## 3 Results and Discussion
### Spontaneous Growth of Passive Films on Stainless Steels and Inconel 600
#### 3.2.3 SKPFM Measurements  
In Figure 11, we show topographic AFM and SKPFM potential images of native films grown on AISI316 at three selected immersion times: at t = 1/2 h (before coalescence), at t = 31/2 h (oxide islands coalescence) at t = 72 h (the native film is complete). Notice that AFM and SKPFM images are taken simultaneously at the same place. As can be seen on the potential images (F, J) and contrarily to thermally grown oxide layers [21], we observe that the surface potential is very homogeneous over the entire analyzed surface (size: 10 \(\upmu\)m \(\times\) 10 \(\upmu\)m) since only a very weak spatial variation around a mean value of the surface potential (less than 10 mV) was observed for the native films grown on AISI316 and for all immersion times. This result is clearly highlighted by the different sections (see Figures 11G and 11K, respectively) along the lines of potential images Figures 11F and 11), respectively. The same results (a very weak spatial variation around a mean value of the surface potential) are obtained for Inconel 600 as for AISI 404 304 samples. Therefore, we can plot the mean value of the surface potential as a function of immersion time, as shown in Figure 12D,E,F for the three substrates. As explained in the literature, the SKPFM techniques give direct access to the work function difference between the sample surface and the tip. By taking some care, this technique allows for a chemical analysis of the surface composition at the micrometric scale. The results of surface potential measurements as a function of the square of immersion time (t\({}^{\
icefrac{{1}}{{2}}}\)) are shown in Figure 12 for all substrates. Clearly, in every case, the bare substrate has much higher potential (410 mV for AISI304 and AISI316, 920 mV for Inconel 600). It seems clear that the formation of the first small oxide grains at t = 1/2 h conduct to a significant decrease of the potential value until coalescence is reached. Afterward, the potential increases again, reaching a maximum of 360 mV (320 mV for AISI316) at t = 24 h and decreases slowly to 250 mV at t = 72 h for AISI304. For AISI316, the maximum is reached later at t = 4 h, and the potential decreases at around 200 mV, then stabilizes. The potential evolution is different for Inconel 600 since, after coalescence, the potential stabilizes faster than for AISI304 and AISI316 to a mean value of around 250 mV. Our interpretation of the SKPFM measurements is based on the literature [49; 50]. Thus, at t = 1/2 h (early stages of passivation), the significant decrease in Volta potential seen in Figure 12 was caused by the growth of small grains of iron oxide. The high value of the potential observed in our images is due to the presence of a chromium-enriched phase on the surface of the alloys. However, it is important to remember that the SKPFM technique only probes the first nanometer of the surface. This is why we attribute the cause of the decrease in the value of the surface potential--after longer immersion times (t >24 h for AISI304 and Inconel 600 and t >3 h 1/2 for AISI 316) to the obtaining of a compact homogeneous film in which the iron oxide layer progressively covers the chromium oxide. This explanation is consistent with the literature description of passive film growth mechanisms [19; 20]. The films formed have a bilayer structure: the inner layer, which is in direct contact with the alloy, is highly enriched in chromium oxide and the outer layer, which is in contact with the electrolyte, is essentially composed of iron oxide. Figure 11: _Cont._  
Figure 11: (Large scale (10 \(\upmu\)m \(\times\) 10 \(\upmu\)m) topographic AFM and corresponding SKPFM images for three different immersion times measured for the AISI316 sample.