Bimetallic Corrosion of Iron

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 Fe +(aq)  

Iron(II) is only slowly oxidized further to iron(III), partly because Eh for oxygen reduction at pH 7 (-f-0.815 V) is only marginally more positive than that for the iron(II)/iron(III) couple 

The example just given assumed a constant pH of 7. Simultaneously with Fe +(aq) release at the anode, however, hydroxide ion is formed by reduction of oxygen at the cathode (reaction 16.5), increasing the local pH, and diffuses out to mix with Fe + in the bulk aqueous phase. The solubility product of Fe(OH)2 is about 8 x 10 , which means that concentrations of several micromoles Fe per liter could build up before Fe(OH)2 is precipitated. At the same time, increased pH will accelerate the oxidation 

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Figure 16.2 Bimetallic corrosion of iron. The immediate anode product is Fe2+(aq), which reacts with dissolved oxygen and OH- (from the cathode) in bulk solution to form iron(III) oxide/hydroxide which precipitates away from the anodic surface.

With reference to the discussion of the bimetallic corrosion of iron given in Section 16.1, confirm (a) that the solubility of iron(II) hydroxide is 5.8 /zmol L-1 if OH- is produced along with iron(II) according to reaction 16.5 and the reverse of reaction 16.6 and (b) that the timescale of the oxidation of iron(II) in solution at pH 7 is as given following Eq. 16.8. Hint Incorporate [O2] and pH into a rate constant ki for a first-order process, then calculate the half-period h/2 = (ln2)/fci. 

Bimetallic corrosion of nickel-iron alloys may be of significance in welding operations. Ni-45 Fe alloys are used as filler materials in the welding of cast irons but the favourable area relationship of weld metal to base plate... 

Figure 1.27 A mixed potential plot for the bimetallic couple of iron and zinc. The figure also explains the higher corrosion rate of iron than zinc in hydrochloric acid solution. Despite the more positive reduction potential of iron, the evolution of hydrogen on iron has a high exchange current density (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

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). 

Bimetallic cylinder n. In most modern extruders and injection machines, the barrel is lined, by centrifugal casting from the melt, with any of several white irons containing chromium and boron carbides and having hardnesses near Rockwell C65. Aster finish-grinding and polishing, the liner, about 1 mm thick, provides excellent resistance to wear or corrosion or both, depending on the formation. The best known trade name is XALOY . 

The clearance between the barrel and screw flights is typically 0.08 to 0.13 mm (0.003 to 0.005 in). To reduce barrel wear, barrels are nitrided or bimetallic liners are inserted into the barrel. Nitriding is the surface hardening of the barrel. This process initially produces higher hardness (Rc 70), but loses that advantage as the barrel wears. Nitriding also provides poor abrasion and only moderate corrosion resistance. In contrast, a bimetallic liner is a 1.5-mm (0.060-in)-thick sleeve that fits in the barrel. As shown in Table 5.5, liner materials depend on the polymer and its additives. Iron/boron materials are used as general-purpose liners, whereas nickel/cobalt liners... 

With hot water tanks, the bottom is more liable to attack than the sides because deposits on the bottom (such as iron filings from holes drilled elsewhere in the tanks) produce bimetallic or differential aeration cells. Such cells also can form on the sides in some waters, especially near the waterline, however. If the water is made more aggressive (e.g., by the presence of copper salts, possibly dissolved from elsewhere in the water system), corrosion cells will form more readily. The presence in water used in domestic... 

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. 

Under the reaction conditions, we don t know whether the cationic iron, added as a selectivity promoter, will be reduced or not to the metallic state in the presence of platinum particles and hydrogen pressure. This observation led us to experiment the properties of a bimetallic Pt-Fe under the form of an alloy if possible (ref. 3). Charcoal was chosen as a support Ssecause of its good resistance to corrosion. 

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