Carbon corrosion normalized concentrations

Calcium carbonate has normal pH and inverse temperature solubilities. Hence, such deposits readily form as pH and water temperature rise. Copper carbonate can form beneath deposit accumulations, producing a friable bluish-white corrosion product (Fig. 4.17). Beneath the carbonate, sparkling, ruby-red cuprous oxide crystals will often be found on copper alloys (Fig. 4.18). The cuprous oxide is friable, as these crystals are small and do not readily cling to one another or other surfaces (Fig. 4.19). If chloride concentrations are high, a white copper chloride corrosion product may be present beneath the cuprous oxide layer. However, experience shows that copper chloride accumulation is usually slight relative to other corrosion product masses in most natural waters. 

FIGURE 5.32 (a) Normalized hydrogen and oxygen concentrations, and the shape of the membrane phase potential along the channel, (b) The ORR and carbon corrosion (COR) cnr-rent densities on the cathode side of the cell, (c) The HOR and ORR cnrrent densities on the anode side of the cell, (d) The differences between HOR -ORR" and ORR -COR are coincidental and equal to the z-component of proton current density in the membrane. The shaded region indicates the domain where the approximation of e = 0 fails. For aU the cnrves, s = 0.04 and = 0.63329 V. 

FIGURE 5.36 (a) The normalized methanol concentration and the shape of the membrane phase potential along the chaimel. (b) The ORR current density in linear and logarithmic scales, (c) The MOR and carbon corrosion current densities. For aU the curves, the mean current density is 100 mA cm, s — 0.01, and the cell potential = 0.5168 V. 

The electrolyte is normally a concentrated solution of a strong acid or alkali this will have a high conductance and will help to minimise concentration overpotential (q.v.). Potassium hydroxide is the first choice as it is less corrosive than concentrated acids, but its usefulness is limited by its reactivity towards carbon dioxide if used in a hydrogen-air cell, the air must be purified, and for any cell designed to use a hydrocarbon fuel it would be unsuitable as CO2 would be one of the products of the cell reaction. The alternatives are concentrated sulphuric acid, which is only suitable at moderately low temperatures, and phosphoric acid. 

Carbon steel is not normally a suitable piping material for concentrated sulfuric acid because of high corrosion rates in flowing acid. However, where temperatures and flow rates are low, heavy waU steel pipe is sometimes used for transferring product acid. 

Nitric acid is normally stored in flat-bottomed, roofed tanks that are made from low-carbon, austenitic stainless steel. Most concentrations of nitric acid are available in tank cars and by truck. Stainless steel is necessary for concentrations up to 80% to 85%. Stronger solutions are less corrosive and may be stored in aluminum. Approximately 90% of nitric acid production is consumed on site to make ammonium nitrate (AN) fertilizers and some industrial explosives. Thus, the merchant market for nitric acid accounts for only 10% of the total. The production of AN fertilizers and most industrial explosives do not require acid concentrations in excess of the azeotropic composition of 68.8%. ... 

Storage and Distribution. Nitric acid is normally stored in flat-bottomed, roofed tanks that are made from low-carbon, austenitic stainless steel. Most concentrations of nitric acid are transported in tank cars and by truck. Stainless steel is necessary for concentrations up to 80 to 85 percent. Stronger solutions are less corrosive and may be stored in aluminum. 

In sulfuric acid production, acid brick lining of membrane coated mild steel tanks and reaction vessels is considered the most durable and versatile construction material for the sulfuric acid plant. Such linings wiil reduce the steel shell temperature and prevent erosion of the normally protective iron sulfate film that forms in stagnant, concentrated (oxidizing) sulfuric acid. Dilute (red uC ing) sulfuric acid solutions are very corrosive to carbon steel, which must be protected by impermeable (e.g., elastomeric) membranes and acid brick lining systems. Such acid brick linings often employ membranes comprising a thin film of Teflon or Kynar sandwiched between layers of asphalt mastic. 

The question of materials for electrolysis cells has also been mentioned. It should be emphasized again, however, that concentrated solutions of electrolytes in water can be highly corrosive media. This restricts the choice of materials of construction and in later chapters there will be many examples of cell bodies made from unusual materials such as concrete, asbestos, alumina, carbon and nickel as well as more normal structural materials, e.g. steel, polymers. 

The breakdown of an ore with a concentrated acid, on an industrial scale, is only justified if the mineral values are fairly high in concentration and of some economic importance. A high acid wastage upon unwanted minerals, e.g. carbonates, oxides, sulphates, phosphates and silicates, can normally be expected if these are present, and in any case the quantity of acid required merely to wet the ore and form a workable pulp is of necessity high. Corrosion problems are generally more severe than with dilute leaching acids and require more expensive materials of construction. 

Because in the metal oxygen diffuses less readily than hydrogen, this reaction normally takes place only near the surface. During steel production, it can lead to carbon depletion at the surface, requiring a thermal treatment to restore a uniform carbide concentration. From a corrosion point of view the reaction (9.14) is usually not critical. 

Linear polarization resistance can be applied to corrosion systems with electrochemical activation control, such as carbon steels and some stainless steels in low concentrations of sulfuric acid. For corrosion systems with a mass transport control or passivating systems, such as carbon steels in water with a pH between 5 and 10, the hnear polarization equation is not valid. Additionally, the normal fluctuation of corrosion potential during the measurements can significantly affect the accuracy of the measurements. Under most circumstances, a larger polarization than 10 mV may be used to increase the signal/noise ratio. However, it may put the system out of the linear region, introducing some additional errors in the measurements. 

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