Migrating corrosion inhibitors

Another problem arises because often only one, the most volatile, component of the inhibitor blend can be analysed using an amine electrode as in the case of proprietary migrating corrosion inhibitors [1,9]. In this way, the diffusion of the volatile part of the inhibitor blend can be measured but no information on the diffusion of the non-volatile fraction can be obtained. Both components of an inhibitor blend are needed at the steel surface to get an inhibiting effect (Figure 13.3). However, only the volatile component diffuses easily through the porous concrete. This may explain the discrepancy between solution experiments and mortar or field tests [1,9,11]. It has also been reported that the volatile component of organic inhibitor blends evaporates [1,9]. 

B. Elsener, M. Btirchler, F. Stalder, H. Bohni, "A migrating corrosion inhibitor blend for reinforced concrete - Part 2 Inhibitor as repair strategy , Corrosion, 2000, 56, 121. 

L. Fedrizzi, F. Azzolini, P.L. Bonora, The use of migrating corrosion inhibitors to repair motorways concrete structures contaminated by chlorides, Cem. Concr. Res. 35 (2005) 551—561. 

A migrating corrosion inhibitor decreases somewhat the early-age concrete strength, while the ultimate compressive strength seems to be improved. However, in combination with BFS cement, a decrease in the ultimate compressive strength might be obtained. 

Vapour phase inhibitors or migrating corrosion inhibitors have been used to impregnate packaging, greases and waxes for many years to protect steel machinery and components, particularly before use. An American company... 

Recently migrating corrosion inhibitors have been proposed as surface applied liquids (Mader, 1994). It is claimed that the inhibitor will migrate both in the gas and in the liquid phase to the reinforcement. The results reported are conflicting a recent laboratory study on precorroded (both chloride induced and carbonated) mortar samples showed practically no effect in reducing the corrosion rate even after several immersion cycles in the inhibitor solution (Elsener et al., 1999b). Despite some field tests (Laamanen et al., 1996) and an increasing number of applications, very few documented and conclusive results exist on the inhibitor efficiency. A first field test with different surface applied inhibitors on a well characterized and instrumented side wall of a tunnel started in Switzerland in 1998. 

Elsener, B., Buchler, M., Stalder F. and Bdhni, H. (1999) Migrating Corrosion Inhibitor Blendfor Reinforced Concrete. Part 1 Prevention of Corrosion, Corrosion 55. 

A further longer term wet lay-up alternative is through the use of volatile corrosion inhibitors (VCIs) such as dicyclohexylamine acetate. These are dissolved in the water at a temperature below 60 °C, and the water is circulated for 4 to 5 hours. The boiler does not need to be completely filled because the VCI migrates to all parts of the boiler and reaches equilibrium in each of the void spaces. With traditional lay-up chemicals, the oxygen scavenger may become depleted easily (which is why the reserve usually is so high) and corrosion protection is quickly lost however, with VCI programs, there is always a volatile buffer available that maintains equilibrium and hence corrosion protection. 

The main aim of SAIE is the complete elimination of heavy metals from the coating systems an approach that primarily relies on tenacious water-insensitive adhesion and good barrier characteristics of a primer has been taken. It should be pointed out that this approach is theoretically incompatible with the approach that utilizes the primers with corrosion inhibitors, e.g., chromated primers. This is because a primer with super barrier characteristics would not allow the migration of inhibitors and would not provide enough water for their electrochemical reaction to form corrosion protection products, as described in Chapter 28. In order to further elucidate the SAIE concept, both chromated and nonchromated spray primers were employed to generate two types of plasma coating-modified systems, and their corrosion protection behaviors were investigated in this study. 

Migrating inhibitors. Substances have been recently developed that, once applied on the concrete surface, are expected to migrate through the concrete cover and reach the steel surface where they can act as corrosion inhibitors. The properties and the effectiveness of migrating inhibitors are discussed in Chapter 13. 

Corrosion inhibitors. Corrosion inhibitors may in principle improve the protection of the steel reinforcement. Mixed-in inhibitors added to repair mortar may increase the chloride threshold or delay the chloride penetration in the repair material. Migrating inhibitors appHed on the surface of the original concrete are intended to reduce or stop corrosion and thus make concrete removal and its replacement unnecessary. The role of inhibitors and their effectiveness are discussed in Chapter 13. It is, however, useful to remember that if a corrosion inhibitor is used, the concentration that is needed in the vicinity of the reinforcement should be specified and suitable means for demonstrating that such conditions are actually achieved and maintained for an adequately long time should be proposed [11-... 

Uses Corrosion inhibitor drier for paints, in food-contact coatings, migrating from food pkg. 

Transport Phenomena Memy corrosion inhibitors are higher molecular compounds, and are usually present in small concentrations. If the objective is localized corrosion inhibition (crevice corrosion, imder-dep>osit corrosion, etc.), the ability of the inhibitor to migrate where it is needed becomes a major concern that needs to be included in the design of the test methodology. 

By their physical-chemical properties and protection principle there are volatile, contact and creep inhibitors of atmospheric corrosion. Contact Cl are the substances whose protecting effect is exhibited only on direct application on the metal surface. Many of them migrate via diffusion and undergo creeping. Volatile Cl with relatively high volatility can easily transfer into vapor and be adsorbed onto metal surfaces to protect them from corrosion. 

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-... 

Figure 13.3 Corrosion potentials of rebar samples in saturated CafOHjj solution containing the two components (volatile dimethylethanolamine and non-volatile benzoate) of a migrating inhibitor [9]...

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. 

The results concerning the barrier properties of the coating are arranged in the same way as the solubility of the cerium species. From these results, it can be concluded that the barrier function of a coating is vitally influenced by the solubility of a possible inhibitor. It may be speculated that this kind of inhibitor is dissolved as soon as an electrolyte enters the pores of the coating material. The dissolved inhibitor will then be present in the direct neighbourhood if any corrosion reaction is caused by the electrolyte. On the other hand, the cerium compounds with limited solubility will not migrate in the electrolyte to the metal surface and thus are not able to inhibit the corrosion processes efficiently. 

---