Passive steels

Hot Dip Tin Coating of Steel and Cast Iron. Hot dipping of tin [7440-31 -5] has been largely superseded by electrolytic coating techniques, especially for sheet. However, hot dipping can be the method of choice for complex and shaped parts. Very thin layers of tin are extensively used to passivate steel used for canned goods. Tin is essentially nontoxic, is nearly insoluble in almost all foods, and easily wets and completely covers steel with a pinhole-free coating. 

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]. 

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

Fig. 19-1 Experimental setup for the cathodic protection of an active steel concrete-passive steel cell.

V, (c) without cathodic protection, in electrical contact with passive steel. 

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]. 

Since the object to be protected represents a cell consisting of active and passive steel, considerable IR errors in the cell current must be expected in measuring the off potential. The considerations in Section 3.3.1 with reference to Eqs. (3-27) and (3-28) are relevant here. Since upon switching off the protection current, 7, the nearby cathodes lead to anodic polarization of a region at risk from corrosion, the cell currents and 7, have opposite signs. It follows from Eqs. (3-27) and (3-28) that the 77 -free potential must be more negative than the off potential. Therefore, there is greater certainty of the potential criterion in Eq. (2-39). 

While concentrated nitric acid passivates steel, the phenomenon is too unreliable to permit cast irons to be used with confidence, even for strong nitric acids. The evidence available in relation to mixed nitrating acids. 

Test report Test on passivated steel specimens by the DECHEMA, D-Frankfiirt,... 

Coated reinforcement MacroceU formation may be important in the case of epoxy-coated rebars (Section 15.4) in chloride-contaminated concrete if there are defects in the coating and the coated bars are electricaUy connected with uncoated passive steel bars in deeper parts of the structure. Small anodic areas are created at the defect points of coated rebars in contact with chloride-contaminated concrete, while the uncoated passive rebars provide a cathodic surface of much greater size. In these situations the macroceU can result in considerable anodic current densities and can significantly accelerate the attack on corroding sites. This is why coated rebars should be electrically isolated from uncoated bars. 

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... 

Where oxygen access is low, it can be seen that the macrocell current tends to diminish in time because of oxygen depletion at the surface of the passive steel. The potential of passive steel consequently decreases in time until it reaches a value similar to that of corroding bars. 

This decrease may not occur in the case of structures subjected to wetting/ drying cycles or in conditions where oxygen consumed at the surface of the passive steel is replaced. This may happen in hollow piles of offshore structures, as depicted in Figure 8.2. Similar conditions may arise in tuimels buried or submersed in chloride-containing environments. Rebars on the inside of hoUow (air-filled) structures may be effective cathodes with noble potentials. They increase the potential of rebars closer to the seawater side of the cross section, stimulating corrosion initiation at lower chloride contents than without additional cathodic effects. Subsequently they may increase the corrosion rate at the anodes by consuming the electrons produced. The final corrosion rate will be a function of the ratio between anodic and cathodic areas, which is influenced by the concrete resistivity. 

The most frequent and also most favourable case is that in which the ratio r between the anodic and cathodic areas is near unity. The cathodic polarization, ip (i r), is prevalent with respect to other contributions. In facL the cathodic polarization of passive steel reaches values of about 200-300 mV even for a current density of about 1-2 mA/m (Figure 8.5) which is usually sufficient to dissipate most... 

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. 

Although there is no experience, interaction between AC and DC stray currents cannot be excluded, since AC can influence the anodic behaviour of steel [8]. Therefore attention should be dedicated to possible synergistic effects of AC and DC stray currents that might, under specific circumstances, be able to stimulate the corrosion rate of depassivated steel or promote corrosion on passive steel. 

Passive steel even in the presence of chlorides because it is cathodically protected... 

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). 

Non-carbonated and chloride-free concrete. In concrete that is not carbonated and does not contain chlorides, and in the absence of external cathodic polarization, hydrogen evolution, and thus consequent embrittlement, cannot take place. In this type of concrete, characterized by a pH above 12, hydrogen evolution can only occur at potentials below about —900 mV SCE. Passive steel under free corrosion conditions has much less negative potentials (Chapter 7) in the case of atmospherically exposed structures, the potential is between 0 and —200 mV (zone A of Figure 10.9). 

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-... 

In atmospherically exposed reinforced concrete the potential of passive steel is between +50 and -200 mV CSE (Figure 16.8). If corrosion is ongoing the potential becomes more negative chloride-induced pitting corrosion results in values from... 

There are two types of passivating inhibitors oxidizing anions such as chromate, nitrite, and nitrate, which can passivate steel in the absence of oxygen, and the nonoxidizing ions such as phosphate, tungstate, and molybdate, which require the presence of oxygen to passivate steel. Examples of passivators (anodic inhibitors) include chromate, nitrite, and orthophosphate (Dihua et al. 1999). 

The most common coating inhibitors are zinc chromate and plumbous orthoplumbate (red lead), which passivate steel by providing chromate and plumbate ions, respectively, as well as the zinc and lead cathodic inhibitors. These inhibitors are not effective against attack by seawater or brines because the high chloride concentration prevents passivation of steel. 

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