solvent evaporation. 

The scales of galvanic potentials are meaningless unless the amount of current flowing between dissimilar metals is known. The designer should obtain accurate information on the material composition of aU items. Galvanic corrosion of dissimilar metals can be avoided by preventing the extended presence of hrrmidity (e.g. condensation) at the joints. Bimetallic connections in the proximity of fumes from combustion generators should be avoided. 

Experimental work [118-125], especially that using photoelastic techniques, has established the presence of residual stresses in joints but the results have often not been quantitative. For the simpler case of polymeric films coated onto metallic substrates, photoelastic techniques and a method based upon the bimetallic strip principle have often been employed [126-131]. Using the latter method, Danneberg [130] showed that for a wide range of epoxy-based coatings on an aluminium substrate thermal contraction was a major cause of internal stress and that the stresses generated were of the order of 0.08... 

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