Iron, passive state

Potential-current density (E-i) curves, which have been determined for a number of the austenitic cast irons and also for the nickel-free ferritic irons, indicate that in general the austenitic cast irons show more favourable corrosion characteristics than the ferritic irons in both the active and passive states. 

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... 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

The transition from the active state to the passive state is the passivation, and the transition in the reverse direction is the activation or depassivation. The threshold of potential between the active and the passive states is called the passivation potential or the passivation-depassivation potential. Similarly, the transition from the passive state to the transpassive state is the transpassivation, and the critical potential for the transpassivation is called the transpassivation potential. Further, a superficial thin film formed on metals in the passive state is often called the passive film (or passivation film), the thickness of which is in the order of 1 to 5 nm on transition metals such as iron and nickel. 

Anodic passivation can be observed easily and clearly with iron group metals and alloys as shown in Fig. 11-10. In principal, anodic passivation occurs with most metals. For instance, even with noble metals such as platinum, which is resistant to anodic dissolution in sulfuric acid solutions, a bare metal surface is realized in the active state and a superficial thin oxide film is formed in the passive state. For less noble metals of which the affinity for the oxide formation is high, the active state is not observed because the metal surface is alwa covered with an oxide film. 

It is weD known that metallic iron corrodes violently in dilute nitric acid solutions, but metallic iron is passivated in concentrated nitric add solutions as shown in Fig. 11-14(a). This passivation of metallic iron results from a strong oxidizing action of concentrated nitric add that changes the iron electrode irom the active state to the passive state. 

O-PO4 is often used in combined preoperational cleaning and passivation programs and acts, in the presence of oxygen, to promote passivity on iron surfaces, changing it from an active state to a passive state by forming a barrier to corrosive ions (although the starting material may actually be P-PO4). 

In the discussion of E the vs pH diagram for iron in water depicted in Figure 1.70, we noted that, with application of high positive potentials, the system moves into a region of passivity and results in a reduced corrosion rate. The passive film formed should be coherent and insulating to withstand corrosion and mechanical breakdown. Upon formation of the passive state the corrosion rate is reduced. Thus by polarization and applying more positive potentials than corrosion potentials the metal attains passivity and is protected. This is the principle of anodic protection. It is necessary that the potential of passivation be maintained at all times, since deviations outside the range would result in severe corrosion. 

Cold, concentrated nitric acid renders iron passive in this state it does not react with dilute nitric acid nor does it displace copper from an aqueous solution of a copper salt. 1 +1 Nitric acid, or hot, concentrated nitric acid dissolves iron with the formation of nitrogen oxide gas and iron(III) ions ... 

Gases, also, may cause iron to assume the passive state. Compressed nitric oxide is a case in point.3 The vapours of concentrated nitric acid have for many years been known to act similarly.4... 

The production of the passive state is attributed to a disturbance of this equilibrium. During anodic polarisation of iron the more reactive or a ions dissolve with greater rapidity than equilibrium can be established, with the result that an excess of noble, inert, or passive jB ions collects on the surface of the metal, tending to render it passive. This explains why passivity is purely a surface phenomenon. Hydrogen ions, like halogens, are assumed to catalytically accelerate the conversion of a into / ions until equilibrium is re-established. 

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. 

Figure 69. Difference images of successive snapshots of the Fe surface (a) during the passive state, (b) during the passive to active transition, (c) immediately after the transition to the active state, (d)-(i) during gradual relaxation to the passive state for the oscillations shown in Fig. 68(A). The superimposed solid circle marks the boundary of the iron electrode. Changes outside this boundary are associated with noise. (After Hudson etal. )...

The Passivity of Metals. It has been known for a long time that a number of metals can exist in two states, in one of which they exhibit greater activity, at least with respect to certain types of chemical reactions, than the other. In the first case the metals are said to be in the active and in the second to be in the passive condition, the two terms being due to Schonbein1 who made some of the early studies of the phenomena. The most familiar case of passivity is that of iron. A piece of iron when placed in dilute nitric acid will normally dissolve with the evolution of hydrogen, and is in its active state. If it is transferred to concentrated nitric acid and returned once more to the dilute acid little or no reaction will take place. It is now in its passive condition. The active state may be regained by touching the surface with a piece of the active metal, and by other means. In the passive state the metal behaves as if it is more noble, L e,3 has a position lower in the electromotive series, than in the active condition. 

The anodic metal dissolution current thus increases with the electrode potential up to a certain potential, called the passivation potential. As shown in Figure 22.7 for iron in acid solution [9,10], in the potential region more positive than the passivation potential, the metal passivates into almost no dissolution current due to the formation of a surface oxide film several nanometers thick, which we call the passive film [11], In contrast to the passive state of the metal, the active state refers to the metal undergoing anodic dissolution at significant rates below the passivation potential. 

Metallic passivity was discovered as far back as 1790, when metallic iron in concentrated nitric acid was found to turn suddenly into the passive state after violent metal dissolution had occurred in the active state [24,25]. It was not until 1960s that we certainly confirmed the presence of an oxide film several nanometers thick on the surface of passivated metals [26]. Passivation was also found to occur with semiconductors in aqueous solution [27]. We may learn the latest overview on the passivity of metals and semiconductors in corrosion literature [11]. 

In the passive state, metal electrodes normally hold extremely small potential-independent dissolution current as shown in Figure 22.7 for metallic iron in acid solution. For some metals such as nickel, however, the passive state changes beyond a certain potential into the transpassive state, where the dissolution current, instead of being potential-independent, increases nearly exponentially with... 

A representative anodic polarization curve for iron in a buffered environment of pH = 7 is shown in Fig. 5.4. The solid curve is representative of experimental observations the dashed curve is an extrapolation of the Tafel region to the equilibrium half-cell potential of -620 mV (SHE) and aFg2- = 10 6. This extrapolation allows estimation of an exchange current density of 0.03 mA/m2. The essentially steady minimum current density of the passive state is ip = 1 mA/m2. 

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... 

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