Titanium alloys, seawater corrosion

These alloys have an hep crystal structure known as alpha phase. The beta-transus temperature of CP titanium alloys is -910 15 °C (1670 27 °F), depending on the oxygen content (Ref 1). These alloys are not strengthened by heat treatment, like some other titanium alloys. They also have excellent corrosion resistance in seawater and marine environments. 

Titanium alloys have the best corrosion resistance of the metals and alloys typically used in seawater. Titanium is a passive film-forming alloy and retains excellent resistance in both low and high velocity seawater. Some titanium alloys, specifically the all alpha and near-alpha alloys, do exhibit a susceptibility to stress corrosion cracking [72]. 

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

Titanium is fully resistant to natural seawater regardless of chemistry variations and pollution effects (i.e., sulfides). Twenty-year corrosion rates well below 0.0003 mm y have been measured on titanium exposed beneath the sea and in splash or tidal zones. In the sea, titanium alloys are immime to all forms of locahzed corrosion and withstand seawater impingement and flow velocities in excess of 30 m s Table 8.43 compares the erosion-corrosion resistance of unalloyed titanium with two commonly used seawater materials. In addition, the fatigue strength and toughness of most titanium alloys are unaffected in seawater, and many titaniinn alloys are immime to seawater stress corrosion. 

When in contact with other metals, titanium alloys are not subject to galvanic corrosion in seawater. However titanium may accelerate attack on active metals such as steel, aluminum, and copper alloys. The extent of galvanic corrosion will depend on many factors such as anode-to-cathode ratio, seawater velocity, and seawater chemistry. The most successful strategies eliminate this galvanic couple by using more resistant, compatible, and passive metals with titanium, alltitanium construction, or dielectric (insulating) joints. 

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). 

For the following pairs of alloys that are coupled in seawater, predict the possibility of corrosion and if corrosion is possible, note which alloy will corrode (a) Al/Mg (b) Zn/low carbon steel (c) brass/Monel (d) titanium/304 stainless steel and (e) cast iron/315 stainless steel. Clearly state any assumptions you make about compositions of alloys. 

Exposure to seawater results in decrease in critical stress intensity factor and the susceptibility to SCC68 0.2% Fe improves the resistance to SCC presence of >5 wt percent of A1 increases the velocity of cracking Sn in the alloy decreases SCC resistance chloride bromide and iodide induce or accelerate SCC69 Occurs by trangranular cleavage of a-phase in which a-phase controls the crack propagation rate Intergranular corrosion due to formation of titanium methoxide... 

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. 

Titanium s physical properties (high melting temperature, resistance to corrosion, strength, light weight) make it an ideal additive to alloys used by the aerospace industry in rockets and jet aircraft, for ship components that are exposed to seawater, and for biomedical implants such as artificial joints or pacemakers. Titanium dioxide is utilized as a white pigment in paint, paper, plastics, and cosmetics. It is also used in some stmscreens because of its abihty to absorb ultraviolet (UV) light. 

Example 3 Titanium usually eauses, in spite of its practical nobleness, little galvanic corrosion in contact with steel when the area ratio is about 1 1, because Ti is not an efficient eathode material (exceptions may occur in seawater wifli bacterial films). With a relatively large area of titanium, damaging corrosion may take place on steels, copper alloys ete. 

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