Passivation critical current density

As discussed in Chapter 4, the passivation potential and the passivation critical current density of stainless steel alloys depend upon the stabdity ofCr203. At temperatures higher than 1000-1200 °C, Cr203 forms volatile Cr03, which drastically decreases the alloy resistance to oxidation and its protectiveness [9,10]. The vapor pressure for oxides of refractory metals above 1000 °C are presented in Fig. 11.2 as Arrhenius plots of log (Pmo) vs. 1/T at constant oxygen pressure or log(po2) [8]-... 

Initially, the curve conforms to the Tafel equation and curve AB which is referred to as the active region, corresponds with the reaction Fe- Fe (aq). At B there is a departure from linearity that b omes more pronounced ns the potential is increased, and at a potential C the current decreases to a very small value. The current density and potential at which the transition occurs are referred to as the critical current density, and the passivation potential Fpp, respectively. In this connection it should be noted that whereas is determined from the active to passive transition, the Flade potential Ef is determined from the passive to active transition... 

Fig. 1.41 Schematic anodic polarisation curves for a passivatable metal showing the effect of a passivating agent that has no specific cathodic action, but forms a sparingly soluble salt with the metal cation, a without the passivating agent, b with the passivating agent. The passive current density, the active/passive transition and the critical current density are all lowered in b. The effect of the cathodic reaction c, is to render the metal active in case a, and passive...

The ease with which stainless steels can passivate then increases with the level of chromium within the alloy and so materials with higher chromium content are more passive (i.e. conduct a lower passive current density) and passivate more readily (i.e. the critical current density is lower and the active/passive transition is lower in potential). They are also passive in more aggressive solutions the pitting potential is higher. 

In the case of the stainless steels, or other readily passivated metals, the rapid reduction of dissolved oxygen on the freely exposed surface will be sufficient to exceed the critical current density so that the metal will become passive with a potential greater than whereas the metal within the crevice will be active with a potential less than. The passivation of the freely exposed surface will be facilitated by the rise in pH resulting from oxygen reduction, whilst passivation within the crevice will be impeded by the high concentration of Cl ions (which increases the critical current density for passivation) and by the H ions (which increases the passivation potential E, see Section 1.4). 

For many metals the critical current density for passivation (/ ,) increases with increasing pH of the solution ... 

In de-aerated 10sulphuric acid (Fig. 3.45) the active dissolution of the austenitic irons occurs at more noble potentials than that of the ferritic irons due to the ennobling effect of nickel in the matrix. This indicates that the austenitic irons should show lower rates of attack when corroding in the active state such as in dilute mineral acids. The current density maximum in the active region, i.e. the critical current density (/ ii) for the austenitic irons tends to decrease with increasing chromium and silicon content. Also the current densities in the passive region are lower for the austenitic irons... 

The significance of the Flade potential Ef, passivation potential pp, critical current density /pn, passive current density, etc. have been considered in some detail in Sections 1.4 and 1.5 and will not therefore be considered in the present section. It is sufficient to note that in order to produce passivation (a) the critical current density must be exceeded and b) the potential must then be maintained in the passive region and not allowed to fall into the active region or rise into the transpassive region. It follows that although a high current density may be required to cause passivation ) only a small current density is required to maintain it, and that in the passive region the corrosion rate corresponds to the passive current density (/p, ). 

Table 10.32 Effect on critical current density and passivation potential on alloying nickel with chromium in In and IOn H2SO4 both containing 0-5N K2SO4 (after Myers, Beck and Fontana")...

Nickel (%) Critical current density ( crii.. Am ) Passivation potential [Pg.263]

Table 10.33 Critical current density and current density to maintain passivity of stainless steel (Fe-18 to 20Cr-8 to l2Ni) in different electrolytes (after Shock, Riggs and Sudbury )...

Alloy Acid concentration Temp. ( C) Critical current density ( crit.. Am-2) Current density to maintain passivity (ip a.< Corrosion rate (mm y ) Unprotected protected Passive potential range (V)... 

Example 5 A stainless steel pipe is to be used to convey an aerated reducing acid at high velocity. If the concentration of dissolved Oj is 10 mol dm (10 mol cm ) calculate whether or not the steel will corrode when (a) the acid is static, (b) the acid is moving at high velocity. Assume that the critical current density for passivation of the steel in the acid is 200/iAcm the thickness of the diffusion layer is 0-05 cm when the acid is static and 0-005 cm when the acid flows at a high velocity assume the diffusion coeffi-... 

Figure 10 Polarization curve for Type 302 stainless steel in 0.5% HC1. Note the presence of a cathodic loop on the return scan due to the greatly reduced passive current density. Also, note the lowered critical current density on the reverse scan due to incomplete activation of the surface. (From Ref. 9.)...

Fig. 17M Schematic representation of the corrosion and passivation of iron in sulfuric acid. The primary passivation potential and the corresponding critical current density for corrosion i are shown. Breakdown of the passive film occurs at potentials more positive than E. ...

After a first sweep towards the positive which is not shown in the diagram and which is dominated by the dissolution of the airfoimed oxide layer, a sweep in the positive direction starts at the negative potential end of the cathodic part of the curve. In the first part, from A to the corrosion potential B where the curve becomes anodic, Hj evolution is the most important process. In this region both samples are very similar. The corrosion potential at B is nearly the same for unimplanted, with Cr implanted and with Ar bombarded iron. From B to C the anodic dissolution of the metal takes place and at C the active to passive transition starts. Here one observes the most significant difference between the two samples. The critical current density for passivation of implanted iron is more than one order of... 

Fig. 32 shows a comparison of the critical current density for passivation (point C of Fig. 31) plotted against the percentage of Cr in the alloy. 

Fig. 32. Critical current density for passivation as a function of percentage of Cr in Cr-Fe alloy (after ). o conventional Cr-Fe alloys, a implanted alloys (Cr in Fe)...

The major alloying element contributing to resistance to pitting corrosion in iron- and nickel-base alloys is chromium. The effect of chromium in reducing both the critical current density and the passivating potential of iron in 1 N H2S04 is shown by the polarization curves of... 

Fig. 7.24 Relation between the pitting potential of 17 wt% Cr, 16 wt% Ni steels with elements shown in 0.1 N NaCl + 0.25 N Na2S04 and the critical current density for passivation in 1 N H2S04 + 0.05 N NaCl at 40 °C. Source Ref 36...

Success of the CCS model has been demonstrated by a large number of experimental measurements of low crevice pH values in a variety of alloys [9-14], including a rationalization of the effects of alloying elements in stainless steels by Oldfield and Sutton via the effect of these alloying elements on the pH needed to achieve a critical current density for passivation of 10 pA cm [39]. 

---