Copper bimetallic corrosion

The main incoming mate coniacts are generally made ot copper or brass and are cither hotted or damped on die vertical bus. Since tlic bus is generally of aluminium. Ihe coniacts may form a bimelaltic join wilh Ihe busbars and cause corrosion and pilling of ibe melal. This may result in a failure of the joinl in due course. To mininii/e rnelal oxidation and bimetallic corrosion, the conlacls must be silver plated. 

Most simple inorganic salt solutions cause virtually no attack on aluminium-base alloys, unless they possess the qualities required for pitting corrosion, which have been considered previously, or hydrolyse in solution to give acid or alkaline reactions, as do, for example, aluminium, ferric and zinc chlorides. With salts of heavy metals —notably copper, silver, and gold —the heavy metal deposits on to the aluminium, where it subsequently causes serious bimetallic corrosion. 

Additional metal layers can create bimetallic corrosion cells if discontinuities appear in service. The layer of copper beneath cadmium plate on aluminium (using a zincate plus cuprocyanide deposit technique) can cause corrosion troubles. When aluminium is plated with nickel and chromium, rapid service corrosion in the zinc layer causes exfoliation. 

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. 

Figure 16.1 The basis of the galvanic (bimetallic) corrosion cell. Zinc, which is more readily oxidized than copper, becomes the anode.

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. 

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. 

There is an apparent exception to the necessity for the two metals to be in electrical contact for bimetallic corrosion to occur namely, when a noble metal corrodes slightly and dissolves in water that subsequently flows over a less noble metal, the more noble metal may deposit on the less noble metal, forming a true bimetallic contact. For example, copper can dissolve very slightly in some natural waters and may then deposit on zinc. Bimetallic corrosion may also be experienced when two dissimilar metals, not in direct contact, are nevertheless connected electrically. 

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. 

After nearly 20 years, the official responsible for maintenance at the War Museum reported that the structure was in perfect condition. Just covered by dust. No need for maintenance in the foreseeable future is envisaged. It should be noted that the roof covering is copper but, because the contacts between the galvanized steel and the copper are in the dry and there is no water present to provide an electrolytic path Joining the two metals, bimetallic corrosion does not occur (see British Standard PD 6484). 

Aluminum and its alloys are also suitable for use in marine atmospheres. Care must be taken in the design since under these conditions the aluminum is susceptible to bimetallic corrosion when in contact with copper or carbon steel. If the chloride concentration is high, the aluminum may be susceptible... 

The greater the difference of potential between the two metals, the greater is the magnitude of bimetallic corrosion. Figure 8.4 shows a valve from a condensate pipe. The cast iron valve was incorporated in AISI 304 stainless steel condensate pipe of a copper heat exchanger. The difference of potential between copper, steel and cast iron caused bimetallic corrosion. 

Galvanic corrosion or bimetallic corrosion is important to consider since most of the structural industrial metals and even the metallic phases in the microstructure alloys create galvanic cells between them and/or the a Mg anodic phase. However, these secondary particles which are noble to the Mg matrix, can in certain circumstances enrich the corrosion product or the passive layer, leading to a decrease or a control of the corrosion rate. Severe corrosion may occur in neutral solutions of salts of heavy metals, such as copper, iron and nickel. The heavy metal, the heavy metal basic salts or both plate out to form active cathodes on the anodic magnesium surface. Small amounts of dissolved salts of alkali or alkaline-earth metal (chlorides, bromides, iodides and sulfates) in water will break the protective film locally and usually lead to pitting (Froats et al., 1987 Shaw and Wolfe, 2005). 

Clad Tube Sheets Usually tube sheets and other exchanger parts are of a solid metal. Clad or bimetallic tube sheets are usecito reduce costs or because no single metal is satisfactory for the corrosive conditions. The alloy material (e.g., stainless steel, Monel) is generally bonded or clad to a carbon steel backing material. In fixed-tube-sheet construction a copper-alloy-clad tube sheet can be welded to a steel shell, while most copper-alloy tube sheets cannot be welded to steel in a manner acceptable to ASME Code authorities. 

In the corrosion of iron by neutral aerated water, in a bimetallic couple with, for example, copper or platinum as the cathode (as in Fig. 16.2), the initial product at the anode is Fe2+(aq) ... 

In bimetallic catalysts prepared by catalytic reduction of copper by hydrogen, copper is deposited as three-dimensional agglomerates which are located, at low copper loadings, on the edges, corners, and rims of the parent metallic particles. The mechanism of deposition can be transferred from that proposed in corrosion and involving a local electrochemical cell ... 

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