3.2.1. Epifluorescence Microscopy

After 7 days of incubation, microorganisms were regularly found on the metal coupon surface. Biofilms covered the damaged area. Active cells (green) were mostly found in and around the pits. The outer part of the biofilm is inactive because the surface was often covered by red (inactive) cells (Figure 10). This means that microorganisms initially attached to the entire surface but could only grow in limited areas where they could form a biofilm. These preferred spots for microbial attachment may contain the right (metallic) nutrients to encourage the growth of bacteria or deliver attractive sites for attachment. The electrons given out by iron at a corrosion spot can be harvested by an SRB film via extracellular electron transfer, which accelerates the cathodic depolarization [29]. In such locations they play a role in the local corrosion process.

**Figure 10.** Micrograph of metal coupon (S9) after 7 days of exposure (before cleaning). A biofilm was detected on the metal coupon surface. Active cells (green) were mostly found in the pits whereas red (inactive) cells were located around the pits.
