Corrosion bimetallic

The various forms of corrosion can be classified by their various causes. These are uniform corrosion attack (UC), bimetallic corrosion (BC), crevice corrosion (CC), pitting corrosion (PC), grain boundary corrosion (GBC), layer corrosion (LC), stress corrosion cracking (SCC), cavitation corrosion (CC), and hydrogen embrittlement (HE). 

Such corrosion is usually easy to detect and rectify. The slow corrosion of a metal in aqueous acidic solution is an example of such corrosion. Impurities in a metal can result in local cells which, in the presence of electrolyte, will show corrosive action. 

This type of corrosion, also called galvanic corrosion, is characterized by the rapid dissolution of a more reactive metal in contact with a less reactive more noble metal. For example, galvanized steel (Zn-Fe) in contact wifli copper (Cu) pipe is a conmum household error. A noncmiducting plastic 

Spacer would reduce the corrosion rate in the pipe. The rate of corrosion is partially determined by the difference in the standard cell potentials of the two metals in contact (see Table 9.4). The relative potential of metals in seawater is given in Table 10.1 and represents the driving force of the corrosion which includes the current, or more precisely, the current density, that is, A/cm.  

An electrochemical cell is formed and the anodic metal dissolves. This can be corrected by applying a counter current or voltage or by introducing a more reactive, sacrificial anode, for example, adding magnesium alloy to the above Zn-Fe-Cu system, a procedure commonly used for hot-water pipes in renovated buildings. 

Consider a zinc strip immersed in water. At equilibrium, a small number of Zn + ions will pass into solution per unit time, leaving twice as many electrons behind, while an equal number of Zn + ions already in the water will be redeposited as elemental zinc (reaction 16.1). The rate of this process, in terms of the electrons transferred per unit surface area of the metal, is the exchange current density io for equilibrium 16.1, as explained in Section 15.4  

For a strip of copper immersed in water, a similar equilibrium will be set up  

The driving force for copper deposition (reaction 16.2) is much greater than for zinc precipitation (reaction 16.1), as the corresponding standard electrode potentials of +0.340 and —0.763 V indicate. Consequently, if the copper and zinc strips are placed in the same aqueous medium and are in electrical contact with each other as in Fig. 16.1, reaction 16.2 will proceed in the direction left to right by driving reaction 16.1 in the sense right to left. The zinc will therefore dissolve while the copper ions in the aqueous phase are redeposited. 

This situation cannot persist for long the concentration of copper(II) ions in water will initially be extremely small, unless some other source is also involved, and will quickly be depleted. The important point is that, as soon as electrical contact is made, the zinc becomes an anodic electrode, and the copper a cathode. If another cathodic reaction besides reaction 16.2 is possible, however, then dissolution (i.e., corrosion) of the zinc will continue, while the copper will serve merely as an electrically conducting surface to deliver electrons for the alternative cathodic reaction. In pure water, the obvious alternative reaction is hydrogen evolution (reaction 16.3) for which Eh is -0.414 V at pH 7  

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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. 

Coatings of more noble metals than the substrate metal (e.g., Cu on Fe) are only protective when there are no pores. In other cases severe local corrosion occurs due to cell formation (bimetallic corrosion). Cathodic protection is theoretically possible. This protection combination is not very efficient since the coating usually consumes more protection current than the uncoated steel. 

If the individual materials are separated by insulating couplings but connected to a protection system, the connections must be made through diodes to avoid bimetallic corrosion when the protection system is shut down (see Fig. 11-6). Furthermore, the different protection currents should be adjusted via variable resistors. 

Almost all common metals and structural steels are liable to corrode in seawater. Regulations have to be followed in the proper choice of materials [16], In addition, there is a greater risk of corrosion in mixed constructions consisting of different metals on account of the good conductivity of seawater. The electrochemical series in seawater (see Table 2-4), the surface area rule [Eq. (2-44)] and the geometrical arrangement of the structural components serve to assess the possibility of bimetallic corrosion (see Section 2.2.4.2 and Ref. 17). Moreover the polarization resistances have considerable influence [see Eq. (2-43)]. The standards on bimetallic corrosion provide a survey [16,17]. 

Ten percent of the anode mass is calculated for aluminum ships. The anode supports must also be of aluminum in order to allow them to be welded and to avoid bimetallic corrosion. 

In 1987 at the Weira River, four Kaplan turbines of 2.65 m diameter in two power stations were cathodically protected. The turbines were of mixed construction with high-alloy CrNi steels and nonalloyed ferrous materials with tar-EP coating. Considerable corrosion damage occurred prior to the introduction of cathodic protection, which was attributed to bimetallic corrosion and the river s high salt content of c(CT) = 0.4 to 20 g L... 

The classification given in Table 1.2 is based on the various forms that corrosion may take, but the terminology used in describing corrosion phenomena frequently places emphasis on the environment or cause of attack rather than the form of attack. Thus the broad classification of corrosion reactions into wet or dry is now generally accepted, and the nature of the process is frequently made more specific by the use of an adjective that indicates type or environment, e.g. concentration—cell corrosion, crevice corrosion, bimetallic corrosion and atmospheric corrosion. 

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. 

Fig. 1.62 Potential/current curves for a metal polarised (a) cathodically and (b) anodically. The horizontal intercepts xy, x y, x"y" with AB and CB represent the local cell currents respectively, and yz, y z, y z" the externally applied currents (cathodic and anodic). In bimetallic corrosion yz, y z, etc. will be the galvanic current /gjjv, flowing from to (see...

Most cases of practical bimetallic corrosion in solutions occur under conditions when the solution contains dissolved oxygen. Accordingly, the primary cathodic reaction is the reduction of dissolved oxygen... 

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. 

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. 

Published work relating to bimetallic corrosion in sodium chloride solution is reported in Referencesin sea-water in Refer-ences " in fresh waters in Referencesin mineral acids in References in water/glycol mixtures in Reference ... 

There are many special factors controlling atmospheric bimetallic corrosion that entitle it to separate treatment. The electrolyte in atmospheric corrosion consists of a thin condensed film of moisture containing any soluble contaminants in the atmosphere such as acid fumes from industrial atmospheres and chlorides from marine atmospheres. This type of electrolyte has two characteristics which are summarised in a paper by Rosenfel d . 

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