Aluminum alloys erosion

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. 

Because of its erosively destructive effects, the thermite reaction previously described, must be modified in such a way that instead of a separation into liquefied layers, the whole reacting mass sinters and stays in place. This is achieved by replacing the aluminum, all or in part, by magnesium, whose highly refractory oxide permeates the liquefied mass. Such a composition is said to be useful for the wdd-ing of aluminum or aluminum-alloy wire cables, as explained in a Swiss patent. Formula 199, quoted from this patent, is actually not part of the patent claims, which are concerned with means of consolidating mixtures for use as heat cartridges. 

Furthermore, some of these countermeasures may increase an electrochemical or a physical corrosion of materials. An increase in flow velocity, for example, may increase erosion of constmctional materials, whereas altering the pH may cause corrosion of copper or aluminum alloys. Thus, benefit and potential cost should be evaluated carefully before choosing a certain countermeasure. 

STEAM. H2O. In laboratory tests under static conditions, alloy 3003 was found to be resistant to pure steam over distilled water at temperatures up to 268 C (514°F). In fact, aluminum alloys exposed to steam at these temperatures had improved resistance to corrosion by other environments because of the increased thickness of the oxide film on the surface. In the same tests, steam at 268°C (514°F) was corrosive. High pressure steam can erode aluminum alloys by impingement corrosion erosion, particularly when the jet of steam is perpendicular to the surface. Aluminum alloy equipment including heat exchangers. dryers, steam jacketed kettles, piping have been used to handle steam in the petroleum, chemical and food processing industries. See also Ref (1) p. 144, (2) p. 778, (4) p. 49, (7) p. 175. 

Reinforcing aluminum alloys with ceramic fibers can provide a useful increase in elastic modulus (especially at elevated temperatures) and improve creep strength and heat erosion resistance. The disadvantages are decreased elongation to fracture and more difficult machining characteristics. 

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. 

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. ... 

The importance of prrqter heat treatment on the resistance of aluminum alloys to cavitation erosion is demonstrated in the following example. 

Example It Cavitation Erosion of a Water-Cooled Aluminum Alloy 6061-T6 Combustion Chamber. Equipment in which an assembly of inline cylindrical components rotated in water at 1040 rpm displayed excessive vibration after less than 1 h of iteration. The malftmction was traced to an aluminum alloy 6061-T6 combustion chamber (Fig. 4a) that was part of the rotating assembly. 

Cavitation erosion orrosion of cast 319 aluminum alloy studied by cor-rosion current versus time curves under potentiostatic control at -0.60 V relative to a calomel electrode, (a) Poorly inhibited coolant. b) WelMnhibited cool ant. Source Ref 7... 

S. Vaidya and C.M. Pteece, Cavitation Erosion of Age-Haidenable Aluminum Alloys, MelalL Trans. A, Vol 9,1978, p 299-307... 

Electroplating. Electroplated coatings are applied to aluminum alloys to obtain a spedfic metallic appearance increased resistance to wear, abrasion or erosion increased electrical conductivity improved solderability or improved liictional properties. Although electroplated metal coatings are occasionally... 

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. 

Ship hulls Painting cannot always protect hostile marine conditions, in ships and, areas above keel blocks. Stem and mdder areas suffer erosion and corrosion due to the high turbulence caused by the propeller coupled with the galvanic effects of the noble bronze propeller. Effective cathodic protection of ship hulls and similar marine structures in seawater against corrosion can be apphed using either aluminum or zinc alloy sacrificial anodes. Twenty percent of the anodes required for full hull protection are required for stern protection only. 

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. 

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