Hydroxides 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. 

There are two main reasons for the poor corrosion resistance of many magnesium alloys (Makar and Kruger, 1990) - firstly, internal galvanic corrosion caused by second phases or impurities (Chapter XX in Emley, 1966) and, secondly, the quasi-passive hydroxide film on magnesium is much less stable than the passive films which form on metals such as aluminum and stainless steels. This quasi-passivity results in only poor pitting resistance for magnesium and magnesium alloys. 

This reaction stops in alkaline electrolytes because of the formation of an insoluble film of magnesium hydroxide on the electrode surface which prevents further reaction. Acid tends to dissolve the film. An important consequence of the film on magnesium elecfrodes (see also Chap. 9) is that there is a delayed response to an increase in the load because of the need to disrupt the film to create new bare surfaces for reaction. Pure magnesium anodes usually do not give good cell performance, and several magnesium alloys have been developed for use as anodes tailored to provide the desired characteristics. 

Takaya, M (1987), Anodizing of magnesium alloy in potassium hydroxide-aluminum hydroxide solutions . Keikinzoku, 37, 581-586. 

The procedure of Mortier et al. (32) for the determination of boron in aluminium and aluminium-magnesium alloy is as follows The sample is irradiated for 20 min with a 2 uA beam of 7 MeV deuterons, which are degraded to 5.3-5.7 MeV, and a surface layer is removed by chemical etching (2.3.1). To separate the sample is dissolved in an oxidizing mixture of phosphoric acid, sulphuric acid and potassium dichromate. The C02 released is absorbed in 0.5 M sodium hydroxide and the activity measured with a t-t coincidence set-up. A pure decay is obtained. The chemical yield was checked by comparing, in a separate experiment, the activity of an aluminium sample, doped with 5000 tig/g of boron, measured instrumentally and after chemical separation. The yield was 100 %. 

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... 

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

Preparation and Manufacture. Magnesium chloride can be produced in large quantities from (/) camalhte or the end brines of the potash industry (see Potassium compounds) (2) magnesium hydroxide precipitated from seawater (7) by chlorination of magnesium oxide from various sources in the presence of carbon or carbonaceous materials and (4) as a by-product in the manufacture of titanium (see Titaniumand titanium alloys). 

Strontium Chromate. Strontium chromate [7789-06-2] SrCrO, is made by precipitation of a water-soluble chromate solution using a strontium salt or of chromic acid using a strontium hydroxide solution. It has a specific gravity of 3.84 and is used as alow toxicity, yellow pigment and as an anticorrosive primer for zinc, magnesium, alurninum, and alloys used in aircraft manufacture (8) (see Corrosion and corrosion control). 

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. 

Metallic magnesium is produced by either chemical or electrolytic reduction of its compounds. In chemical reduction, first magnesium oxide is obtained from the decomposition of dolomite. Then ferrosilicon, an alloy of iron and silicon, is used to reduce the MgO at about 1200°C. At this temperature, the magnesium produced is immediately vaporized and carried away. The electrolytic method uses seawater as its principal raw material magnesium hydroxide is precipitated by adding slaked lime (Ca(OH)2, see Section 14.10), the precipitate is filtered off and treated with hydrochloric acid to produce magnesium chloride, and the dried molten salt is electrolyzed. 

Procedure. Pipette 5 ml nitrate-N standard solution into the distillation flask, add 1 drop octan-2-ol, approximately 0.5 g Devarda s alloy, 6 ml magnesium hydroxide suspension (or 0.5 g MgO), and steam-distil the ammonia into 5-ml boric acid solution in a 100-ml conical flask. After approximately 40 ml distillate has been collected over a 5-min period, wash the tip of the condenser into the distillate, add 2-3 drops mixed indicator solution and titrate with 0.005 M HjSO until the colour changes from green to purple. Carry out a blank distillation using 5 ml water instead of extract solution and subtract from the standard titration to give a difference of 5.0 ml. 

The bimolecular reduction of aromatic nitro compounds, depending on reaction conditions, may produce azoxy compounds, azo compounds, hydrazo compounds (1,2-diarylhydrazines), benzidines, or amines. Whereas the reduction with zinc and sodium hydroxide leads to azo compounds, zinc and acetic acid/acetic anhydride produces azoxy compounds. Other reducing agents suggested are stannous chloride, magnesium with anhydrous methanol, a sodium-lead alloy in ethanol, thallium in ethanol, and sodium arsenite. 

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