Chromium, passive state

The current-potential relationship ABCDE, as obtained potentiosta-tically, has allowed a study of the passive phenomena in greater detail and the operational definition of the passive state with greater preciseness. Bonhoeffer, Vetter and many others have made extensive potentiostatic studies of iron which indicate that the metal has a thin film, composed of one or more oxides of iron, on its surface when in the passive state . Similar studies have been made with stainless steel, nickel, chromium and other metals... 

For a number of metals the oxidizing action of air oxygen is sufficient to produce the passive state. In their air-oxidized state, metals such as tantalum, titanium, and chromium are very stable in aqueous solutions. 

Metals of the iron group and chromium can be made passive by heating in air, and a semi-passive state, in which the metal exhibits an electrode potential between the values for the completely active and completely passive conditions, can be induced in these metals, as well as in molybdenum, tungsten and vanadium, by mere exposure to air. The corrosion resisting properties of stainless steel and of chromium plate are undoubtedly due to passivity resulting from exposure to air. 

It was mentioned above that a chromium anode continues to dissolve in the passive state forming chromate ions in this case the invisible oxide film may be suflSciently porous to allow ions to penetrate it alternatively, the oxide film may become oxidized to C1O3 which dissolves to form chromate, but is immediately regenerated by oxidation of the anode. 

Chemical analysis of the solutions after anodic dissolution have shown that the oxidation state of chromium in the dissolution products depends on the alloy composition and, correspondingly, on the potential of alloy dissolution. At potentials less positive than the potential of the onset of pure-chromium passivity breakdown, chromium dissolves from the nickel-based alloys as Cr(III). The alloys with chromium contents of not more than 15% dissolve in this manner in NaCl solution. At higher Ea, chromium from the alloy dissolves, for the most part (about 90%), in the form of Cr(VI). This is true for all alloys in Na2SC>4 (or NaNC>3) solution and for the alloys containing more than 25% chromium in NaCl solution. 

The anodic polarization of a given alloy base metal such as iron or nickel is sensitive to alloying element additions and to heat treatments if the latter influences the homogeneity of solid solutions or the kinds and distribution of phases in the alloy. The effect of chromium in iron or nickel is to decrease both EpP and icrit and hence to enhance the ease of placing the alloy in the passive state. The addition of chromium to iron is the basis for a large number of alloys broadly called stainless steels, and chromium additions to nickel lead to a series of alloys with important corrosion-resistant properties. 

Polarization curves for iron, chromium, and alloys with 1, 6, 10, and 14 weight percent (wt%) chromium in iron are shown in Fig. 5.24 the environment is 1 N H2SO4 at 25 °C (Ref 21). Iron and chromium are body-centered-cubic metals, and the alloys are solid solutions having this structure. The passivation potential (Epp), the active peak current density (icrit), and the passive state current density (ip) are decreased... 

The extents of the passive potential regions have been reduced for all materials except pure chromium, and the curves for 90 and 100 wt% nickel indicate that an active-to-passive state transition no longer occurs. The magnitude of the influence of the chloride ions is emphasized by comparing the current densities for each alloy at 200 mV (SHE) with and without chloride ions present. 

W. Hittorf concluded that the passivity of chromium is not due to the formation of an oxide film, but rather to the metal assuming a different electrical. state the metal in the passive state is in a strained or coerced condition— Zwangzustand—so that instead of dissolving as a bivalent element it dissolves as a sexivalent element. The film hypothesis, discussed in connection with the passivity of iron, best fits the facts. C. W. Bennett and W. 8. Burnham stated that the film is best regarded as a film of oxide which is rendered stable by adsorption into the metal. The oxide is usually unstable, but becomes stable when adsorbed by the... 

The minimal residual anodic current at the electrode in the passive range is caused by the slow dissolution and constant reformation of the passive film. The passivation potential and the dissolution in the passive range differ for each metal or alloy. Of economic significance are metals with a moderate passivation potential and minimum dissolution in the passive state. Figure 20.18 shows current-potential curves of iron and chromium in sulfuric acid. 

Metals in the passive state (passive metals) have a thin oxide layer on their surface, the passive film, which separates the metal from its environment. Metals in the active state (active metals) are film free. Most metals and alloys that resist well against corrosion are in the passive state stainless steel, nickel-chromium based superalloys, titanium, tantalum, aluminum, etc. Typically, the thickness of passive films formed on these metals is about 1-3 nm. 

Chromium in its passive state resists well against corrosion, even in the presence of chloride ions. However, chromium metal is usefiil only as a coating, because its brittleness renders it unsuitable for bulk applications. Chromium serves mainly as an alloying element in stainless steels. The addition of chromium to steel facilitates the eslabhshment of the passive state in neutral and acidic environments. When the... 

Stainless, high-alloyed steels have a passive surface state in seawater as in nearly all neutral or low-acidic electrolyte solutions. To achieve this passive state, steels require a chromium content of at least 12%. Passive materials show practically no surface corrosion, but if the passivation layer is subjected to a constant local disturbance they may suffer from local corrosion. 

The passivity of metals like iron, chromium, nickel, and their alloys is a typical example. Their dissolution rate in the passive state in acidic solutions like 0.5 M sulfuric acid may be seriously reduced by almost six orders of magnitude due to a poreless passivating oxide film continuously covering the metal surface. Any metal dissolution has to pass this layer. The transfer rate for metal cations from this oxide surface to the electrolyte is extremely slow. Therefore, this film is stabilized by its extremely slow dissolution kinetics and not by its thermodynamics. Under these conditions, it is far from its dissolution equilibrium. Apparently, it is the interaction of both the thermodynamic and kinetic factors that decides whether a metal is subject to corrosion or protected against it. Therefore, corrosion is based on thermodynamics and electrode kinetics. A short introduction to both disciplines is given in the following sections. Their application to corrosion reactions is part of the aim of this chapter. For more detailed information, textbooks on physical chemistry are recommended (Atkins, 1999 Wedler, 1997). 

The most important condition is that the metal must be in a passive state for pitting to occur. Passive state means the presence of a film on a metal surface. Steel and aluminum have a tendency to become passive, however, metals which become passive by film formation have a high resistance to uniform corrosion. The process of pitting destroys this protective film at certain sites resulting in the loss of passivity and initiation of pits on the metal surface. It may be recalled that passivity is a phenomenon which leads to a loss of chemical reactivity. Metals, such as iron, chromium, nickel, titanium, aluminum and also copper, tend to become passive in certain environments. 

For example, in experiments on chromium-nickel steel plasticization by means of anodic polarization within the region of passive state potentials, it obviously prevails over possible manifestations of the barrier effect [53]. A linear dependence of hardness loss on the logarithm current density has been established over all the ranges of active and passive state potentials (Fig. 9.2). This points to the predominant role of chemomechanical effect despite the formation of passive film which is transparent for dislocations. 

When protective films are easily formed on the metal surface by exposure to air, the metal is naturally in the passive state and corrosion resistant. Many metals, however, have to be protected by alloying with one of the naturally passive metals, e.g. chromium or aluminium. In other words, the naturally passive metals have very low passivation current they form their protective coating rather quickly when current is applied to them. When they are alloyed with corroding metals, they lower the passivation current of the latter and help to form a pore free inhibitive oxide layer. 

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