Galvanic coupling test specimen

Fig. 19.30 Plate and Fastening type galvanic couple test specimen...

Some investigatorshave advocated a type of accelerated test in which the specimens are coupled in turn to a noble metal such as platinum in the corrosive environment and the currents generated in these galvanic couples are used as a measure of the relative corrosion resistance of the metals studied. This method has the defects of other electrolytic means of stimulating anodic corrosion, and, in addition, there is a further distortion of the normal corrosion reactions and processes by reason of the differences between the cathodic polarisation characteristics of the noble metal used as an artificial cathode and those of the cathodic surfaces of the metal in question when it is corroding normally. 

Corrosion of the bolts and frames must be considered for those geometries using self loading, such as by bolts in b, c, g, and i or by frames in a, d, and f. Corrosion products from the supports can affect the corrosion of the test specimens, and particularly, galvanic coupling between dissimilar metals could shift the corrosion potential of the test specimen from the value that it would have under uncoupled conditions. 

The RSL is an economical test, as only one specimen may be required to obtain a measure of the threshold value, as compared to the dozen or so normally required to conduct sustained load run-out tests as with the cantilever beam type (ASTM E 1681). The test method takes advantage of using small specimens to measure the fracture toughness [47] and utilizes potentiostatic test methods to simulate conditions of galvanic coupling on the specimen [48]. 

In the specimens with the larger siuface area, a hole is drilled. The galvanic couples and the imcoupled reference specimens are moimted in duplicate on wooden planks by means of an insulated bolt passing through the hole. PTFE washers insulate the test specimens fix>m the plank and the bolt head. 

Apart from the selection of the crevice former device, it is important to have in mind that the severity of the test may depend on the availability of cathodic current. Thus, it is important to use specimens witii large free surface area, particularly if the environment has high electric conductivity that allows long-distance galvanic coupling (seawater, for example). 

An obvious method for studying galvanic corrosion either with or without supplementary electrical measurements is to compare the extent of corrosion of coupled and uncoupled specimens exposed under identical conditions. Such measurements may use the same techniques for estimating corrosion damage, such as mass-loss determinations, as have been described in connection with ordinary corrosion tests. 

A convenient method of carrying out such a galvanic test in the laboratory has been described by Wesley in which the vertical circular-path machine is used. Each assembly includes two pairs of dissimilar metals—one pair coupled galvanically while the other pair is left uncoupled in order to determine the normal corrosion rates under the same environmental conditions. The type of motion provided (specimens moving in a vertical circular path) enables electrical connections to be made without mercury cup or commutator and the leads can be connected to a calibrated resistance for current measurements attached to the specimen carrier. 

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