Showing papers in "Corrosion engineering in 1975"

Effects of Alloying Elements on the Pitting Corrosion of Stainless Steels

Abstract: Effects of alloying elements on the pitting corrosion resistance of 17Cr-16Ni steels with and without 4% Mo were evaluated by the corrosion rate in 10% FeCl3.6H2O and 4% NaCl added with hydrogen peroxide and by the polarization measurement in 0.1N NaCl+0.1-0.25N Na2SO4 at 40C. The elements, the content of which was varied, were carbon, nitrogen, silicon, phosphorus, sulfur, manganese and nickel. The supplementary alloying elements were aluminum, titanium, vanadium, cobalt, copper, zirconium, niobium, tin and tungsten. Tungsten, like silicon, was found beneficial in minimizing the pitting corrosion if added together with molybdenum. Carbon was favorable if no chromium carbides were formed. Copper added to the Mo-free steel was beneficial, but not effective in the Mo-bearing steel. Manganese was proved to be harmful in both Mo-free and Mo-bearing steels, but increasing its content more than that of ordinary stainless steels did not enhance its harmful effect. The very favorable effect of nitrogen was also recognized. The effects of the other alloying elements are summarized in a table. The more noble the electrode potential at a relatively high c.d., i.e. 10mA/cm2, in anodic polarization curves measured in sodium chloride solution, the lower was the corrosion rate in the ferric chloride test if a second phase was absent. The presence of a second phase increases especially the corrosion rate in ferric chloride solution. It was indicated that the alloying to increase the passivating ability of the steel would shift the pitting potential to the more noble direction by passivating the depassivated sites before they can grow as ordinary pits.

Dissolution of Hydrous Metal Oxides in Acid Solutions

Abstract: The dissolution rate of hydrous oxides of iron, chromium and nickel in acid solutions was expressed as follows; v=kaH+2aAm, where aH+ is the proton activity, aA the anion acttvity, l the reaction order for proton and m the reaction order for anion. The value of l is close to 0.5 irrespective of the species of hydrous oxides but the value of m depends upon the anion species. The dissolution mechanism of hydrous oxidewas discussed from electrochemical point of view. The dissolution reaction may be regarded as consisting of the following two coupling and coupled reactions.

Volatile Corrosion Inhibitors

Abstract: 1. は じ め に 気化性 さび止め剤 とは, Volatile Corrosion Inhibitor (VCI)な い しはVapor Rust Inhibitor(VRI)と い う英語 に対応 す る日本語 として, 最 も一般的 に用 い られ ている ものであ る。 もちろん, これ らの英語に対 しては さまざ まな 日本語訳 が考え られ るが, 気化性 さび止 め剤 は 日本 工 業規格に も規定 された言葉 であ り, 本稿 では この訳語 を採用 す ることと した。 気化性 さび止め剤 とは, 常温 で気 化性 を もつ, 金 属の 腐食抑制剤を総称 してい う言葉 であ り, そ の形態 は必ず しも固体 とは限 らず, 液体 であって も良い。それが他の 腐食抑制剤 と異 なる最大 の特 徴は, 密閉空間内の抑制剤 とは離れた 位置 にある 金属表面 まで 蒸 気 と なって 移行 し, それ を保護す ることにあ る。 この特徴をいか し, 金 属製 品の保護包 装, いわゆ るさび止め包装に使用 され る こ とが圧倒的に多いが, その際には, 包装材料に塗布 ま た は含浸 した形で 用 い るか, あ るいは 粉体 のま ま用 い る。それ 以外の使 用方法 としては, 水に溶解 し, ボイ ラ ー とか水 圧機 な どの, 水 と接 してい ない部分 を も含 めた

Membrane Potentials of Nickel Hydroxide Precipitate Membranes

Abstract: Measurements of membrane potentials across nickel hydroxide precipitate membranes have been made to estimate the transport number of anions, the order of selectivity for anions, and the fixed charge concentration. The membranes show a high selectivity for anions; from the measurements of the membrane potentials, the transport number for anions evaluated to be 0.98 in sodium chloride, nitrate, perchlorate, and sulphate solutions. The order of the selectivity for anions determined with the biionic potentials is OH->SO42->Br->I->Cl->NO3->ClO4-. This order of the selectivity is the same as that of the mobility in aqueous solutions, but the difference in the selectivity for these anions is greater in the membranes than in aqueous solutions. Nickel cations on nickel hydroxide precipitates constitute the fixed ion matrix characterizing the membranes as an anion exchanger, and its concentration is determined to be about 0.4g equiv./liter by comparing the experimental membrane potentials with the theoretical values according to the fixed charge theory of membranes.

Effect of C and P Addition on the Corrosion of Steel by Molten Zinc

Abstract: Immersion tests of 0.004-0.20% C steels containing less than 0.002% P and 0.18-0.20% C steels alloyed with 0.002-0.031% P were carried out at 460-540C in molten zinc. Vacuum melted specimens were used to avoid the influence of other elements, for example, silicon. After dipped in liquid zinc for a given time, specimens were pickled in 10% HCl and weighed to measure iron loss. Addition of carbon up to 0.20% lowered the height of the maximum peak at 500C, which appears generally in the curve expressing the relationship between iron loss and temperature. The reaction rate at 500C followed a linear law for 0.004% C steel, but was almost parabolic for 0.20% C steel. The o1P layer of the former was completely broken up at grain boundaries, which is responsible for the linear attack. On the contrary that of the latter was stable and possessed protective property. According to scanning electron micrographic examinations the o1P layer formed on 0.20% C steel at 460C is hardly destroyed on etching in a vital solution. Carbon was found to be distributed in alloy layers as mixed carbide, Fe3ZnCx. In view of the fact that the break-up is caused by both the chemical reaction with molten zinc and the shear stresses existing at grain-boundaries, it can be concluded from above mentioned observations that precipitated carbides improve the grain-boundary strength to prevent fracture. Since a continuous g layer protects o1P layer, carbon has no effect at 460C. At 500C due to the absence of g layer o1p layer reacts with molten zinc. 0.004% C steel is attacked violently, whereas 0.20% C steel shows good corrosion resistance. Phosphorus added to 0.18-0.20% C steels accelerated corrosion at 500C. In other words a high concentration of phosphorus, which makes grain-boundaries fragile, cancels the effect of carbon.

Pitting Corrosion of Aluminum in Sodium Chloride Solution and Analyses of Surface Films

Abstract: The anodic polarization behavior and time dependence of anodic current of 99.99% aluminum in 0.5N NaCl solution (pH 4, 6, 8.5, 10 and 12) were examined potentiostatially. The surface structure and the composition of corrosion product films formed on the surface were also observed by a scanning electron microscope, X-ray microanalyzer and energy dispersion type X-ray spectrograph, and the pitting mechanism of pure aluminum was discussed. Pitting corrosion of pure aluminum in 0.5N NaCl at pH 8.5 seemed to be caused by Clwhich enters through cracks at the interface between the surface film and a rhombic corrosion product formed at less noble potentials than the pitting potential. This rhombic substance was observed only at pH 8.5, and was not formed at pH 4, 6, and 10 in 0.5N NaCl solutions. In 0.5N NaCl solution at pH 12, the pitting corrosion was affected by chemical dissolution, thus the pitting potential became rather noble. The limiting current observed in the anodic polarization curves in the solutions of pH 8.5, 10 and 12 seems to be controled by the diffusion of OH-. The potential at which pits are induced for pure aluminum in 0.5N NaCl was about -0.743V at pH 4, -0.757V at pH 6, -0.760V at pH 8.5, and -0.710V at pH 10. The main components of the rhombic corrosion product were ascertained as

The Passive Oxide Films on Cobalt in Weakly Alkaline Solution

Abstract: Anodic oxide film formed on cobalt in sodium borate solution at pH=11.0 have been investigated by coulometry and potentiometry. The potential change during galvanostatic oxidation exhibits three plateaus before the potential reaches a steady state value for oxygenevolution reaction. In the first plateau Co(OH)2 is formed on cobalt. Further oxidation at constant current leads to a potential rise which accompanies the formation of Coo layer between the cobalt metal and the Co(OH)2 film. In the second plateau a reaction of Coo. nH2O to Co3O4 occurs with a coulomb equivalent to the reaction. This Co3O4 film is oxidized to Co203.mH2O in the third plateau, the potential of which is 0.575V (she) not depending on the current densities and probably corresponds to the equilibrium potential of Co304JCoOOH. This change of Co304 to Co203.mH2O, however, does not complete during the potential arrest at the third plateau. From the galvanostatic-cathodic reduction of the films formed at constant potential and at constant current, the primary passive film formed in the potential range from -0.455Vto+0.22V (she) is estimated to be Coo.nH2O, the secondary passive film in the range from +0.22 V to +1.1 V to, be mainly Co304, and the transpassive film at potentials more positive than 1.1 V to be Co304 and Co2O3.mH2O. In the potential region of the secondary passivity (0.2V to 0.45V), both Cob. nH2O and Co3o4 are formed under potentiostatic conditions and the ratio in amount of Co3O4 to Coo.nH2O increases with rising potential. In this potential region, CoO.nH2O is first formed and then gradually changes to Co3o4 at constant potential.

Corrosion of Metallic Materials in Liquid Alkali Metals (II)

Abstract: 液 体 アル カ リ金属 の精製に用い られ る コール ドトラ ッ プは, 温 度が低 くな るほ ど不純物 の溶解度が小 さ くなる 性質を利 用す るもので, 金網, バ ッフルな どを 内蔵 した タ ンクに液体 アル カ リ金属を連続的に流 しなが ら, m. p. に近 い適 当な温度 に制御 して不純 物を トラ ップす る。 コ ール ドトラップ温 度で溶解度が大 きい ものを除 いた多 く の不純 物の除去に適 してい るが, と くにNa20の よ うに m. p. 近 くで溶解度が小 さ くな る塩型化合物 の除去に有 効 であ る。 また, ホ ッ トトラ ップは, 活性が強 い金属 の 高温 におけ るゲッター作用を利用す るもので, コール ド トラ ップに よる精製 では不 十分な場合に用い られ る。 ゲ ッター材にNa系 ではお もにZr箔 が用 い られ, Li系 で は状況に応 じてTi, Zr, V, Nbな どを使い分け る必要が あ る。特殊 な用途 に蒸 留に よる精製 も行なわれ るが, 一 般的な方法 ではな い。最近, CTRの 液体Liブ ランケ ッ トで生成す るLiTの 溶融塩抽 出が研究 され てい るが, LiTと 同様に塩型化合物 であ るLi2O, Li3Nお よびLi2C2 はLiよ り溶融塩 との親和 力が強 いか ら, 溶 融塩抽出が Liの 精製 法 として発展す ることも考え られ る21)。 8.2 液体Naお よびLi中 の不純 物の定量