Aluminum erosion-corrosion

High Water Velocities. The abiUty of high water velocities to minimize fouling depends on the nature of the foulant. Clay and silt deposits are more effectively removed by high water velocities than aluminum and iron deposits, which are more tacky and form interlocking networks with other precipitates. Operation at high water velocities is not always a viable solution to clay and silt deposition because of design limitations, economic considerations, and the potential for erosion corrosion. 

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. 

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

Galvanic corrosion may also occur by transport of relatively noble metals, either as particulate or as ions, to the surface of an active metal. For example, ions of copper, perhaps resulting from corrosion or erosion-corrosion at an upstream site, may be carried by cooling water to the surfaces of aluminum, steel, or even stainless steel components. If the ions are reduced and deposit on the component surfaces, localized galvanic corrosion may result. 

Erosion/corrosion of the weld bead at the fracture location damaged the aluminum oxide film on the piping, thus allowing the mercury to wet and initiate cracking of the aluminium. 

Elevated speeds have a marked effect on wear, and this is more pronounced if the solution contains some solid particles in suspension. Aluminum forms films of aluminum nitrate or oxide in fuming nitric acid. At low flow rates there is no attack whereas for speeds greater than 1.22 m s the protective layer is removed and erosion-corrosion occurs more readily.16... 

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. 

Bronzes are copper-tin alloys which upon prolonged contact with the atmosphere form a dark patina that is much appreciated in the art world. In presence of certain pollutants such as chloride the dark patinas eventually turn to green. Aluminum-containing bronze forms surface films containing AI2O3 which improves the resistance to erosion corrosion compared to copper or brass. 

Figure 15.4 Mapping electrochemical material loss against mechanical erosion rates for a nonpassivating surface carbon steel (AISI1020) along with two potentially passivating surfaces of nickel aluminum bronze (NAB) one that has been thermally sprayed by high-velocity oxy-fuel deposition as a coating on carbon steel ( j and another which has been cast (A.). These results were obtained from jet impingement erosion-corrosion tests. Reprinted from Ref. [7]. Copyright (2007) with permission from Elsevier.

Standard Test Method for Cavitation Corrosion and Erosion-Corrosion Characteristics of Aluminum Pumps with Engine Coolants... 

Soft and low-strength metals such as copper, aluminum, and lead are especially susceptible to erosion corrosion, as are the metals and alloys that are inherently less corrosion resistant, such as carbon steels. 

Aluminum brass is resistant to erosion-corrosion and dezincification. It is used as condenser heat exchanger tube material for marine applications. A good example is the use of Al-Brass (76-79 Cu, 1.8-2.5 Al, 0.005 As, 0.007 Pb) as in marine service. 

Aluminum bronzes containing 7% Al, 2% Ni, show an outstanding resistance to de-alloying and cavitation corrosion in most fluids and seawater, because of nickel addition which is highly resistant to corrosion. Aluminum bronze, such as 76 Cu-22 Zn-2 Al, are used for marine heat exchangers and condenser because of its excellent corrosion resistance. Aluminum is responsible for increased corrosion resistance. But the velocity must not exceed a safe threshold to avoid erosion-corrosion. 

Dillon R.L., Hope R.S., Erosion-corrosion of aluminum alloys, REV, rapport HW-74359, April 1953. 

Affected metal surfaces will often contain grooves or wave-like marks that indicate a pattern of directional attack. Soft metals, such as copper and aluminum alloys, are often especially prone to erosion-corrosion, as are metals such as stainless steels, which depend on thin oxide films for corrosion protection. 

Most metals and alloys are susceptible to erosion-corrosion damage. Many depend upon the development of a surface film of some sort (passivity), for resistance to corrosion. Examples are aluminum, lead, and stainless steels. Erosion-corrosion results when these protective surfaces are damaged or... 

Erosion or abrasion corrosion occurs when there is a relative movement between a corrosive fluid and a metallic material immersed in it. In such cases, the material surface is exposed to mechanical wear, leading to metallically clean surfaces, which results in a more active metal. Most sensitive materials are those normally protected by passive oxide layers with inferior strength and adhesion to the substrate, such as lead, copper, steel and some aluminum alloys. When wearing particles move parallel to the material surface, the corrosion is called abrasion corrosion. On the other hand, erosion corrosion occurs when the wearing particles move with an angle to the substrate surface. ... 

Another test described in ASTM D 2809, Test Method for Cavitation Erosion-Corrosion Characteristics of Aluminum Puir ts with Engine Coolants. This test can be used to evaluate automotive water pump materials (aluminum castings) or the abOity of a coolant formulation to prevent metal loss. Chance (Ref... 

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. 

Another fouling mechanism that can occur is corrosion of boiler tubing and erosion of refractories due to formation of acids and their buildup in the combustion units from conversion of sulfur and chlorine present in the fuel. Fortunately, the amounts of these elements in most biomass are nil to small. The addition of small amounts of limestone to the media in fluidized-bed units or the blending of limestone with the fuel in the case of moving-bed systems are effective methods of eliminating this problem. Other sorbents such as dolomite, kaolin, and custom blends of aluminum and magnesium compounds are also effective (Coe, 1993). 

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