Passivity in nitric acid

Anodic protection is particularly suitable for stainless steels in acids. Protection potential ranges are given in Section 2.4. Besides sulfuric acid, other media such as phosphoric acid can be considered [13,21-24]. These materials are usually stable-passive in nitric acid. On the other hand, they are not passivatable in hydrochloric acid. Titanium is also a suitable material for anodic protection due to its good passivatability. 

The corrosion potentials are in keeping with the experimental observations steels attain passivity in nitric acid and ferric sulfate sulfuric acid solutions and undergo corrosion in 5% sulfuric acid solutions. 

The equilibrium contact angles of cp Ti and Ti-6A1-4V, both passivated in nitric acid, were 52 2° and 56 4°, respectively (Keller et al, 1994). Wettability was measured employing water drops. This similarity in contact angles reflets the similarity in oxide film composition found in the same work. However, the film on the alloy surface was significantly thicker (8.3 1.2nm) than that on the cp Ti (3.2 0.8nm). 

Nitric acid has a high oxidizing capacity and stainless steels will remain passive in nitric acid solutions. Although sulfuric acid is corrosive to stainless steels, it may be innocuous in low concentrations and in the presence of aeration. Similar performance can be expected in phosphoric acid and in many of the organic acids. 

The distortion of components by the heat treatments can occur but this problem can be solved by controlling the uniformity of heating. Another undesirable effect of the heat treatment is the formation of surface oxide scales which have to be removed either chemically (acid) or mechanically (sand-blasting). After the scales are removed the surface of the component is polished to a mirror or mat finish. The surface is then cleaned, degreased, and passivated in nitric acid (ASTM Standard F86). The component is washed and cleaned again before packaging and sterilizing. 

Sulfuric acid. As a 10 to 20% v/v solution, sulfuric acid can be used to clean 300 series SS, as well as other steels and metals, but not galvanized steel or magnesium. The cleaned SS can then be passivated with nitric acid. In practice, sulfuric acid is seldom used, except by specialist cleaning companies, because of its high heat of dilution and terrible burning effect on skin and other tissues. Add acid to water. 

Beryllium does not react with water, even when red hot its protective oxide film survives even at high temperatures. Magnesium reacts with hot water (see Fig. 13.22), and calcium reacts with cold water (Fig. 14.21). The metals reduce hydrogen ions to hydrogen gas, but neither beryllium nor magnesium dissolves in nitric acid, because both become passivated by a film of oxide. 

Anodizing employs electrochemical means to develop a surface oxide film on the workpiece, enhancing its corrosion resistance. Passivation is a process by which protective films are formed through immersion in an acid solution. In stainless steel passivation, embedded ion particles are dissolved and a thin oxide coat is formed by immersion in nitric acid, sometimes containing sodium dichromate. 

Hong et al. examined the effect of nitric acid passivation on type 430 ferritic stainless steel using potentiodynamic polarization, EIS, and Auger electron spectroscopy (AES) (18). Passivation treatments were carried out on wet polished surfaces by immersion for 60 minutes in nitric acid solutions ranging from 1 to 61% at 50°C. Pitting potential and the magnitude of the total impedance were positively correlated with surface Cr concentration. In response to this study,... 

In nitric acid of density 1 45 iron remains bright and refuses to dissolve.5 When removed and immersed in solutions of copper sulphate, no change takes place. In other words, the metal has been rendered noble 55 or passive.55... 

Dry nitrogen peroxide induces a more intense passivity than nitric acid when allowed to come into contact with iron—an observation which suggests an explanation for the fact that iron is only rendered passive by nitric acid which is either yellow or red, whilst passive iron is actually rendered active by immersion in colourless nitric acid solutions. Apparently only acids of such concentrations as are capable of yielding nitrogen peroxide in contact with the metal are able to exert a passivifying action. This is further supported by the fact that by passing nitrogen peroxide into those concentrations of nitric acid m which iron is normally active, the metal becomes passive.5... 

Testing for Passivity. — According to Heathcote,6 iron may be regarded as passive when no chemical action can be detected by the unaided eye after immersing, shaking, and finally holding motionless a piece of the metal in nitric acid of density 1-20, at the room temperature (15° to 17° C.). This is a preferable method to that of Schonbem,7 who employed nitric acid of density 1 35 in a similar manner, because this latter concentration of acid is sufficient to render active iron passive,8 whereas acid of density 1 20 does not do so, at the room temperature. 

In nitric acid solutions more concentrated than 7N, germanium becomes passive due to the formation of an insoluble oxide layer fprobably GeC>2). The thickness of this layer varies from 150 A or less in the concentrated solutions to 1000 X or more in the less concentrated solutions. The conductivity and type of germanium play no role in the passivity process. 

Passing on to the discussion of very thin layers of 10 to 100 X, such as those forming during oxidation of metals and alloys and also passive layers such as those formed on iron in nitric acid at low temperatures (0 to 200°C). one must consider additional phenomena which are characteristic of the reaction mechanisms involved. Our discussion of these phenomena are based, with certain limitations, on the theory of Mott and Cabrera. 

Some metals (Al, Cr, and Fe) are not oxidized by concentrated nitric acid even though they would be expected to be oxidized based on their oxidation potentials. These metals form thin oxide layers that protect the metal core (passivation). As a result, alloyed steel equipment can be used in nitric acid technology. 

The behavior of iron in nitric acid underscores the importance of recognizing the nature of passivity. Iron is resistant to corrosion in nitric acid at concentrations around 70%. Once passivated under these conditions, it can also exhibit low rates of corrosion as the nitric acid is diluted. However, if this passive film is disturbed, rapid corrosion will begin and repassivation is not possible until the nitric acid concentration is raised to a sufficient level. 

Except for Be, which is rendered passive by nitric acid, the metals dissolve readily in dilute mineral acids. Unlike the other four metals, Be, like Al, is soluble in caustic alkalis. 

The precise mechanism resppnsible for the passivity conferred on metals by anodic inhibitors, such as chromate, is not known. While some early workers thought that a protective salt film (e.g., chromate) was formed, this view is not generally applicable, since passivity can occur in a system where the salt film would be freely soluble (e.g., iron in nitric acid). It is, however, generally accepted that passivity is associated with the formation of a protective film, and current views ascribe the action of anodic inhibitors either to adsorption at anodic sites or to continuous repair of the protective film. The former view has received attention in recent publications by Cartledge ), while the latter is favored by Evans (2). However, work on aluminum has suggested that true passivity is associated with the crystal structure of the film, which in turn determines its stability. This principle has recently been introduced by one of the authors (3) and is developed below into a general theory of passivity. 

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