著者

多賀谷 正義

出版者

社団法人日本鉄鋼協会

雑誌

鐵と鋼 : 日本鐡鋼協會々誌 (ISSN:00211575)

巻号頁・発行日

vol.24, no.8, pp.730-753, 1938-08-25

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In the previous reports the corrosion tests of many kinds of cast iron having various compositions and structures were carried out, and determined that the corrosion amount in a dilute nitric acid solution (0.5 N) is very small with the gray cast irons these contain free ferrite or greater amount of sulphide in pearlite matrix, this kind of gray cast iron evolves a pretty amount of gas in such a dilute nitric acid while the gray cast irons of pearlite matrix do not evolve any gas in the nitric acid solution of the same concentration. Then the rate of evolution of gas was measured and the chemical composition of the gases were determined. The gases evolve always periodically and greater part of them are hydrogen. From this fact and other experimental results, the writer concluded as following. In a dilute nitric acid gray cast iron first evolves hydrogen as a result of the electrolytic action caused by graphite (Cathode) and ferrite (Anode) while the ferrite is anodically oxidized and temporarilly brought to passive state, so that the dissolution ceases. In the next stage the passive state is destroyed by the acid and direct reaction between iron and nitric acid takes place, in this stage ferrite is severely attacked but non gas is evolved and instead of hydrogen ammonia and hydroxylamine these are soluble in the solution or complex iron salt are formed. In the 3rd stage hydrogen is evolved again by electrolytic action as the former case. These changes are repeated periodically in the progress of dissolution of gray cast iron in a dilute nitric acid. In another experiment the electrode potentials of various structural constituents of gray cast iron were measured in a normal ferrous chloride solution against the normal calomel electrode at 30℃. The results are as following ; graphite cathodically treated in 1N hydrochloric +0.2750^u, Fe_aC -0.2357^u, Fe_3P -0.2971^u, MnS 0^u, FeS -0.3600^u, [60% MnS+40% FeS] (solid solution) -0.4407^u. Also the E. M. F. of galvanic cells consist of ferrite and one of these constituents were measured in 0.5N hydrochloric acid solution at 30℃, and the following results were obtained ; graphite^+-ferrite^- 0.9810^u, graphite^+-ferrite^- (containing 0.34% Cu) 0.9535^u, Fe_3C^+-ferrite^- 0.3508^u, Fe_3C^+-ferrite^- (containing 0.34% Cu) 0.3064^u, Fe_3P^+-ferrite^- 0.2100^u, [MnS+FeS]^+-ferrite^- 0.3260^u. In the 1st and 2nd report the writer concluded that gray cast iron dissolves in a dilute hydrochloric acid mainly with the electrolytic reaction caused by local galvanic cells consist of ferrite and graphite or other noble constituents. These present experimental results gree with the former results of corrosion test as described in the previous reports.