The present investigation has been carried out to ascertain the cause of the age-hardening of Duralumin and 24 S-type Super-duralumin. These alloys age-harden marked by at room temperature when they are quenched from such a temperature as 500°, but the Al alloys containing Cu or Mg₂Si do not show at room temperature such a marked age-hardening. The cause of the age-hardening of Duralumin and 24 S-type Super-duralumin is explained to be due to the decrease of the solid solubility of Cu or Mg₂Si in Al with the fall of temperature similarly as those of the alloys containing Cu or Mg₂Si. However, this elucidation does not apply to the age-hardening of Duralumin at room temperature, and to the necessity, of tempering of the alloys containing Cu or Mg₂Si at 150-170° to attain the max. hardeness. The chill cast alloyss corresponding to the 8 sections cut through the Al-axis were prepared, and after annealing at 500° for 16. hours they were quenched in water and aged at room temperature. The change of hardness during the ageing was measured with a Brinell tester. The hardness test indicates that the alloys containing Cu only do not show any marked in-crease of hardness by the ageing at room temperature, but the addition of Mg to Al-Cu alloys gave a considerable influence upon the natural age-hardening, and that the phenomenon of ageing of the alloys containing more Cu than the ratio 4:1.5 in Cu and Mg is somewhat different from the alloys containing more Mg than the same ratio. The alloys containing more Mg than this ratio show less age-hardening at room temperature. By tempering at 50° and 100° immediately after quenching, the alloys containing Cu more than the ratio Cu: Mg=4:1.5 did not show as much hardening as at room temperature, but the alloys containing Cu and Mg in the ratio 4:1.5 to 1:1 were hardened rather remarkably by the tempering at 100°. From the diagram of Al-Cu-Mg alloy system pub-lished previously, the author concludes that the age-hardening of Al-rich Al-Cu-Mg alloys takes place on the course of the precipitation of a ternary compound Al₁₃Cu₇Mg₈ denoted S, or CuAl₂ and this ternary compound from Al-solid solution.

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An experiment of surface-passivity is made on 14% and 21% Cr stainless steels in a solution of 10% FeCl₃, the former is severely attacked while the latter unattacked, retaining metallic lustre provided that the portion of the contact surface of the specimen and glass supporter is attacked to form a pit. When the specimen of 21% Cr stainless steel was immersed in the solution for a long time, the portion severely attacked formed a cavity, while the passive film on the surface of the inner-corroded portion slacken ed. The “inner-corrosion” of the 21% Cr steel occurs also in the case of the surface of the specimen in contact with non-cond uctive substances other than glass supporter, and a discussion of this phenomenon is also made, taking the non-metallic inclusions, pits and fissures on the surface of the specimen into account. These undesirable inclusions, pits and contact portion with non-conductive substances act as capillary crevices on the surface which becomes anodic while the other portions of the surface is subjected only to the action of oxidation and remain in a passive state. It is also ascertained that when the surface of the specimen is covered with glass cylinder having a capillary crevice and the specimen is held vertically, all the surface of the specimen was attacked and no passive film was formed. The interesting phenomenon of inner-corrosion occurs also in the solutions of (75g/L CuSO₄+50g/L HCl) and 10% CuCl₂, while in the solution of 10% FeCl₂, the surface of the specimen was attacked, and in the solutions of (CuSO₄+H₂SO₄) and (_??_M K₂Cr₂O₇+10g/L HCl) all the surfaceof the specimen became passive state. The activity or passivity of the specimen is, of course, depends on the amount of chromium content in the specimen and the kind of acidic solution, however, the cause of the passivity of the surface and the inner-corrosion on a special portion of 21% Cr stainless steel is due to the following factors, - (1) the passivity depends on the strengths of the acidity and oxidation-power of the solution used on the surface of the specimen. In this experiment the existence of metallic cations of higher valency in the solutions of HCl and H₂SO₄, for example, Cu⁺⁺, Fe⁺⁺⁺, Cr⁺⁺⁺ and Hg⁺⁺ give an oxidizing power to the solution, and therefore the passivity of the 21% Cr steel in the solutions of FeCl₃, CuCl₂, (CuSO₄+HCl) and (CuSO₄+H₂SO₄) is produced. (2) The inner-corrosion of the specimen in which its surface is passive state is caused by the presence of capillary crevice on the surface which is in contact with the glass supporter, and it is also due to the existence of some amounts of chlorine ions in the solution. (3) The mechanism of the deep attack in the process of the inner-corrosion is elucidated by an experiment that if the portion on the surface, on which a capillary crevice is existing, is attacked by chlorine ions on account of poor oxidation of the solution, it will be dissolved by the solution and afterwards it cannot be so oxidized to form a passive state by the formation of reducing hydrogen, i.e. due to the impossibility of the formation of passive film on the portion of the specimen.

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The effect of addition of manganese on the corrosibility of pure magnesium, Mg-Zn, Mg-Sn, Mg-Al and Mg-Zn-Al alloys both in cast and annealed states was studied, and also that of addition of silicon on pure magnesium, Mg-Zn, Mg-Sn and Mg-Al alloys in cast state was studied.The result obtained shows that the alloys of the systems Mg-Mn, Mg-Zn-Mn, Mg-Sn-Mn, Mg-Zn-Si and Mg-Zn-Al-Mn show high resistance against corrosion in the range of
Mn>2% (as cast), Mn>0.6% (annealed at 470°)
Zn 2_??_6%, Mn 0.6_??_2%
Sn 2_??_8%, Mn 0.5_??_2%
Zn 2_??_6%, Si 0.05_??_0.8% Zn 4%, Al 6%, Mn>1%
respectively and that the effect of annealing at 470° is very harmful in general, except for Mg-Mn binary alloy which is very effective in reducing the rate of corrosion.

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For the synopsis of this work see the Report I. [This journal 1 (1937), 43]

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For the synopsis of this work see the Report I. [This journal, 1 (1937), 26]

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