Impurities and corrosion

Water Treatment. Water and steam chemistry must be rigorously controlled to prevent deposition of impurities and corrosion of the steam cycle. Deposition on boiler tubing walls reduces heat transfer and can lead to overheating, creep, and eventual failure. Additionally, corrosion can develop under the deposits and lead to failure. If steam is used for chemical processes or as a heat-transfer medium for food and pharmaceutical preparation there are limitations on the additives that may be used. Steam purity requirements set the allowable impurity concentrations for the rest of most cycles. Once contaminants enter the steam, there is no practical way to remove them. Thus all purification must be carried out in the boiler or preboiler part of the cycle. The principal exception is in the case of nuclear steam generators, which require very pure water. These tend to provide steam that is considerably lower in most impurities than the turbine requires. A variety of water treatments are summarized in Table 5. Although the subtieties of water treatment in steam systems are beyond the scope of this article, uses of various additives maybe summarized as follows ... 

Residual radioactivity. As it was pointed out in [7.4], the amount of the long lived radioactivity generated in sodium by neutrons is negligible. Activation of sodium reaches equilibrium state in about ten years of the first cycle of its use and will never exceed this level. The long-lived radionuclides furnished by fission products, sodium impurities and corrosion activation products are chemical elements alien to sodium, that makes possible its external contamination at the reactor plant decommissioning stage. 

To overcome these difficulties, a practical pH scale for supercritical systems was defined somewhat arbitrarily, " based upon available estimates of dissociation constants Ki) and activity coefficients (y) for solutions of some common 1-1 electrolytes, such as HCl and NaOH, in much the same way as has been done for aqueous systems at low subcritical systems. The concentration was chosen to be reasonably low, to allow for complete solnbility and to yield reliable model estimates for Ka and y, but at the same time the concentration of the buffering system should be much higher than the concentrations of potential impurities and corrosion products. As a compromise of all of these factors, 0.01 m solutions of HCl and NaOH were proposed as the primary pH standards. "... 

The compressor intake air must be clean and free from solid and gaseous impurities abrasive dust and corrosive gases are particularly harmful. Exhaust fumes present a hazard if compressed air is required for breathing purposes. The possibility of contamination of the intake by discharge from pressure-relief devices of other plant must be taken into consideration and changes of wind direction must not be overlooked. 

In cases where passivity is impossible, corrosion can be prevented if the metal can reach equilibrium with the melt (case 1). The system usually undergoes some corrosion initially, when traces of oxidising impurities are reduced and the redox potential of the melt falls (Fig. 2.33). Finally, after a certain amount of corrosion has occurred, the metal becomes immune and corrosion ceases. In Fig. 2.33 complete equilibrium between metal and melt was still not quite reached even after several hundred hours exposure. 

In addition, it has fairly recently been recognised that impurities and alloying elements will also tend to segregate to free surfaces. The implications of this for corrosion resistance and particularly for passive-film formation have received relatively little attention. 

Yet more research and development effort concentrates on the diaphragm cell caustic evaporator, finding ways to aid the evaporation of the 10-12% caustic soda in brine to make it into a saleable product. Work is directed into methods of removing the salt products and impurities and preventing corrosion of the equipment. Recovery of the salt from the evaporated caustic soda is an important part of a diaphragm cell plant as the recovered salt is used in the strengthening of the feed brine. 

Organic toxic pollutants and chromium are present in the raw wastewater and normally consist of raw materials, impurities, and metals used as cooling water corrosion inhibitors. 

Efficient refining of the more volatile actinide metals (Pu, Am, Cm, Bk, and Cf) is achieved by selective vaporization for those (Pu, Am, Cm) available in macro quantities. The metal is sublimed at the lowest possible temperature to avoid co-evaporation of the less volatile impurities and then deposited at the highest possible temperature to allow vaporization of the more volatile impurities. Deposition occurs below the melting point of the metal to avoid potential corrosion of the condenser by the liquid metal. Very good decontamination factors can be obtained for most metallic impurities. However, Ag, Ca, Be, Sn, Dy, and Ho are not separated from Am metal nor are Co, Fe, Cr, Ni, Si, Ge, Gd, Pr, Nd, Sc, Tb, and Lu from Cm and Pu metals. 

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

Analysis of most crystalline scale deposits and mineral foulants taken from the waterside of cooling systems shows, not unexpectedly, considerable variation in composition. The analysis normally reveals the presence of mixtures of several salts and metal oxides, originally derived from water impurities or corrosion processes. 

---