Corrosion testing bimetallic

Determination of bimetallic corrosion in outdoor exposure tests , BS 6682 (1986)... 

ISO 7441-84, Corrosion of Metals and Alloys Determination of Bimetallic Corrosion in Outdoor Exposure Tests, Geneva, Switzerland, 1984. 

Problems associated with size can be assessed by the principles for scaling corrosion tests. A report has been published by the European F eration of Corrosion Working Party on Thysicochemical methods of corrosion testing - fundamentals and application"[5] which includes information on specimen size for testing uniform, bimetallic and pitting corrosion and stress-assisted environmental cracking. 

Zinc compounds became the most favored mud scavengers in the 1970s after copper compounds were abandoned because of bimetallic corrosion. In the early 1970s, many zinc compounds and mixtures were tested. 

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. 

TABLE 5—Test methods for evaluating galvanic (bimetallic) corrosion. 

Corrosion of metals and aUo5 —Determination of bimetallic corrosion in outdoor exposure corrosion tests Corrosion of metals and alloys—Stress corrosion testing—Part 1 General guidance on testing procedures... 

Additional corrosion racks were provided to the CRP participants in March 1998 at the second RCM. Most of these racks had been immersed in the individual basins by mid-1998. The surveillance racks were monitored visually for corrosion, and when corrosion was detected, the coupons were removed from the water and analysed. As found in earlier testing, water quality proved to be the key to good performance. Crevice corrosion was seen between most of the crevice couples as expected, because the pH was lower by 0.5-1.0 unit in the crevice. In poorer quality water, further corrosion was observed, espedally between bimetallic crevice coupons, to the extent that coupons had to be forced apart. The results of the individual participating laboratories were presented at the third and final RCM, held in Bangkok, Thailand, in October 2000. 

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