Aluminum degradation

Mercuiy is the primary liquid metal that degrades aluminum. Liquid mercuiy does not wet an aluminum oxide surface, but if the natural oxide film in penetrated aluminum dissolves in the mercury to form an amalgam starting a very rapid reaction. The dissolved aluminum oxidizes immediately in the presence of moisture and more aluminum dissolves. This reaction is assisted by the presence of halides. The mercury penetration tends to proceed along grain boundaries, and if tensile stresses are present in the metal, drastic splitting and the exposure of further film free metal occurs. Mercury can plate out of aqueous solutions to produce this effect. A mercuiy content of greater than 0.01 ppm is cause for concern. Detection of even lesser amounts of mercury may indicate a problem, since mercury tends to evaporate and low levels are difficult to analyze. Common sources of mercuiy are broken thermometers and mercury vapor bulbs, or mercury manometers that have been over-pressurized. 

Flame spray metallising is widely used for the protection of metal against corrosion, especially for in situ protection of stmctural members. The principal metal used for spraying of plastics is sine. Aluminum and copper are also used. If the distance from the part is too great, the zinc solidifies before it touches the part and adhesion is extremely poor. If the molten zinc oxidizes, conductivity and adhesion are poor. If the distance is too short, the zinc is too hot and the plastic warps or degrades. These coatings are not as dense as electrically deposited coatings because of numerous pores, oxide inclusions, and discontinuities where particles have incompletely coalesced. 

Because pure aluminum is n picaUy too soft to be drawn into a fine wine, it is often alloyed with 1° o sihcon or 1° o magnesium to provide a sofid solution-strengthening mechanism. The resistance of Al-1° o Mg wine to fatigue failure and to degradation of ultimate strength after exposure to elevated temperatures is superior to that of Al—1° o Si wine. 

Methylphenol is converted to 6-/ f2 -butyl-2-methylphenol [2219-82-1] by alkylation with isobutylene under aluminum catalysis. A number of phenoHc anti-oxidants used to stabilize mbber and plastics against thermal oxidative degradation are based on this compound. The condensation of 6-/ f2 -butyl-2-methylphenol with formaldehyde yields 4,4 -methylenebis(2-methyl-6-/ f2 butylphenol) [96-65-17, reaction with sulfur dichloride yields 4,4 -thiobis(2-methyl-6-/ f2 butylphenol) [96-66-2] and reaction with methyl acrylate under base catalysis yields the corresponding hydrocinnamate. Transesterification of the hydrocinnamate with triethylene glycol yields triethylene glycol-bis[3-(3-/ f2 -butyl-5-methyl-4-hydroxyphenyl)propionate] [36443-68-2] (39). 2-Methylphenol is also a component of cresyHc acids, blends of phenol, cresols, and xylenols. CresyHc acids are used as solvents in a number of coating appHcations (see Table 3). 

The anodized surface is often subjected to additional treatment before the radiation-sensitive coating is appHed. The use of aqueous sodium siUcate is well known and is claimed to improve the adhesion of diazo-based compositions ia particular (62), to reduce aluminum metal-catalyzed degradation of the coating, and to assist ia release after exposure and on development. Poly(viQyl phosphonic acid) (63) and copolymers (64) are also used. SiUcate is normally employed for negative-workiag coatings but rarely for positive ones. The latter are reported (65) to benefit from the use of potassium flu o r o zirc onate. 

Reactions other than those of the nucleophilic reactivity of alkyl sulfates iavolve reactions with hydrocarbons, thermal degradation, sulfonation, halogenation of the alkyl groups, and reduction of the sulfate groups. Aromatic hydrocarbons, eg, benzene and naphthalene, react with alkyl sulfates when cataly2ed by aluminum chloride to give Fhedel-Crafts-type alkylation product mixtures (59). Isobutane is readily alkylated by a dipropyl sulfate mixture from the reaction of propylene ia propane with sulfuric acid (60). 

Methylene chloride is one of the more stable of the chlorinated hydrocarbon solvents. Its initial thermal degradation temperature is 120°C in dry air (1). This temperature decreases as the moisture content increases. The reaction produces mainly HCl with trace amounts of phosgene. Decomposition under these conditions can be inhibited by the addition of small quantities (0.0001—1.0%) of phenoHc compounds, eg, phenol, hydroquinone, -cresol, resorcinol, thymol, and 1-naphthol (2). Stabilization may also be effected by the addition of small amounts of amines (3) or a mixture of nitromethane and 1,4-dioxane. The latter diminishes attack on aluminum and inhibits kon-catalyzed reactions of methylene chloride (4). The addition of small amounts of epoxides can also inhibit aluminum reactions catalyzed by iron (5). On prolonged contact with water, methylene chloride hydrolyzes very slowly, forming HCl as the primary product. On prolonged heating with water in a sealed vessel at 140—170°C, methylene chloride yields formaldehyde and hydrochloric acid as shown by the following equation (6). 

The most important reactions of trichloroethylene are atmospheric oxidation and degradation by aluminum chloride. Atmospheric oxidation is cataly2ed by free radicals and accelerated with heat and with light, especially ultraviolet. The addition of oxygen leads to intermediates (1) and (2). 

In the presence of aluminum, oxidative degradation or dimerization supply HCl for the formation of aluminum chloride, which catalyzes further dimerization to hexachlorobutene. The latter is decomposed by heat to give more HCl. The result is a self-sustaining pathway to solvent decomposition. Sufficient quantities of aluminum can cause violent decomposition, which can lead to mnaway reactions (1,2). Commercial grades of trichloroethylene are stabilized to prevent these reactions in normal storage and use conditions. 

Catalyst Deactivation. Catalyst deactivation (45) by halogen degradation is a very difficult problem particularly for platinum (PGM) catalysts, which make up about 75% of the catalysts used for VOC destmction (10). The problem may weU He with the catalyst carrier or washcoat. Alumina, for example, a common washcoat, can react with a chlorinated hydrocarbon in a gas stream to form aluminum chloride which can then interact with the metal. Fluid-bed reactors have been used to offset catalyst deactivation but these are large and cosdy (45). 

Etched beryllium light absorbers are somewhat more robust than Martin Black 54 but, as shown m Fig. 10, are meffective above certam wavelengths. Moreover, both beryllium and aluminum are sensitive to envuonmental degradation and may degrade thermally due to their low melting temperatures. 

Although the above experiments involved exposure to the environment of unbonded surfaees, the same proeess oeeurs for buried interfaces within an adhesive bond. This was first demonstrated by using electrochemical impedance spectroscopy (EIS) on an adhesive-covered FPL aluminum adherend immersed in hot water for several months [46]. EIS, which is commonly used to study paint degradation and substrate corrosion [47,48], showed absorption of moisture by the epoxy adhesive and subsequent hydration of the underlying aluminum oxide after 100 days (Fig. 10). After 175 days, aluminum hydroxide had erupted through the adhesive. 

Chemical Reactivity - Reactivity with Water No reaction Reactivity with Common Materials Reacts violently with aluminum. May cause fire on contact with common materials such as wood, cotton, straw. Iron, steel, stainless steel, and copper are corroded by bromine and will undergo severe corrosion when in contact with wet bromine. Plastics are also degraded/ attacked by bromine except for highly fluorinated plastics which resist attack Stability During Transport Stable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization Not pertinent Inhibitor of Polymerization Not pertinent. 

Exfoliation corrosion is especially prevalent in aluminum alloys. The grain structure of the metal determines whether exfoliation corrosion will occur. In this form of corrosion, degradation propagates below the surface of the metal. Corrosion products in layers below the metal surface cause flaking of the metal. 

The correct structure (3) for this compound was first proposed in 1922 by Pieroni and Moggi on the basis of the isolation of succinic acid by chromic acid oxidation. Full confirmation of this structure was more recently obtained by Potts and Smithby the degradation outlined in Scheme 1. The dipyrrylbutane was synthesized by the lithium aluminum hydride reduction of the known dipyrrylbutane-... 

Aluminum Chlorhydrate (ACH), A12(0H)5C1 ACH reacts very similarly to PAC and is available only as a solution. It is a very highly basic product (80%). Upon dilution, PAC and ACH solutions tend to act as highly cationic polymers. After dilution and with time, they also begin to degrade and act like alum. This period may be on the order of 1 to 10 minutes, depending on pH level and temperature, The dose rate is similar to that of PAC. Good turbidity and color removal are achieved with ACH. 

---