Issue 50

A.G. Lekatou et alii, Frattura ed Integrità Strutturale, 50 (2019) 423-437; DOI: 10.3221/IGF-ESIS.50.36 427 The majority of the anodic polarization curves in Fig.1a form negative hysteresis loops (i.e. higher current densities at reverse scanning in comparison with the forward scanning for the same potential); hence, it is suggested that 304L has suffered from localized corrosion. Yet, all voltammograms except those corresponding to 0 and 25 wt.% FA, present anodic-to-cathodic transition potential (E a/c tr ) values that are higher than or almost equal to the corrosion potential (E corr ) values; this indicates nobler or equally noble surfaces at E a/c tr than or to those at E corr (forward polarization). Fig.1a reveals comparably large surface areas of the negative hysteresis loops at 0 and 25 wt.% FA, evidence of marked localized corrosion. All forward anodic portions except that of 0 wt.% FA include regimes of significant current density decrease; furthermore, the current limiting current densities are lower than 0.1 mA/cm 2 , implying surface films of low conductivity. Fig.1a shows that the corrosion potential (E corr ) increases as the FA content increases, indicating nobler steel surfaces with FA. This increase has previously been explained by the pH increase with FA [34]. The pH increase with FA could be the out- come of hydration reactions of the Ca-silicate components of FA that readily yield OH - ions when reacting with water [41]: 3CaO·SiO 2 + 3.90H 2 O → 1.68CaO·SiO 2 ·2.58H 2 O + 1.32Ca(OH) 2 (2) (Here it should be noted, that the Ca(OH) 2 amount of the solution produced by hydration should be counterbalanced by the Ca(OH) 2 consumption from other phases in the FA [32]. However, in the low pH electrolyte, the Ca 2+ content of the electrolyte is not enough to provide the necessary Ca(OH) 2 to react with the phases of FA). Fig.1a also reveals that addition of FA (up to 20 wt.%) has resulted in a shift of the forward polarization curves to lower current densities. Moreover, the anodic curves corresponding to 10-20 wt.% FA present a good resistance to localized corrosion as suggested by the small surface areas of the negative hysteresis loops or even the positive hysteresis loops. This decrease in the corrosion kinetics is expected considering the decrease in the H + concentration of the electrolyte with FA increasing. However, the trend of corrosion resistance increasing with FA content is reversed at 25 wt.% FA. Not only is the hysteresis negative but the hysteresis loop presents a surface area markedly larger than those of the 10-20 wt.% FA anodic voltammograms. In addition, the current densities of the “25 wt.% FA” anodic voltammogram appear notably higher than those of the 20 wt.% anodic voltammogram, implying accelerated corrosion kinetics. Cyclic polarization in the high pH electrolyte Fig.1b reveals a different corrosion status than that in the low pH electrolyte. All anodic curves reveal passive regimes of high potential range and very low current density values, much lower than 0.1 mA/cm 2 , evidence of true passivity. Reverse polarization has led to positive hysteresis for the “0-20 wt.% FA” voltammograms, and a slightly negative hysteresis for the “25 wt.% FA” voltammogram. The positive hysteresis is more extensive (in terms of loop surface area) in the cases of 15 and 20 wt.% FA. All curves present E a/c tr values nobler than the respective E cor values (or equally noble in the case of 25 wt.% FA). Therefore, it is concluded that localized corrosion has not taken place. Regarding the case of 25 wt.% FA, the very low current values during the passive regime, imply that localized corrosion is not an issue either. Nevertheless, the relative trends of the voltammograms with respect to the FA contents are very similar to those observed in the low pH electrolyte, namely: increase in the corrosion resistance with FA increasing up to 20 wt.% FA and deterioration at 25 wt.% FA. The latter is implied by the shift of the current densities to lower values, the slightly negative hysteresis and the insignificantly lower E a/c tr value as compare to the respective E cor value. Fig.1b shows that the corrosion potential increases as the FA content increases, as in Fig.1a. The increase in E cor with FA (up to 20 wt.% FA) can be justified by the respective increase in the pozzolanic activity. Nobler half-cell potentials of steel rebars embedded in 3.5 wt.% NaCl have been attributed to higher pozzolanic activity [25]. In the “25 wt.% FA” case, 304L exhibits “worst” passive behavior than that corresponding to the “20 wt.% FA” case, in compatibility with the trends in the low pH electrolyte. However, conversely to the steel in the low pH environment, the steel has not suffered from localized corrosion. The most plausible explanation for the relatively high conductivity of the passive film in the case of 25 wt.% FA is the extensive deposition of highly hydrated films on 304L, as described in Intro- duction. Moreover, the particular FA, due to its high Ca and fineness, has a high ability for water binding [25,32]. At this point, it is considered appropriate to make a comment on the insensitivity of pH to the FA addition: It seems that the aforementioned counterbalance between the Ca(OH) 2 amount generated from reaction (2) and the Ca(OH) 2 consumed from other phases in the FA [32] exists in the high pH electrolyte, resulting in pH values that do not change with the increase in the FA content. Also, retainment of the pH of the solution at 11.7-11.8 regardless of the FA content suggests that the pozzolanic reaction occurrence was maintained during the polarization tests. This suggestion is based on the fol- lowing consideration: For the initiation and maintenance of the pozzolanic reaction, a sufficient quantity of Ca 2+ and a high pH (>12) are necessary, since at the pH of ~12, the Si 4+ and Al 3+ ions have sufficient solubility to support the pozzolanic reaction [42].