Anodic polarization curves stainless steel

Fig. 4.16 Effect of alloying elements on stainless steel anodic polarization curve.

Figure 16.9 Anodic polarization curves for typical stainless and ordinary steels. At low Eh, stainless steel may become active, in which case it corrodes more rapidly than ordinary iron or steels.

The effect of pH on the polarization of iron is shown in Fig. 5.6. The effect ofpH on the polarization of type 304 stainless steel (nominally 18 to 20 wt% Cr, 8 to 10.5 wt%Ni, 0.08 wt% C maximum) in environments based on 1 M Na2SC>4 with additions of H2SO4 and NaOH to control the pH is shown in Fig. 5.31 (Ref 28). The influence of chromium and nickel in moving the anodic polarization curve of iron to lower current densities persists over the indicated pH range with the corrosion rates being very low for pH >4.0. 

Changes in anodic polarization curves with pH for type 304 stainless steel in 1 M Na2S04 solutions. Redrawn from Ref 28... 

Fig. 7.13 Anodic polarization curve showing current bursts at potentials below the breakdown potential. Type 304 stainless steel in 200 ppm chloride ion solution at room temperature, pH = 4...

Fig. 7.19 Effect of temperature on the anodic polarization curves of a modified austenitic stainless steel containing 5.6 wt% Mo in 3.5 wt% NaCl at pH = 3. Redrawn from Ref 31...

A schematic summary of the alloying metals that affect the anodic polarization curve of stainless steel is shown in Fig. 4.16. The addition of 8% nickel to an alloy containing 18% chromium forms austenitic structure SS Type 304. The addition of Mn and N increases the stability of austenitic steel. The chromium content of stainless steel affects the anodic polarization curves as shown in Fig. 4.16. Nickel promotes repassivation in a corrosive environment, but concentrations higher than 30% reduces the passivation current, the critical current density, and increases the critical pitting potential. Nitrogen... 

For stainless steels and Cr the transpassivity phenomenon is indicated by a branch on the anodic polarization curve as shown for an 18-8 CrNi steel in 1 N H2SO4 in Fig. 5.3. The reason for this indication of transpassivity is that the oxide is generally dissolved and Cr207 and possibly Fe " are formed at potentials below the region where oxygen is developed. 

Figure 7.14 Polarization curves for the CFRP-metal cells in simulated sea water. 1 = Composite cathodic curve at 25°C, 2 = composite cathodic curve at 50°C, 3 = aluminium anodic curve at 25 °C, 4 = brass anodic curve at 25°C, 5 = stainless steel anodic curve at 50°C, 6 = stainless steel anodic curve at 25°C, 7 = titanium anodic curve at 25°C. (Reproduced from [114] by kind permission of TV Chukalovskaya and lAPC Nauka )...

As mentioned in Section 6.4, some metals, such as iron and stainless steels, can also be protected by making them anodic and shifting their potential into the passive region of the anodic polarization curve (see Fig. 6.1, Section 6.2). The passive potential is automatically maintained, usually electronically, by an instrument called the potentiostat. Practical application of anodic protection and use of the potentiostat for this purpose were first suggested by Edeleanu [21]. 

L stainless steel Marginal increase in pitting potential Anodic polarization curves 10% serum (Chawla et al, 1990)... 

FIGURE 14.8. Anodic polarization curve for T pe 446 stainless steel in 10% sulfiiric acid solution at 95°C. 

FIGURE 14.14. Anodic polarization curve of 18-8 stainless steel in acidified NiSOa solution at 95"C [15). 

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... 

Figure 10.11 Variation with applied potential of the friction coefficient between an AISI430 stainless steel and an aluminium oxide pin in 0.5 M sulfuric acid. Also shown is the anodic polarization curve of the steel [5].

The chemical passivation of a rotating stainless steel cylinder immersed in an acidic solution is studied as Fe ions are gradually added to the solution. The Fe " ions are reduced to Fe " " at the limiting current as the metal corrodes. The radius of the cylinder is 2 cm, and the rotation rate is 1000 rpm. The diffusion coefficient of the Fe " ions is equal to 0.7 x 10 cm s the cinematic viscosity of the solution is v = 0.011 cm s In the absence of Fe , the anodic polarization curve indicates a passivation current density of L = 30 mA cm , independent of the rotation rate. Calculate the concentration of Fe " " required to passivate the alloy. 

Figure 1 Anodic polarization curves of the pure metals Fe, Cr, Mo, and Ni and of an austenitic stainless steel, Fel8Crl4.3Ni2.5Mo (at %) (Fel6.7Crl5.0Ni4.28Mo wt%) exposed to 0.1 M HCl + 0.4 M NaCl at 25°C. Sweep rate, 3 mV/s. (From Ref 1.)...

Fig.l. Typical anodic polarization curve of stainless steels in sulfuric acid solutions. Eoc natural corrosion potential, Ep passivity potential, Er rupture potential. 

Consider first the polarization curve (i.e., Tafel plot) for the anodic halfreaction occurring in corrosion of stainless steels (Fig. 16.8). The diagram for the active region is much the same as has been seen for other anodes (Figs. 15.4 to 15.7). As Eh is increased to a certain specific value, however, a sudden and dramatic drop in the anodic current density i occurs, corresponding to formation of an oxide film. At higher Eh, i remains constant at a very low level (the horizontal scale in Fig. 16.8 is logarithmic), and the metal has become passive, that is, effectively immune from corrosion. 

FIGURE 22.30 Schematic polarization curves of a cyhndrical crevice in an anode of stainless steel in neutral solutions of three different chloride concentrations [59] h = crevice depth, Icrev — anodic crevice dissolution current, ccr = chloride concentration in the solution bulk, crcv = crevice protection potential, and /Crev— minimum crevice dissolution current at the critical crevice (protection) potential crcv. 

Standard ASTM potentiodynamic anodic polarization plot for certification of potentiostat performance. Type 430 stainless steel in 1 N H2S04at 30 °C. Scan rate of 600 mV/h. Test curve is to lie within the shaded region. Redrawn from Ref 27... 

These factors can be discussed with reference to the polarization curves for the initial and changing conditions within the occluded region. The combined effects of a potential drop into the pit and the effect of the lowered pH, which raises Epp and increases icrit, are also analyzed by reference to Fig. 7.6 (Ref 20). As previously assumed, the solid anodic curve is taken as representative of a stainless steel in an environment of pH = 1. The dashed extension again represents the anodic polarization behavior in the absence of a passive film. At a potential, Ecorr (or Epot if the potential is maintained potentiostatically), the passive current density would be iCOrr,pass and the active corrosion current density would be iCorr,act- Assume that a small flaw through the passive film is associated with an (IR), drop that lowers the potential in the bottom of the flaw to E,. Since this potential is higher than the passivating potential, Epp, this flaw should immediately repassivate and not propagate. 

Fig. 7.14 Effect of chloride-ion concentration on the anodic polarization of type 304 stainless steel. Dashed lines indicate breakdown potentials, Eb pit. Curves A and B are schematic representations of polarization of cathodic reactions of relatively (A) high and (B) lower oxidizing strength. Based on Ref 27...

A schematic representation of downscan polarization curves using the EPR procedure is shown in Fig. 7.65 (Ref 93). A sensitized stainless steel will result in an anodic loop with size depending on the degree of sensitization. With the specified rapid downscan rate, the passive film... 

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