Protective magnesium alloys

PANI/Ti02 nanocomposites have been also used by Sathiyanarayanan et al. to protect magnesium alloy from corrosion [318]. The ability of the PANI/T1O2 nanocomposite coating to protect against corrosion was found to be more than that of the pure RANI coating, because the uniform distribution of PANI can protect the metal surface more uniformly [318]. 

The special forms consist of the many types of anode which are used for protecting smaller containers. Boilers, heat exchangers and condensers belong to this group. Besides the rod anodes already mentioned with tube screw joints which can be screwed into the container from outside, there are also short and round anode supports as well as more or less flat ball segments which are bolted onto the protected surface with cast-on supports. These shapes are mostly manufactured from magnesium alloys. In addition, there are star-shaped or circular anodes for installation in condensers and pipes. The weight of these anodes lies between 0.1 and 1 kg. 

Contact of brass, bronze, copper or the more resistant stainless steels with the 13% Cr steels in sea-water can lead to accelerated corrosion of the latter. Galvanic contact effects on metals coupled to the austenitic types are only slight with brass, bronze and copper, but with cadmium, zinc, aluminium and magnesium alloys, insulation or protective measures are necessary to avoid serious attack on the non-ferrous material. Mild steel and the 13% chromium types are also liable to accelerated attack from contact with the chromium-nickel grades. The austenitic materials do not themselves suffer anodic attack in sea-water from contact with any of the usual materials of construction. 

In conditions where sea-water spray may be deposited regularly on magnesium articles with no alleviating mechanism for its removal, or where breaking waves may drench the components, the effect is quite different. Corrosion of bare metal will be heavy and will be intensified at junctions with other more noble metals. Unless magnesium alloys can be adequately protected in such combinations it is better to avoid their use. This matter is dealt with under the section on protection. 

In considering the corrosion of magnesium and its alloys it is important to examine the methods available for assessing corrosion tendencies and particularly those known as accelerated tests. Tests carried out by immersion in salt water or by spraying specimens regularly with sea-water are worthless as a means of determining the resistance of magnesium alloys under any other than the particular test conditions. Extrapolation to less corrosive conditions is not valid and even the assessment of the value of protective measures by such means is hardly possible. The reason is to be found in the fact that corrosion behaviour is directly related to the formation of insoluble... 

The proneness or otherwise to corrosion is essentially the same in all the magnesium-base alloys and it is important to note that the requirements of protection therefore do not vary for the magnesium alloy under consideration. If conditions are such that any one of the alloys can be used satisfactorily without protection, then any other of the alloys can be so used. On the other hand, if a given protective scheme is found necessary for a particular alloy, then the same protective scheme will be found necessary (and will be equally effective) with any other magnesium-base alloy. 

Table 4.20 lists a number of the better-known processes for producing protective films on magnesium alloys by chemical and electrochemical processes. 

In recent years use has been made of the strong adhesion, toughness and water impermeability of some of the epoxy resins to secure greatly improved surface protection of magnesium alloys. By this means it has been possible to employ these alloys even in situations where they are drenched repeatedly with sea-water. 

Improved Protection of Magnesium Alloys Against Synthetic Aviation Lubrications at Elevated Temperatures, Rendu, Tawil SAE Paper 880869... 

King, J. F., Adamson, K. G. and Unsworth, W., Impregnation of Anodic Films for the Protection of Magnesium Alloys, Ministry of Defence D.Mat. Report No. 193, February (1973) Adamson, K. G., King, J. F. and Unsworth, W., Evaluation of High Temperature Resistant Coatings for the Protection of Magnesium Alloys, Ministry of Detence D.Mat. Report No. 196, July (1973)... 

Anode efficiency is of little practical significance and can be misleading. For example, magnesium alloy anodes often have an efficiency ca. 50% whilst for zinc alloys the value exceeds 90% it does not follow that zinc alloy anodes are superior to those based on magnesium. Efficiency will be encountered in many texts on sacrificial anode cathodic protection. 

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. 

Etch priming is widely used on aluminium alloy, and is particularly effective on cadmium and zinc. The adhesion to stainless steel and titanium is good. It has also been used quite widely on bare steel and on magnesium alloy, but on these metals its performance is not, in the opinion of some investigators, always quite reliable. For best protection the etch primer coating is followed with a full paint scheme. 

Cathodic protection equipment has been used very successfully in water tanks and HW and steam boilers as anticorrosion devices for 100 years or more. Such equipment comes in many shapes and sizes, and comprises a sacrificial anode of either zinc or magnesium alloy, either bolted directly to a suitable internal water-wetted (cathodic) metal surface, or self-contained by enclosing the anode with a suitable cathode (such as a silver plated base metal). Usually several devices are required for any boiler, more for larger units and less for smaller ones, and these require replacement every one to two years. 

Magnesium alloys are very lightweight, and are being used in the aerospace industry. Because they are very reactive, these alloys need to be protected from corrosion. Dr. Birss holds a patent on a new approach to the electrochemical formation of protective oxide films on magnesium alloys. Dr. Birss also works on developing new catalysts for fuel cells, and studies the factors that lead to the breakdown of fuel cells. 

The film of magnesium hydroxide formed can give rise to passivity. This is attacked by anions such as chloride, sulfate and nitrate. The passive film formed gives reasonable protection from corrosion in rural, marine and industrial atmospheres, as evidenced by the corrosion rate data given in Table 4.69. It is obvious from the data that the corrosion performance of magnesium alloy lies between aluminum and carbon steel. 

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