1. Aluminium light alloys are all very weak for sea water corrosion. Even Duralumin, the best rolling alloy, and Silumin, one of the best casting ones, are not good on this point. Some alloys are really non-corrodible but not used due to their inferior mechanical properties and difficulties in casting, rolling or forging A new alloy was invented in our laboratory 6 years ago, industrial researches were completed and the products are now used in some works. It was named “Chlumin” as it contains chromium, but a few per cent of magnesium and iron are also added. A brief description will be given of it in the following pages.
2. Non-corrodibility. Castings of Chlumin and of an alloy of composition Cu 8%, Al 92 %, the most common one, both made by metallic mould, were immersed in 5 NaCl solution. Photo. I shows the result after 4 years. Chlumin gave only little deposit retaining even some luster, while, the other was severely corroded precipitating considerable mass of aluminium hydroxide. The machined surfaces of Chlumin and Silumin plates were exposed to air being sprayed by salt water every day. Photo. 2 shows the result after 2 months. Many black spots appeared on Silumin but Chlumin remained bright. This difference was observed also on specimens immersed in sea water for a year. Experiments were next carried out somewhat quantitatively. Specimens were immersed in 5% NaCl solution in separate bottles. The quantity of deposit and the weight decrease of specimen were measured after 40 days. The results are summarized in Fig 1. Chlumin was a little better even than aluminium. Silumin, Y-alloy, Duralumin and all others containing Cu or Zn were attacked severely. The variations of mechanical properties due to corrosions are shown in Fig. 2. Many test pieces of wire were immersed in 5 % NaCl solution and the solution was renewed at times to expose them to air for a day to accelerate the corrosion. Chlumin and aluminium wires, annealed or not, lose scarcely their strength by 5 months' immersion. The elongation decreases in some cases. While, Duralumin, variously heat-treated or not, loses both qualities rapidly. After 3 months the strength reduces to 80 to 20% of the original value and the elongation to 50 to 20%.
3. As a casting alloy. The mechanical properties of a cast alloy depend much on casting conditions. Fig. 3 shows the data on ingots made similarly using same metallic mould. Silumin was in perfectly “modified” condition. Chlumin is a little inferior to Y-alloy in strength and yield point but is superior to it in elongation and impact resistance. It is better than Silumin in every respect and is more easy to cast and safe to use as no special process (modification) is wanted on manufacturing. Any complex form can be cast resisting to severe water pressure test. Pistons of internal combustion engine and parts of electric motor have been used for more than a year with better result than Y-alloy.
4. As a rolling alloy. Wires of Chlumin are compared with those of pure aluminium and Duralumin in Fig. 4, being annealed at various temperatures. Duralumin aged after quenching from 510°C into water is exeedingly good indeed when annealed at temperatures below 200°, but becomes worse than Chlumin at 300°. Besides, it may not be a safe process of manufacturing to quench an alloy of melting point of 620° from above 500°. It may be very dificult to heat a large article uniformly above 500°. A little over heating will set in local melting, and when the temperature is too low some parts must meet the worst heat-treatment of Fig. 4. Heating on welding will spoil the good properties given by quenching and aging and it is often impossible in practice to quench, the article again.