Representative Polarization Behavior of Several Commercial Alloys

Representative Polarization Behavior of Several Commercial Alloys 

Type 430 stainless steel (Fe, 16 to 18 wt% Cr, 0.12 wt% C maximum) is used as an ASTM standard material to certify the performance of potentiostats in accurately and reproducibly determining polarization curves (Ref 27). The environment is 1 N H2SO4 at 30 °C, and the scan rate is specified as 600 mV/h. To meet the standard, a measured polarization curve determined using the reference standard must fall within the band shown in Fig. 5.30. An advantage in using this alloy is the large 

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. 

The effects of acid concentration and temperature on the anodic polarization of a commercial nickel-base alloy (Hastelloy C, nominal composition 54 wt%Ni, 2.5 wt% Co, 15.5 wt% Cr, 16 wt% Mo, 4 wt% W, 5.5 wt% Fe, 0.06 wt% C maximum) are shown in Fig. 5.32 (Ref 29). Qualitative conclusions from these curves indicate that the changes in corrosion rate on increasing the acid concentration from 1 to 10 N should be relatively small but that the effect of increasing the tempera- 

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

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