Seeking information on the properties of metal passivation

The general laws of electrochemical corrosion kinetics indicate that when a metal dissolves following the normal anodic reaction pathway, the higher the electrode potential, the greater the rate of metal dissolution. This is the case for nickel and iron when subjected to anodic polarization in hydrochloric acid. However, in many cases, opposite results can also be observed. If the electrode potential of a metal shifts in the positive direction due to an applied anodic current or a local anodic current, beyond a certain value the rate of dissolution of the metal decreases sharply. This phenomenon can be observed when iron and stainless steel are anodically polarized in sulfuric acid. This “anomalous” phenomenon during the dissolution of a metal anode is known as the passivation process of the metal. 　　If an iron sheet is placed in it, it will dissolve rapidly, and the dissolution rate of iron increases sharply as the concentration rises. When the concentration reaches 30–92%, the solubility reaches its maximum value; however, if the concentration is increased further (above 50%), the solubility of iron drops by a factor of ten thousand, resulting in a special state of surface treatment. At this point, even if it is transferred to sulfuric acid, it will no longer be corroded by the acid, as the metal has become passivated. 　　If the medium contains strong oxidizing properties that cause the metal to become passivated. They are collectively referred to as passivators, although the occurrence of passivation does not depend solely on the strength of the oxidizing power of the passivator. Obviously, this is related to the effect of anionic pairs on the passivation process. 　　Although the occurrence of passivation is usually associated with the action of oxidizing media, some metals can undergo passivation in non-oxidizing media. Molybdenum and niobium can be passivated in hydrochloric acid, while mercury and silver can be passivated in the presence of chloride ions. 　　In summary, if the passivation phenomenon occurs as a result of the natural interaction between the metal and the passivator, it is referred to as \"chemical passivation\" or \"autopassivation\". Metals such as chromium, aluminum, and titanium are easily passivated by oxygen in air and in many oxygen-containing solutions, which is why they are known as \"self-passivating metals\". 　　Experimental results show that in electrolyte solutions free of active chloride ions, the passivation of metals can also be induced by anodic polarization; for example, 18-8 type stainless steel dissolves rapidly. However, when an external current is applied to cause anodic polarization, and the potential is polarized to -0.1V (SCE), the dissolution rate of the stainless steel drops sharply to a fraction of tens of thousands of its original value. Moreover, it maintains a high level of stability within the range of 0.1 to +1.2 V; this phenomenon is known as \"anodic passivation\" or \"electrochemical passivation\". Metals such as iron, nickel, chromium, and molybdenum can all undergo electrochemical passivation due to anodic polarization. 　　“\"Anodic passivation\" and \"chemical passivation\" are essentially the same, as both phenomena occur due to some kind of change in the surface of the metal that was originally activated and dissolved. This mutation causes the anodic dissolution process of the metal to no longer follow the Tafel law, and its dissolution rate drops sharply as a result. Therefore, passivation refers to such a sudden change in the surface condition of a metal. The state in which a metal is after passivation is called the passive state, and the properties possessed by a metal in its passive state are known as passivity. 　　There is also a term called \"mechanical passivation,\" which refers to the deposition of a thick, yet more or less porous layer of salts on the surface of a metal in certain environments. This layer of salt, which is usually non-conductive, actually acts as a mechanical barrier to separate the reactants, thereby reducing the rate of metal corrosion. Such passivation phenomena clearly do not require the metal’s electrode potential to shift in the positive direction. Even when the solubility product of salt is very low, the electrode potential can still shift in the negative direction. This is the case with lead in sulfuric acid, magnesium in aqueous solutions, and silver in chloride solutions. 　　Studying the passivation phenomenon has great practical significance. Since a metal in a passive state has a very low dissolution rate, it can potentially be used to slow down metal corrosion. By adding certain metals that are prone to passivation to iron (such as chromium, nickel, molybdenum, titanium, etc.) to produce various stainless steels, these materials become highly resistant to passivation in strongly oxidizing environments; thus, such alloy steels can be used as substitutes for precious metals in the manufacture of chemical equipment that comes into contact with strongly oxidizing media

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