Passivity steel

For steel, passivation is achieved by the surface formation of a tough, adherent mixture of oxides. The passive film is primarily gamma-magnetite (y-Fe203) but also contains gamma-hydrated ferric oxide (y-FeOOH). The film thickness is perhaps 15 A to 30 A (angstrom units). 

Anodizing employs electrochemical means to develop a surface oxide film on the workpiece, enhancing its corrosion resistance. Passivation is a process by which protective films are formed through immersion in an acid solution. In stainless steel passivation, embedded ion particles are dissolved and a thin oxide coat is formed by immersion in nitric acid, sometimes containing sodium dichromate. 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

The reason for the protective action is known to lie in the fact that the nitric acid, even in a very small concentration, enforces passivation of the stainless steel. Passivation is based on the formation of a thin layer of chromium (III) oxide, which brings the attack to a virtual halt ... 

Fig- 7.65 Schematic EPR (electrochemical potentiokinetic reactivation) curves for three amounts of sensitization of an austenitic stainless steel. Passive film formed at (1). Downscans pass through maximum attack at (2). Environment 1 N H2S04 + 0.01 M KSCN at 30 °C. Curve (3) is observed if passive film continues to form on downscan. Source Ref 93... 

K. OsozawaandN. Okato, Effect of Alloying Elements, Especially Nitrogen, on Initiation of Pitting in Stainless Steel, Passivity and Its Breakdown in Iron and Iron-Base Alloys, R.W. Staehle and H. Okada, Ed., National Association of Corrosion Engineers, 1976, p 135-139... 

Other metals also can be "passivated." For example the presence of 12 percent chromium in steel renders the steel passive in an oxygenated environment because it promotes the formation of a thin but tightly-bound oxide layer. The presence of chromates in the electrolyte will encourage the formation of y-FeaOa on iron surfaces, isolating the surfaces from corrosion. 

Structures immersed in seawater. Macrocells may form between rebars reached by chlorides and passive rebars on which, for any reason, oxygen is available. Macrocell current is then controlled by the amount of oxygen that can be reduced on the passive rebars. The galvanic coupling lowers the potential on these rebars and produces alkalinity on their surface. Therefore the macrocell contributes to maintaining the steel passive. 

Very important information on steel passivation in concrete can be assessed from the measurements of anode and cathode polarization potential in the function of current density [60, 63],... 

Type of iron or steel Passivation region (V, versus SCE) Passivation state current (mAcm )... 

The effect of chlorate on the corrosion rate of Fe was studied [34] and it was reported that iron and steel corrode in 50% NaOH at 80°C at concentrations above 1% or below 0.01%. The corrosion rate decreases rapidly at concentrations of >1% and <0.01% NaQOs, because of the formation of passive surface layers (Fig. 14.31). Iron and steel passivate in concentrated NaOH at noble potentials in the presence of dissolved oxygen. However, the passive film is pitted in the presence of chloride ions [35], as shown in Fig. 14.32. 

In acidic media, the metals iron, nickel and chromium have passivation current densities that increase in the order Cr < Ni < Fe. In Figure 6.11, the anodic polarization curves for the three metals in 0.5 M sulfuric acid (25 °C) are compared. Chromium has lower values of both ip and Ep than the other two metals. By alloying increasing amounts of chromium to steel one therefore improves the corrosion resistance. Experience shows that above a chromium concentration of 12 to 13%, a steel passivates spontaneously in contact with aerated water. It becomes "stainless", meaning it does not rust easily. Figure 6.12 gives the corrosion potential of different... 

Carbon steels rust when they are in contact with humid air (Chap. 8) and therefore they are usually protected by a coating. In aqueous solutions, their corrosion rate depends on the pH (Figure 12.6). At low pH, proton reduction takes place and the corrosion rate becomes higher as the pH decreases. In neutral solution, oxygen transport controls the rate of corrosion, which therefore does not vary with pH. Finally, in an alkaline solution steel passivates and the corrosion rate decreases to very low values. This explains for example, why steel reinforcements in concrete do not deteriorate as long as the pH stays high (pH > 13) but may rust if the pH in the concrete drops to a lower value because of carbonation reactions of cement. 

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