Copper alloys bimetallic corrosion

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. 

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. 

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. 

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. 

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... 

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). 

Corrosion caused by the connection of two or more different metals also occurs underground. This electrochemical corrosion cell is commonly referred to as bimetallic or galvanic corrosion. Typical examples include brass or bronze valves connected to steel or cast iron pipes and stainless steel fasteners coimected to steel or cast iron. These couplings of dissimilar metals will locally affect the corrosion rate. Aluminum can be severely corroded if directly connected to most other engineering alloys, such as steel, iron, copper, or stainless steel—dielectric isolation must be used. 

When aluminium is alloyed with appreciable amounts of copper it becomes more noble and when alloyed with appreciable amounts of zinc or magnesium it becomes less noble. These remarks apply to bimetallic contacts and not to inherent corrosion resistance. Such effects are mainly of interest when the aluminium alloys are connected with each other. 

---