Steel in Mortar

174 Handbook of Cathodic Corrosion Protection 5.3.2 Corrosion of Steel in Mortar 

Due to both carbonization and penetration of chloride ions, steel will pass from a passive to an active condition and (consequently) may corrode. If the mortar is completely surrounded by water, oxygen diffusion in wet mortar is extremely low so that the situation is corrosion resistant because the cathodic partial reaction according to Eq. (2-17) scarcely occurs. For this reason the mortar lining of waste pipes remains protective against corrosion even if it is completely carbonated or if it is penetrated by chloride ions. 

The cathodic effectiveness of the passive steel in cement mortar can be seen in Fig. 5-13. The cell current is measured between a mortar-coated DN 100 pipe section and an uncoated steel ring 16 mm broad as anode. It can be clearly seen that the cell current immediately falls and after 100 days goes toward zero. The same result is obtained by removing the specimens and aerating the mortar coating and repeating the experiment with the same components [51]. 

Cathodic protection can be used to protect steel in concrete (see Chapter 19). There is no fear of damage by H2 evolution due to porosity of the mortar. Local corrosion attack can be observed under extreme conditions due to porosity (water/ cement ratio = 1) and polarization (f/jq = -0.98 V) with portland cement but not with blast furnace cement, corresponding to field IV in Fig. 2-2 [53]. However, such conditions do not occur in practice. 

Anodic polarization can occur in the presence of stray currents. Oxygen is evolved on the passive steel according to  

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Figure 13.1 Statistical distribution of time to corrosion initiation of steel in mortar specimens with different admixed percentages of Ca(N02)2 inhibitor with respect to mass of cement, exposed to seawater [20]...

Laboratory studies of steel in mortar showed that by applying several intense flushings before the ingress of chlorides [6], the onset of corrosion during the test duration of 90 days could be prevented even at chloride concentrations as high as 2 % by mass of cement. A critical concentration ratio MFP/chloride greater than 1 had to be achieved, otherwise the reduction in corrosion rate was not significant [6]. MFP acts as a corrosion inhibitor in carbonated concrete as well. 

C. M. Hansson, Corrosion inhibitors in concrete. Part III Effect on time to chloride-induced corrosion initiation and subsequent corrosion rates of steel in mortar . Cement and Concrete Research, 2001, 31, 713. 

FIGURE 6.22 Corrosion of steel in mortar beam and ECC beam. (After Sahmaran, M. et al., ACIMaterials Journal, 105(3) 243-250, 2008. With permission.)... 

Since those first systems were applied in the 1970s, systems have been developed and applied to bridge decks, substructures and other elements, buildings, wharves and every conceivable type of reinforced concrete structure suffering from corrosion of the reinforcing steel. More recently systems have also been applied to steel in mortar in stone brick and terracotta clad structures. 

Steel in cement mortar is in the passive state represented by field II in Fig. 2-2. In this state reinforcing steel can act as a foreign cathodic object whose intensity depends on aeration (see Section 4.3). The passivity can be lost by introduction of sufficient chloride ions or by reaction of the mortar with COj-forming carbonates, resulting in a considerable lowering of the pH. The coordinates then lie in field I. The concentration of OH ions can be raised by strong cathodic polarization and the potential lowered, resulting in possible corrosion in field IV (see Section 2.4). 

Sample 2s, like 1 and 3-9, was obtained by sanding with the abrading tool. Sample 2p was prepared by crushing a chip in a steel percussion mortar. 

Figure 5.10 shows the corrosion rate of steel in artificially carbonated mortars in the absence and in the presence of chlorides. It can be clearly seen that the corrosion rate will be neghgible only in conditions of external relative humidity below 75 %, 60 % and even 40 % as the chloride content increases from zero to 1 % by cement mass. 

G. K. Glass, C, L Page, N. R. Short, Factors affecting the corrosion of steel in carbonated mortars . Corrosion Science, 1991, 32, 1283. 

Figure 13.2 Time to corrosion initiation of four steel bars in mortar blocks exposed to cyclic ponding with chloride solutions for different inhibitors admixed to the mortar in three dosages [11]...

Several studies indicate that the inhibitor blends are effective in solutions whereas pure solvents as dimethylethanolamine are not [1]. A commercial migrating inhibitor blend could be fractionated into a volatile (dimethylethanolamine) and a non-volatile (benzoate) component (9). For complete prevention of corrosion initiation in saturated Ca(OH)2 solution with 1 M NaCl added, the presence of both components at the steel surface in a concentration ratio of inhibitor/chloride of about one was necessary (Figure 13.3). Modern surface analytical techniques such as XPS have confirmed that for the formation of a significantly thicker and protective organic film on iron in aUcahne solutions, both components of the commercial inhibitor blend have to be present (10). Experiments with inhibitor added to mortar showed similar results the inhibitor blend admixed in the recommended dosage could delay the average time to corrosion initiation of passive steel in mor-... 

Alkanolamine-hased inhibitors have been tested in similar conditions. For ongoing chloride-induced corrosion with a chloride level of about 1-2 % by mass of cement, in mortar specimens no reduction in corrosion rate was found (Figure 13.6) except at low chloride concentrations. This is confirmed by two other studies [1,11,14] pre-corroded rebars in mortar (w/c 0.75, cover thickness 25 mm) did not show any detectable effect on the corrosion rate of embedded steel once active corrosion had been initiated, despite the fact that the specimens had low cover and porous mortar [14]. It seems that for penetrating or migrating inhibitors the favourable effects found in solution do not occur when applied to hardened mortar or concrete laboratory specimens with ongoing steel corrosion. It is thus necessary to look for information regarding the transport of inhibitor blends in mortar or concrete. 

Fields of applicability. Figure 15.3 depicts the fields of applicability of pickled stainless steels in chloride-contaminated concrete exposed to temperatures of 20 °C or 40 °C. Fields have been plotted by analysing the critical chloride values obtained by different authors from exposure tests in concrete or from electrochemical tests in solution and mortar and taking into consideration the worst conditions [11-28]. Nevertheless, it should be pointed out that values are indicative only, since the critical chloride content depends on the potential of the steel, and thus it can vary when oxygen access to the reinforcement is restricted as well as when stray current or macrocells are present. For instance, the domains of applicability are enlarged when the free corrosion potential is reduced, such as in saturated concrete. Furthermore, the values of the critical chloride Hmit for stainless steel with surface finishing other than that obtained by pickling can be lower. 

Using ordinary and sulfate-resistant Portland cement to represent differing chloride environments, short-term electrochemical monitoring and SEM were used to characterize corrosion behavior [34]. Steel electrodes attained passivity in mortar with high levels of calcium aluminate, up to 1% wt. chloride. At 1.75% wt. chloride, steel electrodes corrode. All chloride levels resulted in steel corrosion for low levels of calcium aluminate. Pore solution was also impacted by mortar exposure conditions. Atmosphere exposure had a high influence on hydroxide concentration in pore solution but no impact on chloride concentration. Carbonation was also investigated samples in a sealed container had a chloride/hydroxide ratio half that of unsealed samples. 

C. MonticeUi, A. Frignani, G. Trabanefli, G. Brunoro, A study on the inhibiting efficiency of a glycerophosphate-nitrite admixture against steel corrosion in mortars, in Proceedings of the 8th European Symposium, vol. 1, 1995, pp. 609-620. 

Milling of equimolar amounts of ninhydrin and malononitrile (Retsch MM 2000 mill, stainless steel vial 10 mL, two 12 mm balls) produced after Ih Knoevenagel product 155 in quantitative yield (Scheme 2.54) [46]. After reaction completed, product did not need any purification. Solution reaction conditions are in variance with solvent-free method which does not require any catalyst. Metwally has shown that heating of ninhydrin and malononitrile in solution (EtOH/AcOH) yielded entirely different product 156, via initial condensation of two molecules of malononitrile. Manual grinding in mortar afforded after 30min products in considerably lower yields. 

E. Zomoza, J. Paya, and P. Garces, Chloride-induced corrosion of steel embedded in mortars containing fly ash and spent cracking catalyst, in Journal of Corrosion Science, Vol. 50, 2008, pp. 1567-1575. 

The drilled holes and chases have an impact on the appearance bnt none on the profile or clearances. The act of drilling holes could require a structural evaluation although spacing of 300-500 mm minimizes structural impact in most cases. The chases between anodes for the wires can lead to a join the dots appearance (see Figures 1 and 5A). Placing anodes in mortar joins of masonry or brick clad steel framed buildings gives minimal visual impact. 

The addition of 0.53 1.1 % v/v short (5 mm) isotropic pitch carbon fiber in mortar applied to steel reinforced concrete decreased the contact resistivity and volume resistivity of the new mortar, enabling a satisfactory electrical contact material to be made for the cathodic protection of steel reinforced old mortar or concrete [44]. 

Large towers have carbon steel shells. Brick linings, a standard in the past, are still widely used. These usually comprise two courses of acid-resistant brick held in place over a suitable protective membrane. Vinyl ester resins are suitable for the membrane and the mortar that holds the assembly together. Many systems use unlined carbon steel in the last (driest) stage. Other linings have begun to find some application these include... 

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