# Steel Corrosion Behavior in Light Weight Fly-Ash Based Alkali Activated Mortars
## 3 Results and Discussion
### Corrosion Behavior of Coated Steel with Mortars  
Evolution of the open circuit potential (E\({}_{\text{cor}}\)) in the M and LWM coated steel samples during exposure in tap water and in 0.2 M NaCl solution for 60 days is reported in Figure 5. Initially, at the beginning of the exposure test, both samples exhibited E\({}_{\text{cor}}\) values in the range between \(-0.15\) V vs. SCE and 0.00V vs. SCE, indicating that steel was in the passive state during mortar curing [18; 21]. Different potential behavior was observed considering the two exposure conditions. Firstly, when coated steel samples with LWM layer were exposed to tap water (the less aggressive environment with average chloride concentration of 27 mg/L [37]), E\({}_{\text{cor}}\) values fluctuated between \(-0.30\) V vs. SCE and \(-0.10\) V vs. SCE for the whole exposure (60 days), as shown in Figure 5a. This potential fluctuation could be related to the changing in pH of the solution due to the leaching of the free alkalis present in the mortars [19]. For this reason, to ensure the aggressiveness of the electrolyte, solutions were renewed every 14 days (highlighted in Figure 5 by vertical dot lines). Similar behavior was observed also for E\({}_{\text{cor}}\) values of M samples. In the latter case, a stronger value fluctuation was detected in the first 10 days of exposure, when E\({}_{\text{cor}}\) reached \(-0.40\) V vs. SCE. This is probably related to the different pore size distribution between M and LWM (as highlighted in Figure 3), in the sense that M is mainly characterized by capillary pores that are usually responsible of a higher penetration depth compared to the larger pores present in LWM [36]. In the case of exposure to chloride-containing solution, both samples exhibited almost constant E\({}_{\text{cor}}\) values between \(-0.20\) V vs. SCE and \(-0.10\) V vs. SCE within 40 days of exposure. For longer time, E\({}_{\text{cor}}\) started to decrease for both samples and reached values around \(-0.50\) V vs. SCE at the end of the exposure. In addition, LWM samples showed more scattered results probably indicating a slightly greater tendency to corrosion compared to M.  
Figure 5: Average corrosion potential values (E\({}_{\text{cor}}\)) for M (square) and LWM (circle) samples during the 60 days partial immersion in (**a**) tap water and (**b**) 0.2 M NaCl aqueous solutions. Vertical dot lines represent solution renewal every 14 days. At the end of the exposure in both the solutions, the mortar layers were removed by the steel plates to observe the steel surface and the possible corrosion products precipitated at the steel-mortar interface. Results are reported in Figure 6.  
On steel samples exposed in tap water, the extent of corrosion products was limited to few surface areas, especially when M was applied as coating. In this sample, only four surface portions, in which corrosion products have been precipitated, were detected. For LWM coated steel samples exposed in water, localized precipitation of corrosion products was observed along the steel surface together with a more extended area of corrosion products close to a sample edge. Conversely, different features were observed for steel samples exposed to chloride-containing solution. Both steel plates covered with LWM and M exhibited a more pronounced corroded surface (i.e., rust) distributed on the steel specimen, compared to tap water exposure. In particular, both samples exhibited the precipitation of rust close to a sample edge, indicating that the peripheral parts of the coated samples are more susceptible to defects that are induced by steel cutting and/or mortar preparation. Moreover, a larger extent of corroded surface was detected in M sample compared to LWM, indicating that, even if E\({}_{\text{cor}}\) values showed a similar trend (depicted in Figure 5b), the corrosion process was characterized by a faster corrosion rate for M sample.