Titanium 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. 

Current on cathode dissolution, particularly due to combination of electroreduction of protective surface films and active generation of hydrogen. This may be especially marked in the case of titanium, due to facile hydride formation. The result may be pitting, embrittlement or exfoliation of the electrode surface. Electrode corrosion may significantly alter electrocatalytic properties (resulting in a reduced selectivity) contaminate electrolytes (and, hence, products) block cells, dividers or manifolds cause electrical shorting, or provide parasitic redox couples (e.g. Fe /Fe ) which decrease current efficiency or promote deposit or bimetallic corrosion efficiency elsewhere. 

Contact with steel can accelerate attack on Al, but in some natural waters and other special cases, Al can be protected at the expense of ferrous materials, particularly when the Al is "passive." Titanium appears to behave in a similar manner to steel. Stainless steel in contact with Al may increase attack on Al, notably in seawater or marine atmosphere, but the high electrical resistance of the two surface oxide films minimizes bimetallic effects in less aggressive environments. Where Al is coupled to copper, or exposed to metallic copper contamination (such as in water systems), corrosion of the Al is very rapid. This is because Cu is particularly efficient at supporting cathodic reactions (e.g., oxygen and water reduction). Limiting cathodic currents measured for pure copper are reported to be in the vicinity of 1.5 mA cm , whereas limiting currents on pure Al are three orders of magnitude lower (0.5-1 pA cm- ) [52]. 

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