Nickel-copper alloys, seawater 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). 

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. 

In seawater, nickel shows a corrosion rate below 0.13mmyr and could be classified as corrosion resistant. In stagnant seawater, formation of a biofilm on the surface is possible and this could lead to microbiological-induced corrosion. In order to prevent this, nickel-copper alloys with 30% Cu are used, because copper has an antibacterial effect. 

Nickel-copper and nickel-chromium-molybdenum alloys are the nickel-base alloys that are t5fpically used in seawater. The nickel-copper alloys have good corrosion resistance in high velocity seawater, but do exhibit localized corrosion in quiescent seawater [79]. Alloy 625, a nickel-chromium-molybdenum alloy, is susceptible to crevice corrosion in both quiescent and flow conditions [97-700]. Other nickel-chromium-molybdenum alloys, such as Alloys C-276, C-22, 59 and 686 have increased seawater crevice corrosion resistance as compared to Alloy 625 [97,98],... 

Materials with the alloy base material nickel-copper show minimum corrosion in seawater at a composition of 67% nickel and 33% copper. Materials with this base material composition are used in many marine engineering applications. The most important of these materials are listed in Table 61. 

Because of their lack of susceptibility to chloride-induced stress corrosion cracking, nickel-copper alloys are used in seawater desalination plants as pipes for evaporators or heat exchangers. An evaporator made of alloy 400 (NiCu 30 Fe, 2.4360) exhibited a corrosion rate of > 0.01 mm/a (> 0.4 mpy) after an exposure time of 225 days to a CaCl2 concentration of up to 35 % and a temperature of 433 K (160 C). The corrosion rate was 0.07 mm/a (2.76 mpy) for a NaCl solution saturated with water vapor and air at 366 K (93 °C) [73]. 

Monel is the Inco trademark of the original nickel-copper alloy developed in the 1930s. Monel has excellent resistance to organic fouling and corrosion in seawater. The most common cast grade is M35-1. Other cast grades are... 

Monel A type of corrosion-resistant nickel-copper alloy that combines high strength with high ductility. It is used for process equipment in salt and seawater applications. 

Uses. Copper—nickel—iron alloys, UNS C 96200 (90 10 copper nickel) and UNS C 96400 (70 30 copper nickel), are used in corrosion-resistant marine (seawater) appHcations. UNS C 96400 is used for corrosion-resistant marine elbows, flanges, valves, and pumps. Leaded nickel—brass, UNS C 97300 (12% nickel-silver), is used for hardware fittings, valves, and statuary and ornamental castings. 

B. C. Syiett and D. D. Macdonald [1979] The Validity of Electrochemical Methods for Measuring Corrosion Rates of Copper-Nickel Alloys in Seawater, Corrosion 35, 505-508. 

Removal of the corrosion product or oxide layer by excessive flow velocities leads to increased corrosion rates of the metallic material. Corrosion rates 2ire often dependent on fluid flow and the availability of appropriate species required to drive electrochemical reactions. Surface shear stress is a measure of the force applied by fluid flow to the corrosion product film. For seawater, this takes into account changes in seawater density and kinematic viscosity with temperature and salinity [33]. Accelerated corrosion of copper-based alloys under velocity conditions occurs when the shear surface stress exceeds the binding force of the corrosion product film. Alloying elements such as chromium improve the adherence of the corrosion product film on copper alloys in seawater based on measurements of the surface shear stress. The critical shear stress for C72200 (297 N/m, 6.2 Ibf/ft ) far exceeds the critical shear stresses of both C70600 (43 N/m, 0.9 Ibf/ft ) and C71500 (48 N/m, 1.0 Ibf/ft ) copper-nickel alloys [33]. 

Copper and some copper alloys do exhibit corrosion under advanced velocity conditions. Their corrosion resistance is dependent on the growth and maintenance of the protective layers formed on the metal surface. The maximum velocity recommended for copper in seawater is 0.9 m/sec. Alloying with nickel or aluminum increases the resistance of copper alloys to impingement attack [72,79]. 

S] Syrett, B. C., The Mechanism of Accelerated Corrosion of Copper-Nickel Alloys in Sulfide-Polluted Seawater, Corrosion Science. Vol. 21, 1981, p. 187. 

Strength at high temperature Inconels, Hastelloys Seawater corrosion resistance Copper, nickel, titanium alloys Creep resistance Steels and nickel alloys... 

Extensive exposure tests in the North Sea on the influence of the elements copper, chromium, aluminium, nickel and silicon on corrosion in seawater showed that the corrosion in the immersion zone in seawater is significantly reduced by suitable combinations of the alloying elements Cr -t Al, Cr -t Al -t Cu and Cr -t Si. At longer exposure times, the corrosion rates can be reduced to as little as 20% of the rates for unalloyed steel. In the tidal zone (TZ), however, only the combination Cr -t- Si results in an improvement, albeit of only 20% after four years. In the splash zone (SZ) improvements by a factor of 2 can be achieved [50]. 

The final group of copper alloys are the copper-nickel (cupronickels) alloys. They exhibit the best resistance to corrosion, impingement, and SCC of all the copper alloys. They are among the best alloys for seawater service and are immune to season cracking. Dilute hydrochloric, phosphoric, and sulfuric acids can be handled. They are almost as resistant as Monel to caustic soda. 

Copper-based alloys. The copper-based alloys are velocity-limited, as impingement attack occurs when the hydrodynamic effect caused by seawater flow across the surface of such alloys exceeds the value at which protective films are removed and erosion-corrosion occurs. Thus, if these alloys are to exhibit high corrosion resistance, they must be used at design velocities below this limiting value. A more detailed coverage of the marine usage of two important copper-nickel alloys is presented in the section on copper alloys. 

The protective film continues to become more protective with time, as indicated by corrosion rate measures made over several years. Studies in quiet seawater show that the time span approaches 4 years before the decrease in corrosion rate becomes negligible. In flowing water, the corrosion rate was found to decrease continually over at least a 14-year period, the effect being similar for both 90-10 and 70-30 alloys. The normal corrosion product film is thin, adherent, and durable. Once fully formed and reasonably mature, the film on copper-nickel alloys will withstand considerable excursions in water velocity, pollution, and other conditions normally adverse to the good performance of copper alloy tubing. Copper-nickel alloys remain resistant to corrosion in deaerated seawater at low pH, as has been experienced in numerous distillation-type desalination plants. ... 

Bailey, G. L., Copper Nickel Iron Alloys Resistant to Seawater Corrosion. Journal of the Institute of Metals, 79 (1951). 

The most common use of nickel is as an alloy metal with iron and steel to make stainless steel, which contains from 5% to 15% nickel. The higher the percentage of nickel in stainless steel, the greater the steel s resistance to corrosion—particularly when exposed to seawater. Nickel is also alloyed with copper to make Monel metal, which was widely used before stainless steel became more economical and practical. It was used for many purposes as varied as household appliances and general manufacturing. Nickel is also used to electroplate other metals to provide a noncorrosive protective and attractive finish. 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

IMaterials and Scaling Issues. Two aspects of the basically simple desalination process require special attention. One is the high corrosivity of seawater, especially pronounced in the higher temperature distillation processes, which requires the use of corrosion-resistant, and therefore expensive, materials. Typical materials in use are copper—nickel alloys, stainless steel, titanium, and, at lower temperatures, fiber-reinforced polymers and special concrete compositions (39). It is noteworthy that in quest of a lower initial cost, the use of inadequate materials of constmction in many locations combined with poor operation by virtually untrained hands led to rapid deterioration and failure of plants long before their estimated design life. Adequate experience suggests by now how to avoid such failures. The other aspect is scale formation (40,41), discussed in mote detail below. 

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