Polarization and weight loss tests on iron-chromium alloys were conducted in saturated monocarboxylic acid solutions. The results obtained were as follows:
(1) The critical current density and passive current density decreased as increasing the chromium content in alloys.
(2) Alloys of high chromium content began to dissolve in the hexavalent condition of chromium at a potential of about E_h=1.2V in the same manner as in inorganic acid solutions.
(3) The corrosion rate of alloys containing up to 18% Cr was greater than that of pure iron in formic and acetic acid solutions. But, in propionic and butyric acid solutions, the corrosion rate decreased gradually as chromium content increased.
(4) The corrosion rate decreased as increasing the carbon content in saturated monocarboxylic acids.

抄録全体を表示

A study has been made of the effect of the passivation treatment upon the stability of passive stainless steel against pit formation. The specimens used were SUS 27 stainless steel which were passivated at definite potential between +0.2- +0.7V (vs. SCE) in 5wt% H₂SO₄ solution.
The stability against pit formation was evaluated by measuring the time elapsed until the steady rise of the passive current (say induction time τ) after addition of definite quantity of chloride ion into the solution.
The experimental results were as follows;
(1) Induction time τ for pit formation was found to become larger, the more basic the passivation potential, particularly at E_crit=+0.4V and above this critical potential τ showed very small and nearly constant value.
(2) The irregular change of the passive current before the steady increase in current was very characteristic for the passive steel kept in lower passive region than +0.4V, while few fluctuation was observed above +0.4V.
(3) τ became remarkably smaller, when the passivated specimens were heated under vacuum at 300°C, and the irregular change of the current was found to decrease.
From these experimental facts, the function of the film for the pitting resistance of the stainless steel was discussed, referring to the bound water and the chemical composition of the passive film.

抄録全体を表示

この論文は, 塩化ナトリウム溶液中における高張力鋼の環境誘起割れについて, 従来報告されている機構をまとめた。水素脆性割れと局部活性溶解割れ (active path corrosion) とを区別する電気化学的基準を, 初期割れ目 (precrack), 食孔あるいはすきまのような局部腐食領域において溶液が酸性化するという近年明らかにされた事実と関連づけて考察した。改良型12%クロムマルテンサイトステンレス鋼を電気化学的に検討した結果, 従来到底考えられないような電位すなわち可逆水素電位よりも試料全体がより貴な電位においても鋼中への水素の吸収が起こることを明らかにした。また, 実験結果は腐食電位, カソード分極あるいはアノード分極したいずれの電位においても, 割れ成長の活性化エネルギーは, 9.5kcal/mol (±1) になることを示した。この値は, 従来報されているカソード分極することにより脆化した鋼の割れの活性化エネルギーに非常に近い。これらの実験結果は水素吸収に伴う脆化過程が, いずれの設定電位においても割れ成長の主役を演じていることを示している。可逆水素電位よりも卑な電位に分極すると, 試料面全体で直接プロトンのカソード放電が起こり試料中への水素の吸収が行なわれる。腐食電位あるいはそれより貴な電位においては, 食孔中におけるアノード溶解生成物の加水分解に伴うpHの低下によって生ずるプロトンの放電により水素の吸収が起こる。貴な電位領域においては, 局部活性溶解は割れ成長の機構ではないと考えられる。しかし, アノード溶解は水素脆化機構による割れ成長にとって必要条件である。機械的な予備亀裂 (precrack) が存在し, 孔食あるいは間隙腐食に伴う加水分解が起こる場合, 塩化ナトリウム溶液中においては, 他の高張力鋼の割れ挙動も同様の機構によって説明できると推察される。

抄録全体を表示