Aqueous magnesium alloys

The characteristics of these alloys make them ideal for boat and shipbuilding, for which a long history of satisfactory performance is on record for the higher magnesium alloys. Where strength is less critical the lower magnesium alloys may be used with similar success and are recommended for aqueous conditions. 

In considering the corrosion behaviour of magnesium alloys, therefore, it is of the utmost importance to know the nature of the medium to which the metal is to be exposed. In general, atmospheric attack in damp conditions is largely superficial aqueous solutions bring about attack which varies not only with the solute but with the volume, movement and temperature... 

Various concentrations and purities of aqueous ammonia are on the market. The typical concentration is 25% to 30% ammonia and the iron content is less than 10 ppm. If the ammonia content is above 25%, it must be shipped in pressure vessels because of its elevated vapor pressure. For more stringent purity requirements for aqueous ammonia, the containers should be made of seawater-resistant aluminum (magnesium alloyed) or austenitic steels74. 

A. Eliezer, et al., Mechanoelectro-chemical Behavior of Pure Magnesium and Magnesium Alloys Stressed in Aqueous Solutions,/. Mater. Synth. 

Certain metals, such as silver and gold, are extremely unreactive, while sodium is so reactive that it will react with water. Zinc is unreactive with water. It will, however, react with acid. Why will magnesium metal react with copper sulfate solution, while copper metal will not react with aqueous magnesium sulfate In Chapter 3, you learned that an alloy is a solution of two or more metals. Steel is an alloy that contains mostly iron. Is its reactivity different from iron s reactivity ... 

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). 

In aqueous exposures, only strong alkalis, concentrated hydrofluoric acid, and chromic acid are considered not to induce SCC in magnesium alloys [5]. In other exposures from deionized water to a wide variety of aqueous s t solutions SCC failures have been observed in full, partial, or alternate immersion [32]. The most severe acceleration of failures has been found to occur with NaCl + K2Cr04 solutions, which have been used in many laboratory studies of magnesium SCC however, the results have been found to correlate poorly with service experience [28,31,33]. 

Sivaraj D, McCune R, Mallick P K and Mohanty (2006), Aqueous corrosion of experimental creep-resistant magnesium alloys , 2006 SAE World Congress, SAE Paper No. 2006-01-0257. 

Chen J, Wang J, Han E, Dong J and Ke W (2008), States and transport of hydrogen in the corrosion process of an AZ91 magnesium alloy in aqueous solution , Corros. Sci. 50, 1292-1305. 

The atmospheric corrosion of metals is largely dependent on the electrochemical reactions occurring in the thin aqueous layer on the surface and at the interface between the solid substrate and the thin electrolyte layer. The thin aqueous layer on the surface also acts as a conductive medium which can support electrochemical processes on the surface. Due to the presence of different phases with different electrochemical properties in magnesium alloys the anodic and cathodic reactions are often localised in different areas on the magnesium surface. The microelectrodes may consist of different phases present in the microstructure of the alloys. The influence of the microstructure on the atmospheric corrosion behaviour of magnesium alloys will be discussed in more detail further on. In atmospheric corrosion the thin electrolyte reduces... 

E. M. Gutman, Ya. Unigovski, A. Eliezer, E. Abramov, Mechanoelectrochemical behavior of magnesium alloys stressed in aqueous solutions, J. Materials Synthesis Processing, 8, Nos.3/4 (2000) 133-138. 

R. I. Stephens, C. D. Schrader, K. B. Lease, Corrosion fatigue of AZ91E-T6 cast magnesium alloy in a 3.5 percent NaCl aqueous environment, J. Eng. Mat. Techn., 117 (1995) 293-298. 

The combination of immersion, hydrogen collection and weight loss measurement is an easy corrosion evaluation method, particularly for magnesium alloys. The method, first established and used by Song et al. [5] to estimate and monitor the corrosion rate of magnesium in a NaCl solution, has been widely adopted as a common corrosion rate measurement technique for magnesium alloys in various aqueous solutions. The reliability of the method has been theoretically and experimentally demonstrated [24,25] and the details will not be repeated here. 

SCC and the typical plateau appears where the FCGR, da/dN, is no longer depending on the applied AKj. In both 3.8 M KBr and 5.0 M K1 potassium iodide the appearance of SCC is even more evident. However, at higher AKj values of about 17 MPa m, SCC disappears and TCF returns to take the lead. This passage is more evident in Fig. 16.34 relative to a magnesium alloy type ZK 60A-T5 tested in the same aqueous halide solutions of the previous aluminum alloy plus a 1.4 M Na2S04 solution [38]. The test frequency was always equal to 4 Hz and R = 0. Also in this case SCC does not appear in distilled water probably because the load... 

Fig. 16.34 FCGR, da/dN, in type ZK 60 A-T5 magnesium alloy in dry argon, distilled water, several aqueous halide solutions and 1.4 M Na2S04 [38]...

Speidel, M.O., Blackburn, M.J., Beck, T.R., Feeney, J.A. Corrosion fatigue and stress corrosion crack growth in high strength aluminum alloys, magnesium alloys and titanium alloys exposed to aqueous solutions. In Corrosion Fatigue, International Corrosion Conference Series NACE-2 (1972)... 

Continuous immersion test for 7000 series (aluminum-zinc-magnesium) alloys (Ref 36) aqueous solution containing 3% NaCl, 0.5% hydrogen peroxide (30%), 100 m/L 1 N sodium hydroxide, and 20 m/L acetic acid (100%) pH 4.0... 

Impressed-current test for 7000 series (aluminum-zinc-magnesium) alloys (Ref 38) aqueous solution containing 2% NaCl plus O.S% sodium chromate pH 8.1, current density 4.65 x 1(T mA/mm (0.3 mA/ia ) 30 day maximum exposure time... 

Examples of the sacrificial-anode method include the use of zinc, magnesium, or aluminum as anodes in electrical contact with the metal to be protected. These may be anodes buried in the ground for protection of underground pipe lines or attachments to the surfaces of equipment such as condenser water boxes or on ship hulls. The current required is generated in this method by corrosion of the sacrificial-anode material. In the case of the impressed emf, the direct current is provided by external sources and is passed through the system by use of essentially nonsacrificial anodes such as carbon, noncor-rodible alloys, or platinum buried in the ground or suspended in the electrolyte in the case of aqueous systems. 

The unequal attack which occurs in tap water, condensate and other mild electrolytes may lead to perforations of thin-gauge sheet and even to deep pitting of castings. In stronger electrolytes the effect is variable. In chloride solutions such as sea-water, attack on the metal usually results in the pitting of some areas only, but where the metal surface has been rendered reactive, as by shot blasting, attack may be so rapid that uniform dissolution over the whole surface may occur. In either case magnesium-base alloys are not usually suitable for use in aqueous liquids since they are not intrinsically resistant to these electrolytes. 

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