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CeCl_3在NaCl溶液中对LC_4铝合金的缓蚀作用

By 沈长寿

Abstract

本工作研究了CeCl_3在NaCl溶液中对LC_4铝合金(Al-Zn5.0/7.0-Mg1.3/2.8-Cu1.4/2.0-Cr0.10/0.25)的缓蚀作用。LC_4铝合金在NaCl溶液中腐蚀严重,腐蚀过程中有氢气析出,许多产物沉积在溶液底部,去膜后,光学显微显示,试样表面有很深的腐蚀坑。铝合金表面有很多缺陷,无氧化膜覆盖的铝成为活性中心,在NaCl溶液中做为阳极,铝合金中活性较低的杂质组分(如金属间化合物),突出氧化膜,在NaCl溶液中做为阴极,铝合金表面活性中心交替发生点蚀,腐蚀加重。LC_4铝合金在NaCl溶液中腐蚀的阴极去极化过程是氧的去极化。活性中心铝自溶解析氢,Cl~-在活性中心吸附,促进铝阳极溶解,氧去极化的慢步聚有扩散、吸附、电化学反应,控制步骤是扩散和吸附。LC_4铝合金在CeCl_3+NaCl溶液中腐蚀速度很底,腐蚀过程中极少有氢气极出,去膜后,光学显微显示,试样表面仍很均一,没有点蚀的迹象,Ce(+3)在阴极相表面吸附、沉积,阻碍氧的吸附,抑制阴极氧还原,降低腐蚀速度,铝腐蚀产生的H_2O_2可将Ce(+3)氧化到Ce(+4),生成CeO_2:2Ce(OH)_3 + H_2O_2 = 2CeO_2 + 4H_2O在阴极相表面,H_2O_2将Ce(+3)氧化到Ce(+4),消耗H_2O_2,消除H_2O_2对铝腐蚀的加速作用,进一步减慢活性中心铝阳极溶解的速度。LC_4铝合金在LaCl_3+NaCl溶液中的腐蚀速度下降,但局部腐蚀仍较严重,对析氢并无明显影响。La(+3)在阴极相表面吸附、沉积后,阻碍氧的吸附,抑制阴极氧还原,降低腐蚀速度,La(+3)不影响铝腐蚀产生的H_2O_2还原为OH~-的过程,H_2O_2对铝腐蚀的加速作用继续存在,不影响点蚀的孕育、发展,点蚀孔内铝自溶解析氢过程也不会受到影响。The corrosion rate of LC_4 Al alloy (Al-Zn5.0/7.0-Mg1.3/2.8-Cu1.4/2.0-Cr0.10/0.25) in 0.1mol/1 NaCl solution is decreased by the addition of 1000ppm of CeCl_3·7H_2O or LaCl_3·XH_2O. Evolution of hydrogen is seen during corrosion of LC_4 Al alloy in NaCl solution. Corrosion product of LC_4 Al alloy deposits at the bottom of NaCl solution. The surfaces of specimens of LC_4 Al alloy exposed in NaCl solution show severe pitting corrosion and accumulations of a white corrosion product adjacent to the corrosion pits, It is proposed that pits start because of the presence of flaws in the oxide film exposing anodic and cathodic areas and that these pits proceed because the conditions within the pit favour dissolution of the metal. Particles of intermetallic constituent in the Al are more noble than the matrix and therefore act as cathodes. Anodic sites are simulated at breaks in the oxide film. However, new metal is constantly being exposed and corrosion is enhanced. Al dissolution is enhanced by local cathodic activity, with the evolutiom of hydrogen, The oxygen acts as cathode depolariser and the chloride ions generate and acid electrolyte within the pit. The latter forms the anode of a corrosion cell; the aluminium is dissolved by the passage of current, and converted to the hydroxide at the mouth of pit, forming a bulky nodular corrosion product. Adsorption of the chloride ions for sites on the metal surface leads to some accelaration of the Al dissolution. Slow steps of oxygen depolarization are diffusion, adsorption and electrochemical reaction. Diffusion and adsorption of oxygen are rate-determining steps. The surfaces of specimens of LC_4 Al alloy exposed in CeCl_2 + NaCl solution showed little pitting corrosion. No corrosion product of LC_4 Al alloy deposits at the bottom of CeCl_3 + NaCl solution. Adsorption and deposit of Ce(+3) on the surface of cathade phase impede adsorption and reduction of oxygen and decrease the corrosion rate of LC_4 Al alloy. Ce(+3) is oxidized to Ce(+4) by H_2O_2 produced by Al corrosion. H_2O_2, utilized to oxide Ce(+3) to Ce(+4), loses acceleration of Al corrosion. The corrosion rate of Al is greatly decreased again. The surfaces of specimens of LC_4 Al alloy exposed in LaCl_3 + NaCl solution showed severe localized corrosion and no influence to the evoluion of hydrogen. Adsorption and deposit of La(+3) on the surface of cathode phase impede adsorption and reduction of oxygen, But La(+3) doesn't affect reduction of H_2O_2 produced by Al corrosion. So pitting corrosion is sustained

Topics: 腐蚀, 缓蚀, 氯化钠, 氯化亚铈, 氯化镧, 物理化学
Year: 1992
OAI identifier: oai:ir.ciac.jl.cn:322003/35447
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