Marine magnesium alloys

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. 

Magnesium is thermodynamically one of the less noble metals, and it can protect most other metals when used as sacrificial anodes (see Section 10.4). In the atmosphere the metal is covered by an oxide film. Therefore it resists rural atmospheres but is subject to pitting in marine atmospheres. Magnesium alloys are also liable to SCC and erosion corrosion, and are attacked by most acids. Mg alloys are used in automobile engines, aircraft, missiles and various movable and portable equipment, in all cases primarily because of their low density (1.76 g/cm ). 

Already in 1890, naval architects had considered aluminium for reducing weight in vessels. But in order for aluminium to be usable for shipbuilding, metallurgists and corrosion specialists in the 1930s first had to develop aluminium magnesium alloys. These alloys have excellent corrosion resistance in the marine environment, and they are weldable. Since 1960, all high-speed ferries have been built in these alloys. 

Three broad classes of aluminium alloys will be considered here the heat-treatable high-strength aluminium-copper 2000 series and aluminium-zinc-magnesium 7000 series alloys and the non-heat-treatable lower strength aluminium-magnesium 5000 series alloys which are used extensively in marine applications. 

Aluminum and aluminum-zinc alloy anodes have become the preferred sacrificial anodes for the cathodic protection of offshore platforms. This preference is because aluminum anodes demonstrate reliable long-term performance when compared with magnesium, which might be consumed before the platform has served its useful hfe. Aluminum also has better current/weight characteristics than zinc. Weight can be a major consideration for large offshore platforms. The major disadvantage of aluminum for some applications, for example, the protection of painted ship hulls, is that aluminum is too corrosion resistant in many environments. Aluminum alloys will not corrode reliably onshore or in freshwater [37]. In marine... 

Hudson reported lives of about 4y years for 38 /tm thick metal-sprayed aluminium coatings on steel exposed at Sheffield, and more than 1 ly years for coatings 75/tm thick. Sprayed aluminium coatings (approximately 125/tm thick) have also provided complete protection against exfoliation and stress corrosion to aluminium-copper-magnesium (HE 15) and aluminium-zinc-magnesium (DTD 683) alloys in tests lasting up to 10 years in industrial and marine environments . 

Atmospheric corrosion rates will tend to increase with winds directly from the ocean to the site, the lower the elevation, and the closer the ocean is to the specimen as shown in Table 2. The direction and velocity of the wind can affect the accumulation of entrained seawater-related particles on specimen surfaces. Generally, the closer the site to the ocean in the face of a prevailing wind the greater the corrosion rate of metals and alloys. Magnesium and calcium chlorides are hydroscopic and tend to keep surfaces wet or moist. Sulfur dioxide lowers the critical humidity required to activate corrosion [fO] and increases the aggressiveness of the marine atmospheric environment such as found in an industrial marine environment versus a rural marine environment (Table 2). The dew-point temperature and the component/specimen temperature wiU influence the rate of corrosion. 

Alloys with magnesium have an excellent resistance to marine corrosion. They are used for upper works [14]. 

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