Corrosion resistance of aluminum and its alloys in sodium chloride media

Abstract

Many years of practice in the Usol'sk Salt Extraction Plant have shown that the construction material (KS aluminum alloy, containing 95-96% A1, 2% Mg, 1.4% Mn, 0.7% St, and 0.2% Sb) from which the imported salt drying drums were made had very high corrosion resistance in the conditions of salt extraction [1]. This led to a study of utiltzability of aluminum and its alloys manufactured by the Soviet industry in contact with table salt and its solutions. The investigations have been carried out by the All-Union Scientific Research Institute of the Salt Industry. According to data of the All-Union Scientific Research and Planning Institute of the Aluminum, Magnesium, and Electrode Industries the best corrosion resistance in sodium chloride media, could be expected from AMg5 and AMg2 aluminum--magnes inm alloys. Some literature data [2, 3] indicate also that at given conditions (determined pH values, etc.) these alloys should have a high corrosion resistance in chloridecontaining media. The corrosion tests were carried out on A2M aluminum sheets, and AD 1 and AM2 aluminum alloys. The chemical composition of these materials in percent is given in Table 1. The materials were tested under laboratory conditions and in the conditions of the salt-extracting industry. The laboratory corrosion tests were carried out by partial and complete immersion in solutions of chemically purified and mother liquor brines of the Sla~yan Salt Plant, in an artificial brine prepared from table salt from the above plant, and in brine from the Bol'sho[ Kalkaman Lake (Table 2). The specimens were tested at room temperature (20-23~ and at the boiling temperature of the solutions in flasks equipped with reflux condensers. Three specimens of each material were placed in each flask on glass rods. The tests at room temperature were carried out with and without stirring. The corrosion resistance of the materials was assessed by the metal loss in g/m 2 and the corrosion rate in mm/yr. The character of the corrosion was assessed visually and with a binocular microscope. The results of the laboratory corrosion tests carried out on aluminum and its alloys are given in Table 3. The data show that the lowest corrosion rate for A2M aluminum and its AD1 and AMg2 alloys were found in the natural brine from Bol'shoi Kalkaman Lake, which had a pH of 7.2. The surface of the specimens of these metals remained practically unchanged after six months of testing in the lake brine; only a few shallow corrosion spots were detected on the surface of A2M aluminum and AMg2 aluminum--magnes ium alloy specimens. All these materials can be considered as corrosion resistant in the lake brIne.