Critical p H and Critical Current Density for Passivity in Metals
Critical p H and Critical Current Density for Passivity in Metals
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金属钝化的临界 p H 和临界电流密度
DOI：
10.1149/1.2428081
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发表时间：
1961
期刊：
影响因子：
--
通讯作者：
H. Uhlig
中科院分区：
文献类型：
--
作者：
H. Uhlig
基于电化学机理，导出了pH与钝化金属所需的临界电流密度的关系式：i（crit.）其中K和k是常数。因此，log i（crit.）Cr、Ni和14% Cr-Fe合金的pH值均较低。当由于溶解氧而发生钝化时，表明i（crit.）通常等于O~的极限扩散电流密度。存在一个临界pH值，在该值以上钝化是稳定的，但在该值以下则不然。该临界pH值随溶解O3浓度的对数线性下降。14% Cr-Fe合金临界pH值的计算值与实测值符合较好。在不存在O~的情况下，当H~的还原速率超过i（crit.）时，达到钝化。这类金属在相同溶液中的Flade电位必须比氢电极更活跃。更积极的Flade电位伴随着更大的热力学稳定性的钝化膜，部分原因是更大的阻力，由特定的阴离子的钝化击穿。在这类金属中，钛、钽、锆和钼在脱气酸中具有很高的耐腐蚀性。有些金属具有明显的腐蚀倾向，但与环境的反应速率很低，因此被认为是钝化的。对于某些金属，钝化是由于暴露在空气或氧化介质中而产生的;对于其他金属，钝化几乎发生在任何水溶液中。正如目前所考虑的，钝化金属的特征是，它们在钝化状态下的腐蚀电位比它们的活化电位或Flade电位更高，后者标志着将活性区与钝化区分开的电位。例如，铁的弗拉德势是根据经验确定的，它是在被动态自发衰变到主动态之前的电位停滞的最小值。一般来说，氢离子活性是在给定的水性环境中确定钝化性的变量之一，并且存在临界pH，其标志着稳定和不稳定钝化性之间的边界。这个概念首先由罗查和伦纳茨（1）在讨论铬-铁合金系的钝化性时讨论过。本文讨论了临界pH值与Flade电位之间的关系，并从极化和电位行为的角度对临界pH值进行了估算。
Based on electrochemical mechanisms, an equation is derived for the relation of pH to the critical current density necessary to passivate metals: i (crit.) K (H+)~ where K and k are constants. Accordingly, a linear relation beween log i (crit.) and pH is observed for Cr, Ni and 14% Cr-Fe alloys. When passivity occurs by reason of dissolved oxygen, it is shown that i (crit.) usually equals the limiting diffusion current density for O~. A critical pH exists above which passivity is stable, but not below. This critical pH decreases linearly with the logarithm of dissolved O3 concentration. Calculated and observed values for the critical pH of 14% Cr-Fe alloy are in reasonable agreement. In absence of O~, passivity is achieved when the rate of reduction of H~ exceeds i (crit.). Metals in this category must have Flade potentials more active than the hydrogen electrode in the same solution. More active Flade potentials accompany greater thermodynamic stability of the passive film, accounting in part for greater resistance to breakdown of passivity by specific anions. Among the metals in this category achieving high corrosion resistance in deaerated acids are Ti, Ta, Zr, and Mo.Some metals with a pronounced tendency to corrode nevertheless react with their environment at a very low rate and are said to be passive. For certain metals, passivity results from exposure to air or to an oxidizing medium; for others passivity occurs in almost any aqueous solution. It is characteristic of passive metals, as presently considered, that their corrosion potentials in the passive state are more noble than their activation or Flade potentials, the latter marking the potential which divides the active from the passive region. The Flade potential of iron, for example, is determined empirically as the least noble value of the potential arrest just before spontaneous decay of passivity to the active state. In general, hydrogen ion activity is one of the variables in a given aqueous environment determining passivity, and a critical pH exists which marks the boundary between stable and unstable passivity. This concept was first discussed by Rocha and Lennartz (1) in connection with passivity in the Cr-Fe alloy system. The present paper deals with the relation of critical pH to Flade potential and the evaluation of critical pH from a knowledge of polarization and potential behavior.