高温度に於ける金屬に對する硫黄の作用(第3報)アルミニウム合金に對する硫黄の作用

Abstract

The corrosibility of aluminium, aluminium-base cast alloys (Al-Cu, Al-Cu-Zn, Al-Cu-Ni, Al-Cu-Si, Al-Si, Al-Si-Mg, Al-Mg, Al-Mg-Si, Al-Mg-Mn) and aluminium-iron alloys (2, 4, 6, 10, 20, 30, 40% Fe) by sulphur at 300°, 350°, and 440° (boiling point of sulphur) was studied by weight difference due to corrosion. The alloys were placed or dipped for 5 hrs. in vapour or in boiling sulphur. The aluminium-base cast alloys were not corroded to an appreciable degree, as in the case of pure aluminium, and aluminium-rich alloys were also passive, but they were susceptible to corrosion with increasing quantity of iron in aluminium. In order to bring out some lights on the nature of this passivity of aluminium and aluminium-base alloys against sulphur, chemical and X-ray methods of study were undertaken, and it was clarified that the uncorrosibility of aluminium and its alloys is due to their surface protection by the formation of a compact film of Al2S3 in reaction with sulphur. The action of sulphur on aluminium-coated-Flodin iron and-gray cast iron was found to be severer the thicker the coating, due to cracks, which are larger in thickly coated specimens. The attack of aluminium coated irons is attributed to penentration of sulphur vapour through coarse particles in sprayed layer into the mother iron. Therefore, in the case of spraying the thickness must be in optimum and the particles in the sprayed layer must be fine and compact. In the appendix is given a tentative diagram of the Al-S system, which is heretofore un-proposed, based on some experiments and previous work in literature. In this system there exist two kinds of compound, Al S and Al2S3; the former forms from liquid at 2100° and the latter, micrographically verified, by peritectic reaction (L+AlS_??_Al2S3) at 1100°. In aluminium side a conjugate solution forms in a wide range of composition, the monotectic temperature being assumed to be about 1800°. On both aluminium and sulphur sides, horizontal reaction temperatures, 650° and 114°, are assigned; the former was verified to be a eutectic reaction; L_??_Al+AlS, and the latter, the melting point of sulphur. By this diagram the nature of the passivity of aluminium against sulphur can be satisfactorily elucidated.