Bimetallic corrosion effects

Table 4.25 Bimetallic corrosion effects of nickel and nickel alloys (General guidance only other factors, including relative surface areas, often exert an important...

Johnson, K. E., and Abbott, J. S. (1975). Bimetallic Corrosion Effects on Mild Steel in Natural Environments. Report CEL/CC/5/75. Corporate Engineering Laboratory, British Steel Corporation, London, 12 pp. 

Finally, it is important to point out that although in localised corrosion the anodic and cathodic areas are physically distinguishable, it does not follow that the total geometrical areas available are actually involved in the charge transfer process. Thus in the corrosion of two dissimilar metals in contact (bimetallic corrosion) the metal of more positive potential (the predominantly cathodic area of the bimetallic couple) may have a very much larger area than that of the predominantly anodic metal, but only the area adjacent to the anode may be effective as a cathode. In fact in a solution of high resistivity the effective areas of both metals will not extend appreciably from the interface of contact. Thus the effective areas of the anodic and cathodic sites may be much smaller than their geometrical areas. 

For further discussion of cathode to anode area ratio effects see References " and also refer to the section entitled Distribution of Bimetallic Corrosion in Real Systems, p. 1.238. 

Protective measures against bimetallic corrosion should ideally start before the particular installation or equipment is built . Reference should be made to tables showing compatibility of metals, alloys and non-metallic materials (5 Table 1.25) and to the literature. However, it must be emphasised that the environment obviously plays a most important role in bimetallic corrosion, and that there are a number of situations in which apparently incompatible materials in contact can be used without adverse effects. 

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. 

Both metals are applied to copper-base alloys, stainless steels and titanium to stop bimetallic corrosion at contacts between these metals and aluminium and magnesium alloys, and their application to non-stainless steel can serve this purpose as well as protecting the steel. In spite of their different potentials, zinc and cadmium appear to be equally effective for this purpose, even for contacts with magnesium alloys Choice between the two metals will therefore be made on the other grounds previously discussed. 

Bimetallic corrosion and other forms of corrosion continued to cause service failures. In 1962, a report was sent to the British Ministry of Defense stating that a copper alloy end plate had fallen off a seawater evaporator in a submarine because the steel bolts with which it was secured had effectively dissolved through galvanic action. In 1982, the nose wheels failed on two Royal Navy Sea Harriers that had returned from the Falklands War. Studies showed that the galvanic action was responsible for the corrosion that occurred between the magnesium wheel alloy and its stainless steel bearing. 

Bimetallic corrosion is more severe under immersed conditions than in the atmosphere. In the latter, attack occurs only when the bimetallic contact is wet this depends on many factors, such as the presence or retention of moisture in crevices, the effectiveness of drainage, and the speed of evaporation. The relative size of the areas of the metals that remain wet in the vicinity... 

When considering zinc-aluminum alloys, the surface oxide film normally present is likely to reduce any corrosion current. The risk of bimetallic corrosion is small in atmospheric exposure trials by Noranda have been in progress since 1984 on ZA alloys coupled to other common metals. No visual effects were noted at the 5-year examination (Barmhurst and Belisle, 1992). A zinc-25% aluminum-0.05% magnesium alloy coupled to other materials and exposed on the Noranda Research Center roof showed pitting attack on the zinc-based material (but only up to 0.38 mm deep in 10 years) when joined to copper, brass, or steel, but less when joined to stainless steel or lead and least when joined to aluminum. 

The special case of the bimetallic effect between a zinc coating and the substrate that it is protecting is discussed under hot water aqueous corrosion resistance as the normal bimetallic effect whereby zinc protects steel is reversed in some waters, usually at 60-90°C. Bimetallic corrosion of zinc occurs mainly when zinc or zinc-coated steel is protecting uncoated steel or other base metals such as copper. Many of the uses of zinc deliberately invoke this principle, but in other cases an unwanted effect arises as a result of constructional requirements, and avoidance of bimetallic corrosion is needed. 

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. 

As zinc is dissolving sacrificially to protect the steel, the rate of zinc corrosion will be affected by (a) the current passing per unit of area, which in turn is related to the conductivity of the media in which they are immersed, and (b) the relative areas of zinc and steel exposed (see later material on bimetallic corrosion) zinc alloys and zinc will have different rates of corrosion. Sugimoto and Goton (1989) have looked at the relative effect of the zinc-aluminum alloys for gap protection. 

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