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Passivators, steel in concrete

However, if the interpretation of the potentials measured for regions with a covering as uniform as possible and aeration or moisture is extended to estimate the potential gradients corresponding to the explanation for Fig. 3-24, there follows the possibility of classifying the state of corrosion [52-54]. Furthermore, the sensitivity of the estimate can be raised by anodic polarization according to the explanation given for Fig. 2-7, because the depassivated steel is less polarizable than the passive steel in concrete [43]. [Pg.433]

For steel embedded in concrete, it was observed that current densities up to 50 A/m applied for 5 months to passive steel in concrete with up to 0.4% chlorides did not lead to corrosion initiation [5]. Since steel in reinforced-concrete structures is not coated, it is not actually possible to reach such high current densities. It can be assumed, therefore, that interference from AC current cannot induce corrosion on passive steel in concrete. [Pg.143]

The corrosion potential of a non corroding, passive steel in concrete is determined by the passive current density and the current density of the oxygen reduction (Fig. 8-8). The passive current density is very low, thus for aerated concrete structures corr = o2- The main influencing factor is the pH with decreasing pH the corrosion potential becomes more positive. This is important for concretes with additives where the pozzolanic reaction takes place. In practice, variations in the corrosion potential of passive steel of up to 200 mV are observed due to changes in concrete humidity and pH. In aerated alkaline concrete values of... [Pg.952]

After electrochemical realkalization, half cell potential values of about-0.2 V CSE were measured, thus the potentials shifted to more negative values by about 250 mV compared with the values of the untreated control field. This behaviour can be rationally explained bearing in mind that (a) the concrete cover after the ER treatment has a much lower resistivity (sodium bicarbonate in the pore solution) and (b) passive steel in concrete acts as a pH electrode. The potential values measured indicate that the steel/ concrete interface has become more alkaline. [Pg.980]

Zinc will initially react with cement-based materials with the evolution of hydrogen. This reaction can be controlled by the presence of soluble chromate either in the cement (over 70 ppm) or as a chromate passivation treatment to the zinc surface. Zinc can therefore be used to provide additional protection to steel in concrete. It is more effective in cmbonated concrete than in chloride-contaminated concrete. [Pg.53]

Cathodic protection is one of the methods to mitigate the corrosion of steel in concrete Figure 7.24. Some factors to be considered in this connection are remaining service life of the structure should be more than lOyr delamination and spalls should be less than 50% by weight of concrete half-cell potential should be less than —200 mV (indicating breakdown of passive film) the structure should be sound the reinforcing bars should be electrically continuous AC power should be available. [Pg.478]

In non-carbonated concrete without chlorides, steel is passive and a typical anodic polarization curve is shown in Figure 7.3. The potential is measured versus the saturated calomel reference electrode (SCE), whose potential is +244 mV versus the standard hydrogen electrode (SHE). Other reference electrodes used to measure the potential of steel in concrete are Ag/AgCl, CU/CUSO4, Mn02, and activated titanium types. From this point on in the text, unless explicitly stated otherwise, potentials are given versus the SCE electrode. [Pg.112]

Presence of different metals. Rebars of carbon steel in certain cases can be connected to rebars or facilities made of stainless steel or copper. This type of coupling, which in other electrolytes would provoke a considerable degree of corrosion in carbon steel by galvanic attack, does not cause problems in the case of concrete any different from those provoked by coupling with normal passive steel. In fact, the corrosion potential of passive carbon steel in concrete is not much different... [Pg.126]

We have seen that stray current can hardly induce corrosion on passive steel in non-carbonated and chloride-free concrete. However, the potential adverse effects of stray current on concrete structures may become increasingly important with the increased use of underground concrete construction. Stray-current effects are rarely recognised as such. The importance increases further due to the increase of the required service lives (i. e. 100 y or more). [Pg.145]

The alkaline condition leads to a passive layer forming on the steel surface. The passive layer is a dense, impenetrable film, which, if fully established and maintained, prevents further corrosion of the steel. The layer formed on steel in concrete is probably part metal oxide/hydroxide and part mineral from the cement. A true passive layer is a very dense, thin layer of oxide that leads to a very slow rate of oxidation (corrosion). There is some discussion as to whether or not the layer on the steel is a true passive layer as it seems to be thick compared with other passive layers and it consists of more than just metal oxides but as it behaves like a passive layer it is generally referred to as such. [Pg.6]

Once the passive layer breaks down then areas of rust will start appearing on the steel snrface. The chemical reactions are the same whether corrosion occnrs by chloride attack or carbonation. When steel in concrete corrodes it dissolves in the pore water and gives up electrons ... [Pg.7]

In Chapter 2, we discussed the corrosion of steel in concrete and the effectiveness of the alkalinity in producing a passive layer of protective oxide on the steel surface which stops corrosion. In the previous section we observed that alkalinity is neutralized by carbonation. The depassivation mechanism for chloride attack is somewhat different. The chloride ion attacks the passive layer although in this case (unlike carbonation) there is no overall drop in pH. Chlorides act as catalysts to corrosion. They are not consumed in the process but help to break down the passive layer of oxide on the steel and allow the corrosion process to proceed quickly. This is illustrated in... [Pg.21]

The studies of passive film on steel in concrete have shown that it is composed of Fe304-7-Fc203 solid solution. In the case of passive film damage and initiation of corrosion process potential is changing to the more negative values from -400 mV to even -1,000 mV... [Pg.480]

Nevertheless, corrosion of steel reinforcements does occur in structures such as concrete bridge decks and parking garages. These problems have been studied for many years and result from the breakdown of passivity caused, for example, by salt in the environment or in the concrete [59]. Passivity may also be lost after several years if air diffuses through the concrete to the reinforcements, converting alkaline Ca(OH)2 to less alkaline CaCOs. Corrosion processes for steel in concrete are illustrated in Fig. 7.15 [60]. [Pg.144]

The process by which steel in concrete is protected from corrosion by the formation of a passive layer due to the highly alkaline environment... [Pg.19]

The pH of concrete is typically >12.5 with some measurements on squeezed pore water suggesting even higher values [/]. Thus in the absence of a dep>assivating ion such as chloride, or carbonation of the concrete to a lower pH value, steel will remain passive. Note that steel in concrete subjected to high stray voltages could corrode under environmental conditions that would normally be benign. [Pg.405]

Mass loss tests are a common means of testing many metals in atmospheric and submerged exposure. For steel in concrete they are destructive to the concrete and difficult to perform. Laboratory specimens typically have a portion of the embedded bar coated or taped to define a specific test area. These materials are often hard to remove and could gain or lose mass. Furthermore, at the initial stages of corrosion most of the damage is in pits which have relatively low mass loss relative to that lost in the formation of the passive film [12], and thus mass loss is not necessarily a good indicator of corrosion activity. [Pg.406]

A characteristic feature for the chloride induced corrosion of steel in concrete (pitting) is the development of macrocells, that is the coexistence of passive and corroding areas on the same rebar forming a short circuited galvanic element with the corroding area acting as anode and the passive surface as cathode (Fig. 8-11). The cell voltage. [Pg.957]

Figure 8-11. Increased localized corrosion of steel in concrete due to the formation of a macrocell ac-tive/passive. The current (/) is flowing from the local anode to the cathode. Figure 8-11. Increased localized corrosion of steel in concrete due to the formation of a macrocell ac-tive/passive. The current (/) is flowing from the local anode to the cathode.
Galvanized reinforcement, i.e. zinc coatings formed by dipping clean rebars in a bath of molten zinc, can protect steel in concrete from corrosion attack. However, the performance reported in the literature is contradictory (Bentur et al., 1997). Galvanized rebars remain passive in carbonated concrete and the corrosion rate is much lower than with black steel. In situations where chloride induced corrosion prevails, a delay in the initiation of corrosion can be expected, but at high chloride concentrations depassivation cannot be avoided completely. [Pg.967]


See other pages where Passivators, steel in concrete is mentioned: [Pg.140]    [Pg.147]    [Pg.136]    [Pg.141]    [Pg.218]    [Pg.223]    [Pg.288]    [Pg.398]    [Pg.401]    [Pg.529]    [Pg.539]    [Pg.19]    [Pg.50]    [Pg.154]    [Pg.179]    [Pg.274]    [Pg.159]    [Pg.438]    [Pg.950]    [Pg.951]    [Pg.963]    [Pg.966]    [Pg.972]    [Pg.616]   
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