Passivation of steel

Commercially, heteropolytungstates, particulady the heteropolytungstates, are produced in large quantities as precipitants for basic dyes, with which they form colored lakes or toners (see also Dyes and dye intermediates). They are also used in catalysis, passivation of steel, etc. 

Anodic Inhibitors. Passivating or anodic inhibitors produce a large positive shift in the corrosion potential of a metal. There are two classes of anodic inhibitors which are used for metals and alloys where the anodic shift in potential promotes passivation, ie, anodic protection. The fkst class includes oxidking anions that can passivate a metal in the absence of oxygen. Chromate is a classical example of an oxidking anodic inhibitor for the passivation of steels. 

The deposition of contaminants in the surface usually causes an acceleration of the corrosion process however, there should not be excluded the possibility that a given contaminant could diminish corrosion rate, as it could be the case of ammonia and its influence on steel (due to its alkaline properties, it could induce the passivation of steel). 

Passivation of steel in cooling systems does not occur naturally and thus anodic inhibitors are generally employed. This step can be relatively expensive in large cooling systems (as the consumption of chemicals can be quite high in a short period of time), or seen to be a waste of time and therefore unnecessary. This is not the case and proper passivation should always be undertaken. 

This property is used in Russian industry, for example, for the passivation of steel pipes against aggressive waste waters, since CN contained in waste waters coats the interior of pipes with an insoluble protective layer of Iron Blue N.G. Chen, J. Appl. Chem. USSR, 74(1)(1974), pp. 139-142. But it should be noted that this borders on criminal negligence, since toxic cyanides simply do not belong in waste waters. 

For the ion chromatographic analysis of chloride in cement, that cancels out the passivation of steel surfaces in concrete and, thus, is only admitted up to a maximum content of 0.1%, Maurer et al. [93] developed a procedure in which the cement sample is extracted with nitric acid. Since a direct chloride determination is impossible due to the high nitrate concentration in the extract, silver nitrate is added in three-fold excess to the nitric acid extract. The precipitated silver chloride is filtered off and subsequently dissolved in 100 mL of a 0.25% ammonium hydroxide solution. This solution, according to the chloride concentration, can be further diluted with de-ionized water or injected directly. 

Anodic passivation of steel surfaces can be efficiently achieved by metal chromates. Chromates of Intermediate solubility (e.g., zinc chromate and strontium chromate) allow a compromise between mobility in the film and leaching from the film to be achieved. Chromates inhibit corrosion in aqueous systems by formation of a passivating oxide film. The effectiveness of chromate inhibitors in aqueous systems depends on the concentration of other ionic species in solution, for example, chloride. Synthetic resin composition can also significantly influence the effectiveness of chromate pigments. The effect appears to be related to the polarity of the resin (20) chromate pigments appear to be less effective in resins of low polarity. 

The anodic process can be stopped by applying a coating to the reinforcement that acts as a physical barrier between the steel and the repair mortar. For this purpose only organic coatings, preferably epoxy based, should be used. Protection is entirely based on the barrier between the reinforcement and the mortar, and passivation of steel cannot be achieved because contact with alkaline repair material is prevented. This method should be used to protect depassivated areas of the reinforcement only as a last resort, i. e. when other techniques are not applicable and only for small specific applications [1,4]. It may be used, for instance, when the thickness of the concrete cover is very low and it is impossible to increase it to the proper level, so that the repair material cannot provide durable protection to the embedded steel. 

HNO3 (see Equation (6.5)) is an example of a strong oxidizer, which (contrary to CI2) promotes passivation of steel. Because HNO3 lifts the potential strongly, it may cause heavy corrosion on relatively noble metals and alloys (e.g. copper and its alloys). 

Evaluations have shown that the increase of corrosion products amount in the circuit could be as high as hundreds of kg per year. Thus, it is obviously needed to develop special-purpose mass transfer equipment designed for catching corrosion products with permanent passivation of steel surface in contact with liquid metal. 

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. 

In the presence of chloride ions passivation of steel needs higher pH value. Several hypotheses were proposed to explain the mechanism of passive film destmction in the presence of chlorides [342]. Possibility of chloride ions penetration to passive film, the effect of electric field generated around the adsorbed chloride ions, promoting of Fe " ions diffusion from the surface of metal are listed. Other factors will be discussed farther. 

The rate of corrosion process will depend on the conductivity of electrolyte and the difference of potential between the anode and cathode. Particularly the oxygen access, necessary for the cathodic reaction, can be the factor limiting the rate of corrosion [98]. Simultaneously, as a result of corrosion current, the polarization of electrodes occurs (their potentials increase in respect to the equilibrium potential values) and the dynamically maintained potential value has the deciding effect on the corrosion rate. In the case of steel in paste environment strong polarization of anodic microareas occurs, which increase anodic potential, decreasing the difference of potential in respect to cathode therefore, as it results from the curves in E -pH system, the passivation of steel due to the oxides film occurs [98]. 

Messaddeq (1999) AISI316L Zt02 ZrOa -1- RitPlM Zr(0C3H7)4 e = 0.2-0.4 SL C/l 5 mtn 200 C/30 min Polarization curves in H2SO4 Corrosion resistance inaeases with PMMA content Passivation of steel in H2 04... 

This effect is seen even more cleariy when the pH value is iowered below9.0. The anodic polarization curves (Figure 6) predict that passivation is not possible below a pH of 8.6. However, the passivation of steel at pH values as low as 8.0 has been demonstrated. Hausler inferred that EDTA forms an interphase inhibitor layer on steel composed of some sort of insoluble FeEDTA complex. Such a complex layer would change the Iron dissolution kinetics and also possibly influence the passivation behavior. As the free-EDTA concentration increased, this layer would tend to be less stable. The present data confirm such a trend. 

Impermeable dense concrete A Porous concrete A A A A > Passivation of Steel... 

Destruction of passivity of steel by chloride ions and initiation of pits. 

Two major factors destroy the passivation of steels in concrete (a) reduction of pH level by ingress of atmospheric carbon dioxide, and (b) penetration of chloride from the environment. 

The addition of fly ash to cement results in the formation of decreased amounts of calcium hydroxide in the hydration product. This is attributable to the dilution effect and to the consumption of calciiun hydroxide by the pozzolanic reaction with the fly ash. In Fig. 1, the amount of calcium hydroxide formed at different times of hydration in cement containing fly ash is given. The amount of Ca(OH)2 estimated by TG was found to be lower in samples containing fly ash. With the increase in the amount of fly ash, less calcium hydroxide was formed because of the pozzolanic reaction and dilution effect. Even at 60% fly ash, some lime was present in the mortar, and the pH was found to be 13.5. At this pH value, the passivity of steel is assured. It can also be observed that there is more lime at 60% fly ash than at 75% slag addition. 

In practice, electrochemical passivation often determines the corrosion resistance of base metals. Passivation of steel by the reaction in eqn. (6.57), for example, is decisive of the durability of reinforcement bars in concrete. As will be shown later in a calculation example, the strongly alkaline environment in concrete with pH 12.5 furthers the formation of a passivating oxide layer of Fe203 on the embedded reinforcement. 

In water with a pH near 7.0, a low concentration of chlorides, silicates, and phosphates cause passivation of steel when oxygen is present hence, they behave as anodic inhibitors. Another anodic characteristic is that corrosion is localized in the form of pitting when insufficient amounts of phosphate or silicate are added to saline water. However, both sUicates and phosphates from deposits on steel increase cathodic polarization. Thus, their action appears to be mixed, i.e., a combination of both anodic and cathodic effects. 

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