Nitric aqueous corrosion

Nitric acid is one of the three major acids of the modem chemical industiy and has been known as a corrosive solvent for metals since alchemical times in the thirteenth centuiy. " " It is now invariably made by the catalytic oxidation of ammonia under conditions which promote the formation of NO rather than the thermodynamically more favoured products N2 or N2O (p. 423). The NO is then further oxidized to NO2 and the gases absorbed in water to yield a concentrated aqueous solution of the acid. The vast scale of production requires the optimization of all the reaction conditions and present-day operations are based on the intricate interaction of fundamental thermodynamics, modem catalyst technology, advanced reactor design, and chemical engineering aspects of process control (see Panel). Production in the USA alone now exceeds 7 million tonnes annually, of which the greater part is used to produce nitrates for fertilizers, explosives and other purposes (see Panel). 

There is an accelerating trend away from the use of lead-containing solders in contact with potable water. The effects of galvanic corrosion of one of the substitute alloys (Sn3%Ag) in contact with a number of other metals including copper have therefore been studied . The corrosion of tin/Iead alloys in different electrolytes including nitrates, nitric and acetic acids, and citric acid over the pH range 2-6 were reported. The specific alloy Pb/15%Sn was studied in contact with aqueous solutions in the pH range... 

The US Bureau of Mines found the chemical and galvanic corrosion behaviour of both the TZM and Mo-30W alloy to be generally equal or superior to that of unalloyed molybdenum in many aqueous solutions of acids, bases and salts. Notable exceptions occurred in 6-1 % nitric acid where both alloys corroded appreciably faster than molybdenum. In mercuric chloride solutions the TZM alloy was susceptible to a type of crevice corrosion which was not due to differential aeration. The alloys were usually not adversely affected by contact with dissimilar metals in galvanic couple experiments, but the dissimilar metals sometimes corroded galvanically. Both alloys were resistant to synthetic sea water spray at 60°C. 

Although titanium has a large positive E° for oxidation, and T dust will burn in air, the bulk metal is remarkably immune to corrosion because its surface becomes coated with a thin, protective oxide film. Titanium objects are inert to seawater, nitric acid, hot aqueous NaOH, and even to aqueous chlorine gas. Titanium is therefore used in chemical plants, in desalination equipment, and in numerous other industrial processes that demand inert, noncorrosive materials. Because it is nontoxic and inert to body fluids, titanium is even used for manufacturing artificial joints and dental implants. 

With chemical corrosion we mean the decay of a material under the influence of a corrosive substance. When brass contains more than 15 % (m/m) of zinc, the zinc and copper ions dissolve in an aqueous environment at a high temperature. Subsequently the copper ions are deposited on the metal surface. Nitric acid is able to selectively dissolve iron out of certain ceramic materials. Molecules of a sol-... 

The relation between the corrosion rate and pH in various aqueous media is depicted in Figure 4.5 and the corrosion rate is minimal in the pH range 4—8.5. Most of the alloys are inert to concentrated nitric and acetic acids, but are attacked by dilute nitric, sulfuric and hydrochloric acids. The sodium silicate acts as an inhibitor. The sensitivity to acid attack also depends upon the alloy. 

As described in chapter 16.6.1, extractive distillation of diacetyl requires the use of aqueous sulfuric acid as volatility modifier throughout most of the column. Although the process calls for an acid strength of only 0.3 % by weight, with a column made of stainless steel type 316 L this led to considerable corrosion within a year of operation. However, it was found possible to completely eliminate this problem by adding a trace of nitric acid. 

Derivation From tantalum potassium fluoride by heating in an electric furnace, by sodium reduction, or by fused salt electrolysis. The powdered metal is converted to a massive metal by sintering in a vacuum. Foot-long crystals can be grown by arc fusion. Corrosion resistance 99.5% pure tantalum is resistant to all concentrations of hot and cold sulfuric acid (except concentrated boding), hydrochloric acid, nitric and acetic acids, hot and cold dilute sodium hydroxide, all dilutions of hot and cold ammonium hydroxide, mine and seawaters, moist sul-furous atmospheres, aqueous solutions of chlorine. 

Metallic passivity was discovered as far back as 1790, when metallic iron in concentrated nitric acid was found to turn suddenly into the passive state after violent metal dissolution had occurred in the active state [24,25]. It was not until 1960s that we certainly confirmed the presence of an oxide film several nanometers thick on the surface of passivated metals [26]. Passivation was also found to occur with semiconductors in aqueous solution [27]. We may learn the latest overview on the passivity of metals and semiconductors in corrosion literature [11]. 

The outer compartment G is provided with a cylindrical anode E made of corrosion-resistant metal wire mesh. The anode compartment is filled with aqueous nitric acid, around 3 Af. 

In the initial development of the Thorex process [S9], the feed was made acid-deflcient by evaporation until the boiling point reached 155 0. Trouble was experienced with corrosion and with precipitation of solids. The procedure finally adopted [R2] is shown in Fig. 10.20. The dissolver solution is evaporated until its boiling point reaches 135°C, at which point about 70 percent of the original volume has been evaporated and the nitric acid concentration is down to 3 M, Further stripping at constant volume and a constant temperature of 135°C is carried out by adding water and boiling off aqueous nitric acid until the solution is 0.2 M acid-deficient. The solution is finally diluted with water to make it around 0.15 M acid-deficient. 

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