Titanium high-temperature corrosion

In industrial applications the environments usually contain more than one reactant. For example high temperature oxidation occurs in air by the combined attack of oxygen, nitrogen and quite frequently water vapour. However, most of the studies concerning the oxidation resistance are performed in dry oxygen or dry air. The oxidation behaviour of the intermetallic phases of theTi-Al system has recently received considerable attention. The influence of water vapour on the oxidation of titanium aluminides has not been studied intensively. There are only a few studies of the high temperature corrosion of titanium and its alloys. 

Fig. 11.24 Effect of coatings on a titanium alloy in a high-temperature corrosive environment [34].

I. Gurappa, Protection of titanium alloy components against high temperature corrosion. Mat. Sci. Eng. A 356 (2003) 372-380. 

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). 

Other alloys have been developed for use in particular corrosive environments at high temperatures. Several of these are age-hardenable alloys which contain additions of aluminum and titanium. Eor example, INCONEL alloys 718 and X-750 [11145-80-5] (UNS N07750) have higher strength and better creep and stress mpture properties than alloy 600 and maintain the same good corrosion and oxidation resistance. AHoy 718 exhibits excellent stress mpture properties up to 705°C as well as good oxidation resistance up to 980°C and is widely used in gas turbines and other aerospace appHcations, and for pumps, nuclear reactor parts, and tooling. 

Types 321 and 347 have additions of titanium and niobium, respectively, and are used in welding appHcations and high temperature service under corrosive conditions. Type 304L may be used as an alternative for Types 321 and 347 in welding (qv) and stress-reHeving appHcations below 426°C. 

Carbide-based cermets have particles of carbides of tungsten, chromium, and titanium. Tungsten carbide in a cobalt matrix is used in machine parts requiring very high hardness such as wire-drawing dies, valves, etc. Chromium carbide in a cobalt matrix has high corrosion and abrasion resistance it also has a coefficient of thermal expansion close to that of steel, so is well-suited for use in valves. Titanium carbide in either a nickel or a cobalt matrix is often used in high-temperature applications such as turbine parts. Cermets are also used as nuclear reactor fuel elements and control rods. Fuel elements can be uranium oxide particles in stainless steel ceramic, whereas boron carbide in stainless steel is used for control rods. 

Tatsuya, K. and Shuichi, F., Pitting Corrosion of Titanium in High-temperature Halide Solutions , Proc. 2nd Ini. Conf. Titanium Sci. Technol., 4, 2383 (1973)... 

It is in its behaviour to caustic alkalis that zirconium shows itself to be superior to those other elements of Groups IV and V whose resistance to corrosion results primarily from an ability to form surface films. Thus, in contrast to tantalum, niobium and titanium, zirconium is virtually completely resistant to concentrated caustic solutions at high temperatures, and it is only slightly attacked in fused alkalis. Resistance to liquid sodium is good. Zirconium is thus an excellent material of construction for sections of chemical plant demanding alternate contact with hot strong acids and hot strong alkalis—a unique and valuable attribute. 

Zirconium alloys have been much less thoroughly studied than titanium alloys. The main application of interest has been for nuclear reactor components where good corrosion resistance combined with a low neutron capture cross-section has been required. Corrosion fatigue crack growth in these alloys in high temperature (260-290°C) aqueous environments typical of... 

Operation of cells at higher temperatures such as 80°C, as in membrane fuel cells, is not encouraged here because of the corrosion instability of the hardware, manufactured from titanium or titanium alloy. Even without such constraints, however, this high temperature would be unwelcome as the water produced is present as steam - without the conductive bridge of the liquid phase it would be necessary to bond the catalytic particles to the membrane with all the associated problems of technology and cost. 

For most pH values and at most potentials, titanium is passive and no corrosion takes place. This is revealed by the white areas of Fig. 23.2. However, there are some exceptions. Consider alkaline attack (Fig. 23.3). It is occasionally assumed that titanium cannot be used in the alkaline conditions of hypochlorite and excess alkalinity, etc. Corrosion data do not sustain this assumption. It is only at high temperatures and high concentrations that corrosion becomes a factor of concern, as does the increase in erosion under such conditions. 

Given titaniums lightness, strength, and resistance to corrosion and high temperatures, its most common use is in ahoys with other metals for constructing aircraft, jet engines, and missiles. Its alloys also make exceUent armor plates for tanks and warships. It is the major metal used for constructing the stealth aircraft that are difficult to detect by radar. 

HP IR cells need to exhibit high mechanical strength and resistance to corrosion by solvents and reagents. They are often fabricated from austenitic steels (e. g. type 316) which are satisfactory for relatively mild temperatures and pressures but can be corroded by acid or form [Fe(CO)5] and [Ni(CO)4] by reaction with CO. Alternative materials for construction include some titanium alloys (which can be vulnerable to primary alcohols at high temperature) and nickel-molybdenum-chromium alloys (e. g. Hastelloy C-276, Hastelloy B2) which are highly resistant to reducing, oxidising and acidic conditions. 

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