Copper alloys alkaline corrosion

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

Corrosion of industrial alloys in alkaline waters is not as common or as severe as attack associated with acidic conditions. Caustic solutions produce little corrosion on steel, stainless steel, cast iron, nickel, and nickel alloys under most cooling water conditions. Ammonia produces wastage and cracking mainly on copper and copper alloys. Most other alloys are not attacked at cooling water temperatures. This is at least in part explained by inherent alloy corrosion behavior and the interaction of specific ions on the metal surface. Further, many dissolved minerals have normal pH solubility and thus deposit at faster rates when pH increases. Precipitated minerals such as phosphates, carbonates, and silicates, for example, tend to reduce corrosion on many alloys. 

Nickel is usually alloyed with elements including copper, chromium, molybdenum and then for strengthening and to improve corrosion resistance for specific applications. Nickel-copper alloys (and copper-nickel alloys see Section 53.5.4) are widely used for handling water. Pumps and valve bodies for fresh water, seawater and mildly acidic alkaline conditions are made from cast Ni-30% Cu type alloys. The wrought material is used for shafts and stems. In seawater contaminated with sulfide, these alloys are subject to pitting and corrosion fatigue. Ammonia contamination creates corrosion problems as for commercially pure nickel. 

Neutral and alkaline solutions Copper-base materials are resistant to alkaline solutions " over a wide range of conditions but may be appreciably attacked by strong solutions, particularly if hot. Copper/nickel alloys usually give the best results in alkaline solutions. Copper and copper alloys should be avoided if ammonia is present, owing to the danger of both general corrosion and, if components are under stress, stress corrosion. 

Essentially, the pH is controlled to suppress the hydrogen evolution cathodic reaction- The Pourbaix Diagram for iron indicates that high pH values as well as low values may lead to corrosion. The construction of these diagrams for higher than ambient temperatures - shows how the area of the alkaline zones increases considerably under boiler conditions, so that the risk of corrosion is correspondingly higher. Many feed systems contain copper alloys. 

It is generally accepted that brass containing less than 65 percent copper in the alloy, and certain nickel alloys, are suitable for use in acetylene service under normal conditions. Conditions involving contact with highly caustic salts or solutions, or contact with other materials corrosive to copper or copper alloys, can render the above generally acceptable alloys unsatisfactory for this service. The presence of moisture, certain acids, or alkaline materials tends to enhance the... 

Copper-containing deposits may be found in process equipment any time that the fluids come in contact with copper-base alloys. While most copper alloys are relatively resistant to corrosion, very small amounts of copper will dissolve, and then may replate on contact with an iron-base ailoy. Copper that is plated onto the steel will accelerate corrosion because of the galvanic nature of the iron/copper couple. The copper acts as the cathode for the oxidizing agent, while the anodic iron corrodes. Many different techniques have been developed for removing copper deposits. Included are sections on alkaline oxidizers, HCI/complexers, and chelating agent solvents. 

Dichloroethylene is usually shipped ia 208-L (55 gal) and 112-L (30 gal) steel dmms. Because of the corrosive products of decomposition, inhibitors are required for storage. The stabilized grades of the isomers can be used or stored ia contact with most common constmction materials, such as steel or black iron. Contact with copper or its alloys and with hot alkaline solutions should be avoided to preclude possible formation of explosive monochloroacetylene. The isomers do have explosive limits ia air (Table 1). However, the Hquid, even hot, bums with a very cool flame which self-extiaguishes unless the temperature is well above the flash poiat. A red label is required for shipping 1,2-dichloroethylene. 

Wilde, B. E. and Teterin, G. A., Anodic Dissolution of Copper-Zinc Alloys in Alkaline Solutions , Brit. Corrosion J., 2, 125 (1967)... 

Most simple inorganic salt solutions cause virtually no attack on aluminium-base alloys, unless they possess the qualities required for pitting corrosion, which have been considered previously, or hydrolyse in solution to give acid or alkaline reactions, as do, for example, aluminium, ferric and zinc chlorides. With salts of heavy metals —notably copper, silver, and gold —the heavy metal deposits on to the aluminium, where it subsequently causes serious bimetallic corrosion. 

Other metals, such as copper, nickel, or silver, have been used as electrode materials in connection with specific applications, such as the detection of amino acids or carbohydrates in alkaline media (copper and nickel) and cyanide or sulfur compounds (silver). Unlike platinum or gold electrodes, these electrodes offer a stable response for carbohydrates at constant potentials, through the formation of high-valence oxyhydroxide species formed in situ on the surface and believed to act as redox mediators (40,41). Bismuth film electrodes (preplated or in situ plated ones) have been shown to be an attractive alternative to mercury films used for stripping voltammetry of trace metals (42,43). Alloy electrodes (e.g., platinum-ruthenium, nickel-titanium) are also being used for addressing adsorption or corrosion effects of one of their components. The bifunctional catalytic mechanism of alloy electrodes (such as Pt-Ru or Pt-Sn ones) has been particularly useful for fuel cell applications (44). 

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