Magnesium alloys corrosion rate

Nickel-iron alloys are more resistant than iron to attack by solutions of various salts. In alternate immersion tests in 5% sodium chloride solution Fink and De Croly determined values of 2-8, 0-25 and 0-5 g m d for alloys containing 37, 80 and 100% nickel compared with 46 g m d for iron. Corrosion rates of about 0.4 g m d are reported by Hatfield for Fe-30Ni alloy exposed to solutions containing respectively 5 Vo magnesium sulphate, 10 Vo magnesium chloride and 10% sodium sulphate the same alloy corroded at a rate of about 1.2 g m d in 5% ammonium chloride. 

Alloys of aluminium with magnesium or magnesium and silicon are generally more resistant than other alloys to alkaline media. The corrosion rate in potassium and sodium hydroxide solutions decreases with increasing purity of the metal (Fig. 4.9), but with ammonium hydroxide the reverse occurs. 

Table 4.19 Corrosion rates of magnesium-rich alloys (g m d ) in three different...

By the use of many commercial abrasive processes, the corrosion resistance of magnesium alloys can be reduced to such an extent that samples of metal that may lie quiescent in salt water for many hours will, after shot blasting, evolve hydrogen vigorously, and the corrosion rate, as measured by loss of weight, will be found to have increased many hundred-fold. The effect in normal atmospheres is naturally much less, yet the activation of the surface is an added hazard and is the opposite of passivation which is essential if later-applied paint finishes are to have proper durability. 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

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. 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

The polarisation resistance can be related to the rate of general corrosion for metals at or near their corrosion potential, corr. Polarisation resistance measurements are an accurate and rapid way to measure the general corrosion rate (CR) of bioinert Ti alloys, biotolerant austenitic surgical steels or biodegradable magnesium alloys. 

Magnesium and its alloys are heavily attacked in all acids, except hydrofluoric acid (HF) and chromic acid (H2Cr04). In HF, an insoluble layer of Mgp2 is formed, which protects the magnesium from further attack. In dilute solutions of HF (concentration 6-60%), the corrosion rate at 20 °C is around 1 mm yr . ... 

Fig. 30 Effect of alloying and contaminant metals on the corrosion rate of magnesium, determined by alternate immersion in 3% NaCi solution [35].

Tjscc - th the threshold intensity range at the corrosion crack growth rate <10 °m/cycle. The rate daldN in aqueous solutions is much higher (by a dozen times) than in ambient air. However, by suitable choice of solution composition, the CFG growth can be reduced in titanium and magnesium alloys (86, 100). 

It is well known that aluminum as such is fairly passive, because a very dense and uniform aluminum oxide AI2O3 layer is formed onto the metal to protect the metal from corrosion. Highly ductile light weight aluminum alloys that are passed through specific heat treatments can, however, make aluminum susceptible to corrosion. These materials may contain alloying elements such as magnesium and/or copper, which alter and complicate the corrosion behavior of aluminum. Typical forms of corrosion for the alloys are localized and pit corrosion. Due to the dense structure of the aluminum oxide layer, the corrosion rate of aluminum alloys is, however, substantially slower compared with corrosion/dissolution of CRS or HDG steel [15]. 

Low alloy steels, also called weathering steels, contain small amounts of copper, chrome, nickel, phosphorus, silicon, and magnesium (< 1 %, typically). Their resistance to atmospheric corrosion generally exceeds that of carbon steel. Indeed, when exposed to environments that are not too strongly polluted a dark brown patina forms over some years that slows down the corrosion rate. On buildings this natural layer can thus replace a paint coating. For this reason, alloyed steels find numerous applications in architecture. 

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. 

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