Passivating inhibitors, steels

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 second class of anodic inhibitors contains ions which need oxygen to passivate a metal. Tungstate and molybdate, for example, requke the presence of oxygen to passivate a steel. The concentration of the anodic inhibitor is critical for corrosion protection. Insufficient concentrations can lead to pitting corrosion or an increase in the corrosion rate. The use of anodic inhibitors is more difficult at higher salt concentrations, higher temperatures, lower pH values, and in some cases, at lower oxygen concentrations (37). 

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. 

Gabel also investigated the use of corrosion inhibitors. She found that antimony trioxide and dibasic lead phosphite appeared to have a synergistic effect in reducing corrosion with a phenolic-bonded coating. She also found that sodium nitrate, which acts as an oxidising passivator on steel, was effective, but less so than dibasic lead phosphite. 

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. 

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. 

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. 

Cathodic, anodic or passivating inhibitors are commonly used to prevent corrosion of steel reinforcement. In the class of anodic inhibitors, calcium nitrite, sodium nitrite, sodium benzoate and sodium chromate are commonly used. Cathodic inhibitors mainly consist of amines. 

LED3A to inhibit the corrosion of bright mild steel was evaluated. Steel coupons were immersed in 15% HCl at 40°C for 96 hours. The progress of corrosion was followed gravimetrically. The data are presented in Figure 18. The control contained no corrosion inhibitor. The LEDS A reduced the rate of corrosion by a factor of 10. Na LED3A has also been found to passivate stainless steel under highly alkaline conditions. 

Since they are surface active chelates which adsorb strongly onto metals, they act as corrosion inhibitors for bright mild steel and passivate stainless steel under alkaline conditions. Given the broad spectrum of unique properties exhibited by these products, it is likely that they will find widespread use in a wide range of detergent and personal care applications. 

For a given concentration of passivating inhibitor, there is a concentration of chloride and sulfate ions that will cause depassivation. Table 5.1 lists the critical concentrations of sodium chloride (NaCl) and sodium sulfate (NajSO ) required to cause pitting of steel in the presence of various... 

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. 

Steel, like many other metals, is more difficult to passivate in the presence of the chloride ion, therefore, a higher concentration of passivating inhibitor is required. Non-passivating inhibitors must also be used in higher concentrations because chloride ions are strongly absorbed by steel. 

Water also inhibits the stress corrosion cracking of steel in ammonia, and titanium in methanol, as well as attack on titanium by dry chlorine. A trace of water (0.001%) in liquid hydrogen fluoride (HP) behaves as a passivating inhibitor for nickel. This is an extreme example of the importance of solvent-inhibitor interactions. The exact mechanism of inhibition by water in HF is unknown, but the passivating effect is similar to that observed on steel in the presence of chromates in aqueous solution. 

Metal Cleaning. About 204 thousand metric tons of HCl (100% basis) was consumed in 1993 for steel pickling, wherein the hydrochloric acid readily dissolves all of the various oxides present in the scale formed during the hot rolling process. Using suitable inhibitors such as alkyl pyridines, HCl reacts very slowly with the base metal rendering the surface so clean that it must be passivated with a mild alkaline rinse. 

Impurities in a corrodent can be good or bad from a corrosion standpoint. An impurity in a stream may act as an inhibitor and actually retard corrosion. However, if this impurity is removed by some process change or improvement, a marked rise in corrosion rates can result. Other impurities, of course, can have very deleterious effec ts on materials. The chloride ion is a good example small amounts of chlorides in a process stream can break down the passive oxide film on stainless steels. The effects of impurities are varied and complex. One must be aware of what they are, how much is present, and where they come from before attempting to recommena a particular material of construction. 

Carbon steel lines should be pickled, passivated, and coated with rust inhibitor. 

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