Nitrite passivator

Citrate/iron/nitrite passivator The Citrosolve process using air or 0.5 to 1.0% NaNOz or NaBr03. 

Carbonate/nitrite passivator Based on 1.0% Na Oj and 0.5% NaN02. The carbonate may be replaced by caustic, except where there is a risk of caustic cracking. 

Nitric acid readily attacks lead if dilute and the metal should not be used for handling nitrate or nitrite radicals except at extreme dilutions and preferably with a passivating reagent such as a sulphate, which will confer some protection. An example of this is the wash water from cellulose nitrate units. Corrosion decreases to a minimum at 65-70 Vo HNO3 and lead has been used for storage of nitric acid in the cold at this concentration . Resistance to a mixture of 98-85 Vo HjSO and nitric acid of 1 -50-1 -52 S.G. can be excellent °. ... 

Another class of inhibitors in near-neutral solutions act by stabilising oxide films on metals to form thin protective passivating films. Such inhibitors are the anions of weak acids, some of the most important in practice being chromate, nitrite, benzoate, silicate, phosphate and borate. Passivating... 

Nitrite-based formulations are available either as 100% powdered or granular products, or as liquids varying from 20 to 40% available sodium nitrite. Sodium nitrite (NaN02) is most commonly used and acts as an anodic inhibitor and passivator. 

Nitrite formulations are employed for both hot and cold water closed loops (and also occasionally for open cooling systems). Unfortunately, nitrite is easily oxidized to nitrate and is very susceptible to microbiological attack (by Nitrobacter agilis and other microorganisms). Nevertheless, it is a good low-cost passivating inhibitor. 

As an anodic passivating agent, nitrite enhances the formation of magnetite film and produces reducing conditions. It is simple to detect and test, but the high feed level and reserve requirement limit this product to small LPHW and LP steam boiler heating systems. 

The temperature is again raised, aeration begins, or 0.5 to 1.0% sodium nitrite is added to dissolve the copper, which is then com-plexed by ammonia to [CuOIXNFyj. The solvent is circulated for 2 to 3 hours before draining, and rinsing with hot condensate follows this. Rinsing requirements are minimal because of the passivating effects of the alkaline conditions. 

Most recently, these same authors have employed an in situ cell (Fig. 14) for carrying out these experiments. Again they studied nitrite- and chromate-passivated films. The results obtained in this case are quite different from the ex situ measurements. In addition,... 

Figure 15. Derivatives of the near-edge region of spectra for nitrite- (A) and chromate- (B) passivated iron films under in situ ( 4-) and ex situ ( ) conditions. (From Ref. 72, with permission.)...

Citrosolv A two-stage process for removing deposits from steam boilers, using citric acid. The first stage uses ammoniated citric acid at pH 3.5 to 4 to remove iron oxide the second uses a solution containing more ammonia, pH 9.5 to 10, to remove copper oxide, and an oxidant such as sodium nitrite to passivate the surface. 

Anodic inhibitors such as nitrites, chromates and molybdates are strong oxidizing passivators. They strengthen the protective oxide layer over the steel which otherwise would break down in the presence of chloride ions. The mechanism involves a redox reaction in which the chloride and nitrite ions engage in competing reactions the inhibitor is reduced and steel becomes oxidized to iron oxide as follows ... 

The study of the effect of the adsorptions of various additives on the anodic dissolution has been the subject of several studies. For instance, the influence of the adsorption of N species on the anodic dissolution of Ni was studied in [43]. The dissolution and passivation of Ni in nitrite-containing acid solutions were investigated by Auger spectroscopy, AFM, and conventional electrochemical techniques. It was found that the dis-solution/passivation of the Ni surface is consistent with a competition between adsorbed OH and nitrogen-containing... 

In addition to these techniques, there are passive samplers for N02 that have been used for unique situations such as indoor measurements. For example, in the Palmes Tube, N02 diffuses through to a surface coated with triethanolamine and is trapped in the form of NOJ. The nitrite is subsequently measured colori-metrically (e.g., see Boleij et al., 1986 Miller, 1988 and Krochmal and Gorski, 1991). As with most, if not all, such wet chemical methods, interferences can arise, for example, from PAN (Hisham and Grosjean, 1990) and HONO (Spicer et al., 1993b). 

Finally, passive samplers have also been developed for ozone, primarily for use in epidemiological studies. For example, Brauer and Brook (1995) describe the application of a passive sampler in which air containing ozone diffuses through a Teflon membrane and reacts with nitrite. The sampler is then extracted and the nitrate product measured using ion chromatography. 

Sodium nitrite is incorporated into formulations for both open and closed cooling systems and acts as an anodic inhibitor. It is a good passivator but requires a relatively high dose rate to ensure that all anodic areas within a system are protected from the risk of pitting corrosion. The dose rate has to be increased when high chlorides or sulfates are present. 

It is important not to leave the system empty of water for any long period, as rapid surface rusting will take place. As soon as the closed-loop system is declared free of contamination, sufficient corrosion inhibitor is added to provide long-term corrosion protection. The corrosion inhibitor is usually an anodic, passivating formulation, typically based on nitrite or tannin (and often in combination with phosphate, silicate, borate, or molybdate, etc.). Finally, after confirmation that the entire system is adequately treated (which usually requires the inhibited water in the system to be recirculated for a further 16 to 24 hours), the system is signed off and handed over. 

The short-term application of an increased dose rate of chemical inhibitor enhances the corrosion inhibiting potential of an already passivated metal surface. Nevertheless, in order to change the metal surface from an active state to a passive state, the electrode potential must be raised to a level above that of the passivation potential. Typically this is achieved by the use of chromate, nitrite, and/or orthophosphate in the presence of oxygen. 

Nitrite is suitable as an initial passivator in open cooling systems, but it requires dosing at 1000 to 1500 ppm NaN(>2 to compensate for the loss of nitrite in the system, due to conversion to nitrate. 

A useful illustrative example is shown in Fig. 12.8 of iron passivated by chromate and nitrite. Fourier transform of the EXAFS data to give distance-dependent signals shows the similarities and differences between the two passivation methods. 

Fig. 12.8. Passivation of iron by chromate and nitrite. Raw X-ray absorption data of (a) Fe and Fe304 and (b) Fe films after treatment (c) EXAFS data after Fourier transform (adapted from Ref. 31 with permission).

Oxidants such as nitrite and chromate which function by shifting the the surface potential of the metal in the positive direction until the passive zone in Fig. 16.6 (note that if these components are present in insufficient quantity the metal stays in the active zone, with potentially disastrous consequences). 

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