Cathodic prevention

Unfortunately, up to date, no solid evidence was reported to support the hypothesis of the above adsorption mechanism. Moreover, the hydrogen gas produced at the EC cathode prevents the floes from settling properly on leaving the electrolyzer [34], In order to overcome this problem, an EC process followed by an electroflotation (EF) operation can be applied. In this combined process, the EC unit is primarily for the production of aluminum hydroxide floes. The EF unit would undertake the responsibility of separating the formed floes from water by floating them to the surface of the cell. 

The mutual mechanical mixing of anolyte and catholyte in the bell-jar type electrolyzer is prevented by stratification of the electrolyte due to difference in specific gravity of both liquids, so that no diaphragm is necessary. Simultaneously the motion of the electrolyte from the anode toward the cathode prevents the penetration of hydroxyl ions towards the anode which enables to attain... 

On the other hand, anything that makes iron behave more like the cathode prevents corrosion. In cathodic protection, iron makes contact with a more active metal (stronger reducing agent), such as zinc. The iron becomes cathodic and remains intact, while the zinc acts as the anode and loses electrons (Figure 21.2IB). Coating steel with a sacrificial layer of zinc is the basis of the... 

P. Pedeferri, Cathodic protection and cathodic prevention . Construction and Building Materials, 1996, 10, 391-402. 

L. Bertolini, F. Bolzoni, T. Pastore, P. Pedeferri, New experiences on cathodic prevention of reinforced-concrete structures , in Corrosion of Reinforcement in Concrete Construction, C. L. Page, P. B. Bamforth,... 

Figure 10.9 Diagram indicating the field of potential and pH for reinforcement passive A), in aerated (6) and non-aerated (C) carbonated concrete, subject to pitting (D), under cathodic prevention ( ), under cathodic protection (F), and under overprotection (C) [7]...

In Figure 10.9 zones E and F represent the operational conditions of reinforcement in structures to which cathodic prevention and cathodic protection, respectively, have been applied (Chapter 20). Values at which cathodic protection normally operates are not sufficiently negative to induce hydrogen evolution. But even if it should operate in conditions of overprotection (potential below —900 mV SCE), the same diagram shows that up to potentials of—1100 mV SCE on the protected steel, the situation will nevertheless be less critical than on that unprotected reinforcement where pitting attack occurs. 

The cost of various techniques can only be given very roughly, and any estimate will be incomplete, since the actual cost will vary from one application to another. Furthermore, different types of prevention mechanisms are not directly comparable. Beyond this, it can be said that with respect to normal carbon-steel reinforcement, use of galvanized and epoxy-coated bars costs about twice as much, and the cost of stainless-steel reinforcement is about 5 to 10 times higher. The use of nitrite inhibitors in higher doses costs approximately 30 /m of concrete (volume). Coatings may vary from 7 to 50 /m of concrete surface, hydrophobic treatment costs about 10 /m. Cathodic prevention varies from 50 to 100 /m. ... 

Carbon steel Corrosion inhibitor Stainless steel 1.4301 Stainless steel 1.4401 Cathodic prevention... 

G. Sergi, C. L. Page, Sacrificial anodes for cathodic prevention of reinforcing steel around patch repairs applied to chlorideEuropean Federation of Corrosion, Event No. 227, Aachen, 30 August-2 September 1999, 248. 

Electrochemical techniques appHed for controlling corrosion of steel in concrete are cathodic protection, cathodic prevention, electrochemical chloride removal and electrochemical reaUcalization. 

Cathodic prevention (CPre) is applied to new structures that will presumably be contaminated by chlorides the passive reinforcement is cathodicaUy polarized with the aim of increasing the critical chloride content necessary to initiate pitting attack in such a way that it will not be reached within the service life of the structure. 

Both the beneficial effects on the steel as well as those modifying the protective properties of the concrete are obtained by forcing a direct current to circulate between an anode, placed on the external surface of the structure, and the reinforcement (Figure 20.1). The current density imposed varies from 1-2 mA/m for cathodic prevention, to 5-20 mA/m for cathodic protection, up to 1000-2000 mA/m for electrochemical reaUcahzation and chloride removal. 

Once applied, cathodic protection and cathodic prevention must operate for the remaining service life of the structure. Consequently, the design should take into account the durability of the installation, that is, its ability to provide sufficient protection current for a long period. Normally, the quaHty of protection offered by CP and CPre is checked regularly using electrical measurements. 

Recently this technique has been proposed in conjunction with conventional patch repair of chloride-contaminated structures in order to avoid the initiation of incipient pitting around the repaired zones, by utihsing sacrificial anodes embedded near the periphery of the repair patches [20,21]. Cathodic prevention is now being used in several countries. ... 

C. L. Page writes [22] Although cathodic prevention is now being used in several countries, some researchers think there is still a need to convince some engineers of its appropriateness as the follovvdng statement, reproduced from the most recent edition of a widely used reference book on concrete... 

Alkali-aggregate reaction. The increase in alkalinity produced at the cathode can cause damage if the concrete contains aggregates potentially susceptible to al-kah-sihca reaction (Section 3.4). In practice, this may happen only for current densities weU over 20 mA/m [33]. Therefore it may involve electrochemical chloride removal or electrochemical realkahzation, which in fact cause large increases in pH [34, 35, 36] at the surfaces of the reinforcement, but not cathodic protection, and much less, cathodic prevention. 

Anodic acidification. At the anode surface, the anodic process of oxygen evolution takes place 2H2O —>62 + 4H + 4e . In the presence of chlorides, even chlorine develops 2Cr —> CI2 + 2e . Such processes may directly or indirectly produce acidity and may thus lead to destruction of the cement paste in contact with the anode [34]. Experience shows that such deterioration is negligible for activated titanium mesh anodes if the anodic current density does not exceed 100 rtiA/m (or values 3-4 times greater for brief periods). Design of the anodes for cathodic prevention and cathodic protection must respect these hmits. 

To this concern. Figure 20.5 shows the results of application of cathodic prevention to slabs subjected to ponding with a NaCl solution [43]. After about 700 days, initiation of rebar corrosion was detected in the control slab (in the free corrosion condition) at spots where the chloride content at the steel surface had reached more than about 1 % by mass of cement. From about that point in time, the slab receiving a very low current density of 0.4 mA/m showed a similar (instant-off) potential as the control slab and its 4-hour decay values became lower than 100 mV. The slab receiving 0.8 mA/m showed lower decays from about... 

Figure 20.5 Instant-off potential of non corroding steel cathodically polarized with current densities typical of cathodic prevention and chloride content in the control slab. Specimens exposed to saturated NaCI solution alternating...

On the basis of these results, the increase of the chloride threshold brought about by cathodic prevention in practical applications is expected to be sufficient to avoid corrosion initiation throughout the entire service life, even where the exposure is very aggressive and the concrete itself is unable to sufficiently resist chloride penetration. 

The higher throwing power and the less negative potentials needed allow safe application of cathodic prevention with regard to avoiding the risk of hydrogen... 

T. Pastore, P. Pedeferri, Cathodic Protection and Cathodic Prevention in Concrete. Principles and Applications , Journal of Applied Electrochemistry, 1998, 28,... 

L. Bertolini, Cathodic Prevention , Proc. COST 521, Workshop, 28-31 August, D. Sloan, P. A. M. Basheer (Eds.), The Queen s University Belfast, 2000. 

G. Sergi, G. L. Page, Sacrificial anodes for cathodic prevention of reinforcing steel around patch repairs applied to chloride-contaminated... 

P. Pedeferri, E. Redaelli, Cathodic protection of steel in concrete and cathodic prevention . Final project report 15, COST 521, Luxembourg,... 

L. Bertolini, P. Pedeferri, Field application of cathodic prevention on reinforced-concrete structures . Corrosion 96, NACE, Houston, paper 312, 1996. 

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