Galvanic corrosion cathodic efficiency

It is obvious from the data that galvanic corrosion of zinc in rural atmosphere can be five times the rate in corresponding atmospheric corrosion and three times that in marine atmospheres. Mild steel appears to be the most efficient cathode among the metals studied. More detailed discussions are given in the literature.92... 

Table 7.3 shows an example of galvanic corrosion rates of aluminium alloys in 3.5% NaCl solution when coupled to different materials. For instance, it is seen that the contact with low-alloy steel gives considerably higher galvanic corrosion rates on aluminium than does contact with the - from a practical point of view - more noble stainless steels as well as Ni- and Ti-based alloys (regarding material descriptions, see Section 10.1). The table reflects the cathodic efficiency of the various materials coupled to aluminium (with the exception of cadmium, zinc and aluminium alloys) in the actual environment. 

If there is a crevice on a component made of a material liable to crevice corrosion, and this component is connected to a more noble material with free surfaces, crevice corrosion may be intensified strongly. Such a case is a couple of an aluminium component (with a crevice) and a steel plate in water containing some chloride. The corrosion form can be called galvanic crevice corrosion. The crevice corrosion rate will be particularly high if the more noble metal acts as an efficient cathode in the given environment. The explanation is the same as for ordinary galvanic corrosion. 

Titanium in contact with other metals In most environments the potentials of passive titanium. Monel and stainless steel, are similar, so that galvanic effects are not likely to occur when these metals are connected. On the other hand, titanium usually functions as an efficient cathode, and thus while contact with dissimilar metals is not likely to lead to any significant attack upon titanium, there may well be adverse galvanic effects upon the other metal. The extent and degree of such galvanic attack will depend upon the relative areas of the titanium and the other metal where the area of the second metal is small in relation to that of titanium severe corrosion of the former will occur, while less corrosion will be evident where the proportions are reversedMetals such as stainless steel, which, like titanium, polarise easily, are much less affected in these circumstances than copper-base alloys and mild steel. 

One more recent use of aluminium is in cathodic protection. In this application, the alloy sacrificially corrodes, i.e. is anodic and thus prevents corrosion of the structure to which it is electrically connected. In order for these alloys to be efficient, they must undergo little self-corrosion, but corrode uniformly when stimulated by galvanic contact. 

Magnesium and zinc are the predominantly used galvanic anodes for the cathodic protection of pipelines [13—16]. The corrosion potential difference of magnesium with respect to steel is 1 V, which Umits the length of the pipeline that can be protected by one anode. Economic considerations have led to the use of aluminum and its alloys as anodes. However, aluminum passivates easily, decreasing current output. To avoid passivation, aluminum is alloyed with tin, indium, mercury, or gallium. The electrochemical properties of these alloys, such as theoretical and actual output, consumption rate, efficiency, and open circuit (corrosion) potential, are given in Table 15.1. 

The significance of dust is mentioned above. Industrial and urban atmospheres contain more or less solid particles consisting of carbon, soot, sand, oxides, and salts, e.g. chloride and sulphate. Many of these substances attract moisture from the air some of them also attract polluting and corrosive gases. The salts cause high conductivity, and carbon particles can lead to a large number of small galvanic elements because the particles act as efficient cathodes after deposition on the surface. 

Recent experience has confirmed that, by adopting the recommendations of the British Standards Institution or similar codes of practice operating in other countries, the likelihood of corrosion damage to buried structures adjacent to cathodically protected installations is negligible. This is because recently installed cathodically protected structures are usually coated with efficient and durable insulating coverings such as epoxy resins and the protective current applied is consequently small. In many cases the small protective currents that can be applied by means of galvanic anodes is adequate. 

Cathodic protection Because crevice corrosion occurs above a critical potential, cathodic protection is an efficient way to prevent crevice corrosion by maintaining the potential of the free surfaces below the protection potential. This is the case of 17-4-PH stainless steel, which is not resistant to crevice corrosion in seawater but is commonly used because it performs very well under cathodic protection (this protection is often due to galvanic coupling to unalloyed steel). 

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