Aluminum oxide corrosion inhibited

With the increased usage of 120°C cured, rubber modified epoxy structural adhesives for aluminum airframes, certain service problems have been observed which have been attributed to environmental factors. The problems associated with the combined effects of sustained load, elevated temperature and high humidity upon the aluminum substrate, corrosion inhibiting primers, and the structural epoxy adhesive matrix are discussed. A particular type adhesive matrix, based on acrylonitrile/butadiene rubber modified bisphenol type epoxy systems is discussed in detail, and important advances in the preparation of more moisture resistant aluminum (oxide) surfaces are reviewed. 

Corrosion-inhibiting primers based on this technology have been in continuous service since they were first utilized with nitrile epoxies in the late 1960s. These inhibitors function by passivating the aluminum. In this process, water permeating the adhesive bondline carries a certain amount of inhibitor to the oxide surface. 

The mechanism of developing corrosion protective properties in an inorganic coating principally consists of forming insoluble oxides on the netal surface. Additionally, oxides must have certain corrosion inhibition (redox) properties which can protect the nnetal substrate from corrosive species like Cl and 804 . In the case of chromate conversion coating, OCC, the oxides of aluminum and chromium have been responsible for their corrosion inhibitive properties which were derived from their soluble and insoluble portions of the... 

Aluminum, a very active metal, reacts rapidly with O2 from the air to form a surface layer of aluminum oxide, AI2O3, that is so thin that it is transparent. This very tough, hard substance is inert to oxygen, water, and most other corrosive agents in the environment. In this way, objects made of aluminum form their own protective layers and need not be treated further to inhibit corrosion. 

Because paint is such a common means of combating corrosion, many paint companies add chemicals to inhibit corrosion. A professional society of industrial painters lists 20 rust inhibitors that are environmentally approved additives for paints. Although the specific chemical content may vary, most of these corrosion inhibitors contain one of the following ions phosphate, borosilicate, chromate, or phosphosilicate. The precise mode of corrosion inhibition depends on several factors, but all of these ions, when part of a paint coating, form compounds with oxidized iron that inhibit fiirther formation of rust. Much like an oxide coating on the surface of aluminum prevents ongoing corrosion, these coatings stop the corrosion of the iron after only a small amount of material has reacted. This process is called passivation. 

Aluminum oxide (AI2O3) is an important constituent of the corrosion product film on this brass. Its presence markedly increases resistance to impingement attack in turbulent high velocity saline water. Arsenical aluminum brass (alloy C68700) tube used to be widely specified for marine condensers and heat exchangers in which impingement attack was likely to pose a serious problem. This alloy is susceptible to dezincification unless it is inhibited with a small amount of arsenic. 

Metals other than Fe are commonly used in alloys to inhibit corrosion and enhance mechanical properties. They include Mo, Cr, Ni, Cu, Zn, and Cd. The selection of metal species and the quantities in the iron matrices are based on the engineering properties of the materials. Pure metals in common use are limited to Fe, Al, Cu, and Ti. We know very little about biocorrosion of Al and Ti (Gu et al. 2000a). Aluminum (Al) reacts with molecular oxygen (O2) under ambient conditions, forming an oxidized layer of protective aluminum oxide on the outer surface of the material matrix. When Al ions are released, the free Al " is toxic to both the microflora (Appanna and Piperre 1996 Ulmer and Schinner 1999) and animals... 

Aluminum. When aluminum is immersed in water, the air-formed oxide film of amorphous y-alumina initially thickens (at a faster rate than in air) and then an outer layer of crystalline hydrated alumina forms, which eventually tends to stifle the reaction. In near-neutral air-saturated solutions, the corrosion of aluminum is generally inhibited by anions that are inhibitive for iron (e.g., chromate, benzoate, phos-... 

In near-neutral and deaerated solutions, the oxide film on anodized aluminum is stable and protective in distilled water and chloride solutions, as well as in solutions of inhibitive anions. Thus the inhibition of aluminum corrosion by anions differs from that of iron or zinc in that the presence of dissolved oxygen in the solution is not necessary to stabilize the oxide film. In corrosion inhibition by chromate ions, their interaction with the oxide film on aluminum has been shown to result in the formation of an outer layer of the film that is more protective due to its high electronic resistance and low dissolution rate. Chromate ions were also found to prevent the uptake and penetration of chloride ions into the aluminum oxide film. 

Specifically for aluminum corrosion released Cr oxoanions inhibit pit initiation by adsorbing onto aluminum oxides, thereby discouraging adsorption of anions such as chloride and sulfate, which promote dissolution and destabilization of the protective oxides. " Thus, competitive adsorption of chromates with regard to aggressive anions such as chloride and sulfate appears as another major property of chromate conversion coatings. ... 

The mechanism of corrosion inhihition, with these heavy metal chromates, hinges on the fact that they can passivate aluminum. When such a corrosion-inhibited bonded joint is attacked, a mixture of hydrated aluminum oxide and chromic oxide (Cr203) is formed (cf. the Alocrom process). This not only seals the oxide film, repairing the damage caused by the ingress of the electrolyte, but the presence of the stable chromic oxide also reduces the rate of dissolution of the aluminum oxide. The longevity of such a protection is due to the low solubility ( 1.2 g/L at 15 °C) of the chromate in water, which means that the chromate remains active for a considerable period of time. 

Cerium compounds have been found effective in reducing the corrosion rate ot aluminum alloys by inhibition ot the cathodic reaction. The cerium can be incorporated in the passive oxide film to provide a protective conversion coating [36,37]. 

Clean metallic aluminum is extremely reactive. Even exposure to air at ordinary temperatures is sufficient to promote immediate oxidation. This reactivity is self-inhibiting, however, which determines the general corrosion behavior of aluminum and its alloys due to the formation of a thin, inert, adherent oxide film. In view of the great importance of the surface film, it can be thickened by anodizing in a bath of 15% sulfuric acid (H2SO4) solution or by cladding with a thin layer of an aluminum alloy containing 1 % zinc. 

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