Erosion-corrosion titanium

The heat-transfer quaUties of titanium are characterized by the coefficient of thermal conductivity. Even though the coefficient is low, heat transfer in service approaches that of admiralty brass (thermal conductivity seven times greater) because titanium s greater strength permits thinner-walled equipment, relative absence of corrosion scale, erosion—corrosion resistance that allows higher operating velocities, and the inherently passive film. 

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

Metals that depend on a relatively thick protective coating of corrosion product for corrosion resistance are frequently subject to erosion-corrosion. This is due to the poor adherence of these coatings relative to the thin films formed by the classical passive metals, such as stainless steel and titanium. Both stainless steel and titanium are relatively immune to erosion-corrosion in most cooling water environments. 

Because alterations to equipment design can be cumbersome and expensive, a more economical approach may be to change the metallurgy of affected components. Metals used in typical cooling water environments vary in their resistance to erosion-corrosion. Listed in approximate order of increasing resistance to erosion-corrosion, these are copper, brass, aluminum brass, cupronickel, steel, low-chromium steel, stainless steel, and titanium. 

This example of aluminium illustrates the importance of the protective him, and hlms that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective him is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a signihcant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also affect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective him rather than to an increase in hardness. 

The original saltwater condenser tube made of admiralty brass was found to be susceptible to erosion-corrosion at tube ends. Aluminum brass containing 2% aluminum was more resistant to erosion in saltwater. Inhibition with arsenic is necessary to prevent dezincification as in the case of admiralty brass. The stronger naval brass is selected as the tube material when admiralty brass mbes are used in condensers. Cast brass or bronze alloys for valves and fittings are usually Cu-Sn-Zn compositions, plus lead for machinability. Aluminum bronzes are often used as tube sheet and channel material for exchangers with admiralty brass or titanium tubes exposed to cooling water. 

Reasonably, the corrosion form is typical at relatively high velocities between the material surface and flie fluid, and it is particularly intensive in cases of two-phase or multiphase flow, i.e. hquid-gas and liquid-solid particle flow. Components often liable to erosion corrosion are propellers, pumps, turbine parts, valves, heat exchanger tubes, nozzles, bends, and equipment exposed to liquid sputter or jets. Most sensitive materials are those normally protected by corrosion products with inferior strength and adhesion to flie substrate, e.g. lead, copper and its alloys, steel, and under some conditions aluminium/aluminium alloys. Stainless steel, titanium... 

In the absence of suspended particles, the corrosion rate of passive metals such as stainless steel or titanium in neutral media is not markedly affected by hydrodynamic conditions (Table 10.26). However, when exposed to slurries, these metals are subject to erosion corrosion because the suspended particles that impinge on the surface damage the passive film. As a consequence an anodic partial current flows which serves for film repair and repassivation of damaged areas. In the presence of aggressive anions such as chloride, passive film damage can lead to metal dissolution by pitting [23]. 

Solid p2ttticle erosion in salt water was found to increase anodic passivation in titanium. The rate of erosion for pure titanium wtis compared with the same for titanium alloys. It was found that titanium tJloys like Ti-6Al-4V showed significant increased resist2uice to erosion-corrosion. The addition of 0.1 % Ru to Ti-6Al-4V further increased the erosion-corrosion resistance of the alloy. 

The protective oxide film of most metals is subject to being swept away above a critical water velocity. After this takes place, accelerated corrosion attack occurs. This is known as erosion-corrosion. For some metals, this can occur at velocities as low as 2-3 ft/s. The critical velocity for titanium in seawater is in excess of 90 ft/s. Numerous corrosion-erosion tests have been conducted and all have shown that titanium has outstanding resistance to this form of corrosion. 

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. 

Nevilee A. and McDougall B. A. B. (2002), Electrochemical assessment of erosion-corrosion of commercially pure titanium and a titanium alloy in slurry impingement , Proc. Instn. Mech. Engrs., Part L-Materials, Design and Applications, 216,31 1. 

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

Plate anodes were used for corrosion protection in order to avoid damage due to erosion and cavitation. These consisted of enamelled steel bodies in which a metal oxide-coated titanium anode 1 dm in surface area was fitted. The enamel... 

Titanium impellers have been used in pumps employed for the conveyance of corrosive and erosive ore slurries, for organic chlorides containing hydrochloric acid and free chlorine , for handling moist chlorine gas, and in the wood-pulp and the textile-bleaching industry, particularly with sodium hypochlorite . 

Titanium Nitride. TiN is chemically stable. TiN forms an excellent diffusion barrier and has a low coefficient of friction. As such, it is well suited for reducing corrosion, erosion, and galling. It is used extensively as a coating for gear components and tube- and wire-drawing dies. 

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. 

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