Aluminium 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. 

It also follows that if the solution is stirred the rate of arrival of oxygen at the cathode will be increased. This will result in a corresponding increase in the rate of bimetallic corrosion as is shown in Fig. 1.63 for the aluminium-mild steel couple in stirred 1 - On NaCl solution . The increase in galvanic corrosion rate will be in the inverse relation to the slope of the anodic polarisation curve of the more negative metal, provided that the cathodic reaction is not totally diffusion controlled. 

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. 

The mechanical joining of aluminium alloys to steels using rivets and bolts, a combination which is difficult to avoid in the shipbuilding industry, represents a typical example of a situation where subsequent bimetallic corrosion could occur. Similarly, other examples of an ill-conceived choice of materials, which could normally be avoided, can be found in, for... 

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. 

Some of the CRP participants saw no corrosion of their aluminium alloy coupons, while others saw significant pitting. Pitting, crevice and galvanic corrosion were the main forms of corrosion observed. Crevice corrosion was not always accompanied by pitting of the aluminium surfaces within the crevice. Bimetallic corrosion of aluminium alloys coupled to stainless steel generally resulted in accelerated corrosion with pitting. 

Access of air and water will also affect the corrosion rate. Metal inserts in corrosive plastics are most actively attacked at the plastic/metal/air interfaces with certain metals, notably aluminium titaniumand stainless steel, crevice effects (oxygen shielding and entrapment of water) frequently accelerate attack. Acceleration of corrosion by bimetallic couples between carbon-fibre-reinforced plastics and metals presents a problem in the use of these composites. 

The normal thickness of coating is 5-25 jjim and has low frictional properties, making it an ideal surface on fasteners which reduces the tightening torque and prevents jamming. It also provides an effective barrier to prevent bimetallic reaction between steel fasteners and aluminium, e.g. where parts are fixed to an aluminium framework. Cadmium-plated surfaces can be easily soldered without the use of corrosive fluids. 

In 1996 the IAEA initiated a CRP on the corrosion of aluminium clad spent research reactor fuels to help evaluate the state of the spent fuel assemblies and to inform pool/basin operators regarding maintenance and housekeeping procedures to extend the lives of the FAs. The main activities of this programme are related to exposing racks of aluminium alloy specimens (coupons) in different spent fuel basins around the world. Five racks were suspended in the 1EA-R1 reactor pool and were subsequently withdrawn after different time spans to evaluate the extent of corrosion of the coupons as a function of alloy composition, crevices, bimetallic effects and water chemistry. During this period the pool water was monitored for pH, conductivity, chloride ion content and radiometry (Table 6.3). The IAEA CRP racks are denoted as racks 1,2A, 2B, 3A and 3B. 

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. 

Experiments carried out by Keir, van Rooyen and Pryor have clarified the variable behaviour of the Al-Zn couple in chloride solutions. Using high-purity aluminium and zinc electrodes of equal size coupled together in sodium chloride solution, it was found that zinc is initially anodic to aluminium but that within one day the polarity of the couple reverses and remains as such subsequently (Fig. 1.70). This reversal in polarity appears to be due to the accumulation of Zn in solution. Accordingly, with decrease in distance between the electrodes, and in solution-volume electrode-area ratio, the polarity reversal occurs much more rapidly. The accumulation of Zn in solution depresses the potential of the aluminium from an initial value of about -0-5 V to a final open-circuit value of about —1-0 V (vj. S.H.E.). The corrosion rates of both the aluminium and the zinc electrodes are greater than in the absence of bimetallic contact, but the corrosion of the aluminium is changed from the characteristic pitting, usually observed in nearly neutral chlorides, to a desirable mild uniform attack. The polarity reversal is not... 

Figure 1. Evans diagram for estimating the galvanic current density. The bimetallic electrode consists of a carbon fibre embedded in a 6061 aluminium alloy (metal matrix composites). Two galvanic regimes can be reached as function of the aeration of the corrosive media. This analysis does not take into account the spatial distribution of the carbon fibers in the metallic matrix (see Fig. 12). Reprinted with permission from/nrernahono/Mafen-alsReviews, 39 (1994) 245.Copyright 1994 Maney Publishing. ...

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