Aluminum surfaces, corroding

The intent of this paper is to present a methodology for estimating, from available information on concentrations and deposition velocities, the potential effects of anthropogenically derived acidic substances on indoor surfaces. Surface accumulation rates are derived that are applicable to all types of indoor surfaces. The discussion of the possible effects of the accumulated substances will concentrate on zinc and aluminum surfaces because data exists on the behavior of these metals in indoor environments (0. Aluminum forms a passivating oxide which protects against corrosion in most environments, while zinc is expected to corrode at a roughly linear rate over its lifetime. 

Why does iron continue to corrode even after a layer of rust forms on the surface, whereas aluminum stops corroding after a corrosion layer forms ... 

If 1,1,1-trichloroethane is not properly stabilized it forms hydrochloric acid in the presence of aluminum. HCl corrodes aluminum. The presence of free water invalidates the result of this test. An aluminum coupon is scratched beneafli flie surface of a solvent The coupon is observed for 10 min and 1 h and flie degree of corrosion is recorded in form of pass (no reaction) or fail (gas bubbles, color formation, or metal corrosion). The test is important to cleaning operations because aluminum should not be used for parts of machines (pumps, tanks, valves, spray equipment) in contact with corrosive solvent. 

As noted in Chapter 10, creep was one problem with aluminum wire used in the late 1970s. An additional problem concerns corrosion. In contrast to copper oxide, aluminum oxide is not a good conductor. As aluminum wire corrodes, aluminum oxide forms on the wire outside surface. Resistance increases, producing more heat and accelerating corrosion. This problem may lead to fire. Aluminum creep and corrosion multiply the potential for these effects. 

Steel corrodes by electrochemical reactions. In the presence of oxygen, at anodic areas ferric ions and at cathodic areas hydroxyl ions are formed. Aluminum generally corrodes more slowly than steel because of a dense, coherent layer of aluminum oxide. However, aluminum corrodes more rapidly than iron under either highly acidic or basic conditions. Also, salt affects the corrosion of aluminum more than it affects the corrosion of iron. Galvanized steel is protected since zinc acts as a sacrificial anode and a barrier preventing water and oxygen from reaching the steel surface. 

Iron, steel, stainless steel, Monel, and some plastics have proven satisfactory in methy-lamine service. Copper, copper alloys (including brass and bronze), zinc (together with zinc alloys and galvanized surfaces), and aluminum are corroded by the methylamines and should not be used in direct contact with them. Mercury and the methylamines can explode on contact, and instruments containing mercury must never be used with the methylamines. Among gasket and packing materials satisfactory for use with them are compressed asbestos, polyethylene, Teflon, and carbon steel or stainless steel wound asbestos. 

FLUORINE. F. In laboratory tests, 1100 alloy was resistant to fluorine at temperatures up to 450°C (842°F). In the presence of moisture, hydrofluoric acid is formed which corrodes aluminum alloys. Dry fluorine gas has been handled in aluminum alloy equipment. A durable protective coating is formed on the aluminum surfaces contacting the gas. See also Ref (1) p. 133, (2) p. 297. (3) p. 35, (7) p. 95. 

MERCURY. Hg. The action of metallic mercury on aluminum is unique. It tends to amalgamate with aluminum to produce a surface that corrodes at an extraordinary rate in the presence of moisture with the production of voluminous columnar corrosion products. When that reaction is started, the rate of corrosion is dependent upon relative humidity. When dry. metallic mercury reacts only with difficulty because of the oxide film on the aluminum surface. Traces of acidity or halides on the surface give rise to rapid attack. Solutions containing mercury ions tend to cause rapid pitting of aluminum alloys because of plating out of mercury in localized areas. Mercury can be removed from aluminum surfaces by treatment w ith 70% nitric acid. Mercury can be distilled away from an aluminum surface by treatment with steam or hot air. See also Ref (Dp- 136. (2) p. 446. (3) p. 80. 

Nevertheless the potential—pH diagrams are very useful to predict the corrosion tendencies in atmosphere and environments. The protective oxides films formed prevent the outdoor atmosphere corrosion. Films, such as boehmite (p-AlOOH), bayerite [a-Al(OH)3] and hydrag-Ulite [y-Al(OH)3] contain varying amounts of water. These films are amorphous. The crystalline films found on corroded aluminum surfaces are shown in Table 10.13. 

Table 10.13 Films formed on corroded aluminum surface...

Roberge, P. R., and Trethewey, K. R., The Fractal Dimension of Corroded Aluminum Surfaces, Journal of Applied Electrochemistry, 25 962-966 (1995). 

The native passive AI2O3 layer existing on the metal surface provides protection against corrosion. It is weU known that, depending on the nature on the electrolyte anions, this passive layer can be broken down leading to aluminum corrosion at a high potential. The [N(Tf)2] anion shows a corrosive effect on the aluminum collector, corroded around 3.8 V vs. Li/Li+ when the Li[N(T02] electrolyte was contained in lithium-ion batteries (LIBs) or electrochemical double-layer capacitor (EDLC) systems [ 116-119] with an aluminum-coated positive electrode. Therefore, it is very important before any use of RTMS as an electrolyte to control its effect on aluminum corrosion. 

In special cases where the surface hardness must be increased or chemical corrosion resistance is necessary (e.g. plasma etching with chlorine), anodized aluminum surfaces can be useful. Alloying elements, impurities, and heat treatment can influence the nature and quality of the anodized coating - typically, the more pure the aluminum alloy, the better the anodized layer. To build up a thick anodized layer on aluminum, it is necessary for the electrolyte to continuously corrode the oxide, producing a porous oxide layer. ASTM Specification B-580-73 designates seven thicknesses (up to 50 microns) for anodization. 

An equipment manufacturer in the US made a vacuum system with an aluminum chamber. A number of these systems were shipped to the Far East. After several months of use the equipment would not meet the pumpdown specifications and were returned to the factory at great expense. Investigation showed that the aluminum surfaces were corroded - probably from a chlorine-containing cleaner. Proper cleaning procedures for aluminum were then included in the operations manual for the equipment. 

Effect of Water. Except in cases of high-tempa-a-ture oxidation, gas-metal reactions, fretting, and certain hot, anhydrous organic chemicals such as phenol and methanol, alurrrinum does not corrode unless water is present on the surface. The water can appear in the form of isolated droplets, as a thin film of moisture condensed on an aluminum surface below the dew point, or as an aqueous solution. Water in contact with air contains dissolved oxygen, which must be present for corrosion of aluminum to occur. Deaeration usually stops the corrosion reaction. The protective surface filrn on aluminum thickens on exposure to water. This reaction is more rapid in the absence of oxygen. 

In natural environments, including saline conditions, zinc is anodic to aluminum and corrodes preferentially, giving protection to aluminum. Magnesium is similarly protective, although in severe marine environments it causes cathodic corrosion of aluminum because of an alkaline condition produced on the aluminum surface. Cadmium is neutral to aluminum and can safely be used in contact with it. The other stmc-... 

Another way to simulate alcladding is to apply a coating of an anodic alloy to an aluminum surface by thermal spray techniques, such as flame or plasma spray. These coatings act in the same way as the clad-(fing layer on an alclad product and corrode sacrifi-cially to protect the core alloy (Ref 11,12). Similarly, anodic coatings (zinc, pure aluminum or a more anodic aluminum alloy) can be applied by hot dipping or vapor deposition. 

These platings have potentials lower than that of aluminum (Fig. 5) hence, aluminum corrodes preferentially protecting both plating and solder. To provide maximum resistance to corrosion, only those areas covered by solder should be plated, thus allowing maximum exposure of aluminum surface. 

Rolling oil tanks were corroded on surfaces contacting the emulsion. Small pitlike depressions were present beneath aluminum soap deposits. Each pit was surrounded by a lightly etched region exactly mirroring deposit patterns (Fig. 6.26). 

Figure 8.4 Corroded aluminum-alloy casting valve seat. The seat was in contact with water having a pH between 9 and 10 for 3 months. Red marks were drawn on the surfaces to assist metallographic preparation.

This frontier s practical opportunities were first developed with submarines, which until the nuclear ones were limited to depths of only a few hundred feet. Many thousands of feet can now be navigated. The crushing pressures below the surface, which increase at a rate of about V2 psi per foot of depth, make corrosion a major threat to the operation and durability of many materials. For example, the life of uncoated magnesium bolts in contact with steel nuts is less than seventy-two hours, aluminum buoys will corrode and pit after only eleven months at just four hundred feet, and low-carbon steel corroded at a rate one-third greater than in surface waters. 

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