Valve metals, corrosion

On the other hand, metals such as Ta, Nb, Ti, Zr, Al, etc. (the valve metals ) do not exhibit transpassive behaviour, and in appropriate electrolyte solutions film growth at high fields rather than corrosion and/or oxygen evolution is the predominant reaction thus aluminium can be anodised to 500 V or more in an ammonium borate buffer titanium can be anodised to about 400 V in formic acid and tantalum can be anodised to high voltages in most acids, including hydrochloric acid. 

Anodic oxidation of valve metals, particularly, aluminum, has attracted considerable attention because of its wide application in various fields of technology. Traditionally, aluminum is anodized in order to protect the metal against corrosion, to improve its abrasion and adsorption properties, etc.1 The more recent and rapidly growing applications of anodic aluminas in electronics are due to their excellent dielectric properties, perfect planarity, and good reproducibility in production. Finally, ways have recently been found to use the energy potential of aluminum oxidation for chemical power sources of the metal-air type2,3 and other electrochemical applications. 

Titanium as a carrier metal Titanium (or a similar metal such as tantalum, etc.) cannot work directly as anode because a semiconducting oxide layer inhibits any electron transport in anodic direction ( valve metal ). But coated with an electrocatalytic layer, for example, of platinum or of metal oxides (see below), it is an interesting carrier metal due to the excellent corrosion stability in aqueous media, caused by the self-healing passivation layer (e.g. stability against chlorine in the large scale industrial application of Dimension Stable Anodes DSA , see below). 

The isomerization processes created unusual corrosion problems. Aluminum chloride-bearing catalyst proved to be extremely corrosive to steel and other common structural metals. Corrosion resulted in leaks in the reactor, connecting piping, and heat exchangers downstream from the reactor and in failure of block and control valves to function properly. The amount of corrosion depended on the type of aluminum chloride catalyst used and varied from point to point in a plant. In cases where corrosion was severe, Hastelloy B was found to be the only satisfactory resistant metal. 

Owing to their low electrochemical potential the group IVB and VB valve metals Ti, Zr, Hf, V, Nb and Ta readily react with water or oxygen to form a dense, protecting passive layer. This also holds for Al. Because of these protecting oxide films, the valve metals show an exceptional resistance towards corrosion in many aggressive environments. This explains why valve metals are widely used in the construction of chemical apparatus. An extensive number of papers and reviews exist that focus on the corrosion resistance of valve metal oxides [e.g. 1, 2]. 

Ellipsometry can be used to gain information about the films formed in the three uniform corrosion regimes. Details are given here about two of those cases firstly the barrier or valve metal oxide layers, which optically are nearly ideal films, and secondly alkaline corrosion, which gives some insight into the applications of ellipsometry to non-uniform films and roughened metallic surfaces. 

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