Oxygen corrosion aluminum

Aluminum alloys are essentially unaffected by dissolved oxygen in pure water up to 350°F (180°C). Although much of aluminum s corrosion resistance is due to the presence of an adherent oxide film, oxygen is not necessary to form the layer. Direct reaction with water can pro-... 

If sea water is first deaerated, only a small amount of corrosion inhibitor, if any, probably would be needed to prevent attack on steel, or copper-base alloys. For aluminum, oxygen is needed to promote a protective film. 

Formation of interphases incorporating aluminum, oxygen, and alloying elements has been observed in some AI2O3/AI MMCs. Preferential corrosion near fibers and particles is sometimes noticed. 

Thermodynamics tells us that aluminum is a reactive metal. Despite this aluminum has excellent corrosion resistance. Because aluminum has strong affinity for oxygen, the surface aluminum oxide film protects the metallic aluminum from corrosion in many environments. Before mentioning the corrosion behavior, it is necessary to know the structure and properties of surface oxide film formed on aluminum. 

Aluminum and its alloys have been extensively used for structural applications with success. Aluminum resists corrosion from the atmosphere if there is an absence of narrow crevices. Many statues erected, over a hundreds of years ago, have not deteriorated badly which is in contrast with aluminum cables used in seawater. The corrosion resistance of aluminum is due to its tendency to form a compact oxide layer over the surface. The oxide formed offers a high resistance to corrosion. The normal surface film present in air is about 1 nm thick. The film thickness increases at the elevated temperature. The film growth is more rapid in water than in oxygen. 

For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl ) is the most frequently encountered in service. The effect of chloride ion concentration on the pitting potential of aluminum 1199 (99.99-i-% Al) is shown in Fig. 6. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential. In the absence of dissolved oxygen or other cathodic reactant, aluminum will not corrode by pitting because it is not polar-... 

Many alloys react with oxygen and alter their surface microstructures and properties. Figure 4.25 shows the oxide scale and second phase observed in a titanium alloy when it is exposed to oxygen. Corrosive oxidation is an electrochemical reaction where a metal loses its electrons and becomes a cation. Metallurgically, it is a reaction between metal and oxygen to form an oxide. For instance. Equation 4.25 shows the oxidation of aluminum forming aluminum oxide, that is, alumina AI2O3 ... 

Vanadium is resistant to attack by hydrochloric or dilute sulfuric acid and to alkali solutions. It is also quite resistant to corrosion by seawater but is reactive toward nitric, hydrofluoric, or concentrated sulfuric acids. Galvanic corrosion tests mn in simulated seawater indicate that vanadium is anodic with respect to stainless steel and copper but cathodic to aluminum and magnesium. Vanadium exhibits corrosion resistance to Hquid metals, eg, bismuth and low oxygen sodium. 

Foulants enter a cooling system with makeup water, airborne contamination, process leaks, and corrosion. Most potential foulants enter with makeup water as particulate matter, such as clay, sdt, and iron oxides. Insoluble aluminum and iron hydroxides enter a system from makeup water pretreatment operations. Some well waters contain high levels of soluble ferrous iron that is later oxidized to ferric iron by dissolved oxygen in the recirculating cooling water. Because it is insoluble, the ferric iron precipitates. The steel corrosion process is also a source of ferrous iron and, consequendy, contributes to fouling. 

Alcohols may be corrosive to some aluminum alloys. In an aqueous mixture corrosion may still occur because of dissolved ions from residual salts. At high temperatures and in the presence of residual oxygen from the air, glycols are oxidized slowly to the corresponding acids. These acids can corrode metals. 

Steel objects, when exposed to humid atmospheres or when immersed in electrolytes, corrode at a rapid rate. For example, abrasively polished, cold-rolled steel panels will show signs of rust within 15 minutes when immersed in dilute chloride solutions with pH in the range of 7-10. One of the methods used to control this rapid corrosion is to coat the metal with a polymeric formulation such as a paint. The role of the paint is to serve primarily as a barrier to environmental constituents such as water, oxygen, sulfur dioxide, and ions and secondarily as a reservoir for corrosion inhibitors. Some formulations contain very high concentrations of metallic zinc or metallic aluminum such that the coating provides galvanic protection as well as barrier protection, but such formulations are not discussed in this paper. 

It was demonstrated that the change in thickness of these layers depends on the physicochemical properties of water in these thin water layers. It is reported that on iron surfaces, the number of adsorbed water layers is about 15 at RH 55% and 90 at 100%. Similar values are obtained for Copper and Zinc however significant differences are reported for Platinum, gold, aluminum and silver. These monolayers have been calculated only in presence of water (without oxygen) where the corrosion process is very slow and, consequently, in conditions far from the reality. 

The oxidation process affecting iron is harmful. As the rust (or iron oxide) falls off It exposes a fresh surface of iron to the air. But the oxygen captured from the air by aluminum helps to form a protective layer that sticks to the surface of the metal and actually prevents corrosion. However, it robs the metal of its shiny appearance. 

ANODIC OXIDATION. Oxidation is defined not only as reaction with oxygen, but as any chemical reaction attended by removal of electrons. Therefore, when current is applied to a pair of electrodes so as to make them anode and cathode, the former can act as a continuous remover of electrons and hence bring about oxidation (while the latter will favor reduction since it supplies electrons). This anodic oxidation is utilized in industry for various purposes, One of tire earliest to be discovered (H, Kolbe. 1849) was the production of hydrocarbons from aliphatic acids, or more commonly, from their alkali salts. Many other substances may be produced, on a laboratory scale or even, in some cases, on an economically sound production scale, by anodic oxidation. The process is also widely used to impart corrosion-resistant or decorative (colored) films to metal surfaces. For example, in the anodization or Eloxal process, the protection afforded by the oxide film ordinarily present on the surface of aluminum articles is considerably increased by building up this film by anodic oxidation. 

The Office of Saline Water is directing a large number of investigations into the feasibility of new processes for producing fresh water starting with sea or brackish water as a source. It is desired that these plants last for 20 years or more. This paper points up ways in which the economic waste resulting from corrosion in saline water plants can be avoided. The article is based on a review of the corrosion literature and on consultations with marine experts in the field. Of the many materials for distillation plants, steel is the most important. It can be used to handle sea water below 250° F., if proper steps are taken such as the removal of all air (oxygen) from solution. For severe service and better performance metals like titanium, Hastelloy C, Monel, cupro-nickels, aluminum, aluminum brass, or Admiralty brass are used. 

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