Inclusions

3.2.1 SPM measurements

Figure 7 presents both an STM and a dl/ds picture of DIN 1.4301 obtained in 0.01 M H2SO4 at a potential where no passive layer is formed on the surface. The spectroscopic image clearly shows four inclusions (dark spots), which are not seen in the pure topographic image (left).

It is also possible to detect inclusions by dF/ds using AFM, as shown in Fig. 8. The smallest inclusion found by spectroscopic STM and AFM investigations measured approximately 5 nm in diameter, as shown in Fig. 9.

Fig. 7. STM (left) and d//ds picture (right) of DIN 1.4301 in 0.01 M H2S04 at E = 600 mV. Some inclusions are visible in the spectroscopic image (black bar corresponds to 1000 Â).

Fig. 7. STM (left) and d//ds picture (right) of DIN 1.4301 in 0.01 M H2S04 at E = 600 mV. Some inclusions are visible in the spectroscopic image (black bar corresponds to 1000 Â).

Fig. 8. AFM (left) and dF/ds (right) pictures of DIN 1.4301 in 0.01 M H2S04 at 0 E = Some inclusions are visible in the spectroscic image (black bar corresponds to 1 pm).

Fig. 9. Smallest detectable inclusions by STM (left) and AFM (right) (black bar corresponds to 1000 Â).

3.2.2 Microelectrochemical measurements

With microelectrochemical experiments the dissolution of single inclusions of stainless steels has been studied.

Measurements showed that, even in the chloride-free Na2SC>4 solution, MnS inclusions are dissolved [7]. Since the inclusions are dissolved rather slowly and no stable pitting occurs, the dissolution processes can be assigned to single inclusions quite well. Measurements at sites with inclusions are shown in Fig. 10. Two local potentiodynamic polarization curves of the steel DIN 1.4301 (0.017% S) were measured. In the case of an active 10 ^m x 5 |im inclusion, the electrochemical current shows an abrupt increase over a limited potential range (shaded areas). To avoid the dissolution of an inactive 3 jam x 3|tun inclusion in the transpassive range, the measurement was stopped at 1000 mV. Subsequent optical microscopy studies of the same area revealed that the inclusion had been dissolved during the experiment. The SEM pictures indicate the nearly complete dissolution of the oval, active inclusion (Fig. 10, top left), whereas the inactive inclusion of a rounded shape (Fig. 10, bottom) did not dissolve at all.

In the presence of chlorides an even larger number of transients could be observed, indicating that additional nucleation sites were activated. Measurements in solutions

Fig. 10. Local potentiodynamic polarization curves of DIN 1.4301 stainless steel (0.017% S) at an active (10 pm x 3 pm) MnS inclusion and an inactive (5 pm x 5pm) MnS inclusion: 1 M Na2S04, dK/di = 0.2 mV/s.

Potential [mV] (SCE)

Fig. 10. Local potentiodynamic polarization curves of DIN 1.4301 stainless steel (0.017% S) at an active (10 pm x 3 pm) MnS inclusion and an inactive (5 pm x 5pm) MnS inclusion: 1 M Na2S04, dK/di = 0.2 mV/s.

with and without chlorides showed that only in presence of chlorides could a limiting pit growth potential be observed, above which stable pit growth takes place. Obviously chlorides are necessary to stabilize pit growth.

Local potentiodynamic polarization curves of the steel DIN 1.4301 (0.003% S) in a 1 M NaCl solution showed that the pitting potential does not have a constant value. With the usual large-area technique a value of about 300 mV is obtained. Diminishing the exposed surface to an area of 50 pm in diameter leads to an increase of the pitting potential to about 1200 mV (Fig. 11(a)). The pitting potential is usually considered to be independent of the area. There exist specific values for a particular combination of material and electrolyte. However, this work shows that the pitting potential also is an area-dependent value.

Figure 11(b) )shows the polarization curves of three DIN 1.4301 stainless steel with different sulfur contents. All samples were 250 pm in diameter. The pitting potential of the purest steels was about 790 mV. An increase of the sulfur content to 0.017% S and 0.36% S decreased the pitting potential to 380 mV and 170 mV respectively. The low resistance against pitting of the DIN 1.4305 steel in comparison with the DIN 14301 steel was also proved by the examination of fastening elements in the Mont Blanc alpine road tunnel [14]. Small-area experiments showed the same effect as a reduction of the sulfur content. In both cases the number of sulfide inclusions was decreased.

Fig. 11. Local potentiodynamic polarization curves in 1 M NACI, dF/dt = 0.2 mV/s. The diameter of the microcell (a) and the sulfur content (b) are varied.
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