Corrosion Behavior of Aluminum and its Alloys

Aluminum is an active metal and its resistance to corrosion depends on the formation of the protective oxide film. According to the Pourbaix diagram the metal is passive in the pH range —4-9. The protective oxide film formed in water and atmospheres at ambient temperatures is amorphous and a few nanometres in thickness. The stability of the oxide film and its disruption results in corrosion. 

The dissolved oxygen in acid solution causes corrosion of aluminum the hydrogen, nitrogen, carbon dioxide and hydrogen sulfide have no effect. Hydrogen chloride is 

Alloy and temper Type casting Ultimate Yield In 50 mmc 

Fresh water. Aluminum and its alloys are not prone to corrosion on exposure to distilled water up to 180°C. Thus storage tanks, piping and fittings of the alloy can be used for handling distilled water. The composition of natural fresh water is variable. In spite of this restriction, the alloys are not attacked, even at 180°C in natural waters. It should be noted that pitting may occur when a small thickness of the sample is exposed. In this case Alclad 3003 is recommended for use to avoid failure due to pitting. 

Seawater. The recommended alloys for exposure to unpolluted seawater are 5XXX wrought alloys and 356.0 and 514.0 cast alloys27. It is likely that some pitting corrosion may occur and rates of the order of 3-6 pm/yr during the first year and 0.8-1.5 pm/yr averaged over a 10-yr period have been observed. The depth of seawater exposure of samples appears to be of no relevance. 

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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. 

The metallurgical characteristics of the aluminum oxide layer also depend on its physical metallurgy, such as defects and metallurgical structure included in the oxide layer. For instance, when intermetallic compound particles as secondary phases are exposed on the surface, a discontinuous oxide film with various defects is often produced at the metal-particle interface. This discontinuous oxide film is weakly or non-protective chemically and physically. Because corrosion is a chemical and electrochemical reaction on the surface, corrosion behavior is readily influenced by surface morphology. The aluminum surface is usually adsorbed or contaminated by water, gases and many kinds of micron-sized substances. Microscopic heterogeneous structures such as vacancies, steps, kinks, and dislocations, and macroscopic heterogeneous structures such as scratches, pits and other superficial blemishes influence the corrosion behavior of aluminum and its alloys to different extents. 

It is recognized that elements in solid solution are less detrimental to the corrosion of aluminum, but that the existence of secondary phases in the mass are harmful, because a discontinuous and non-protective oxide film is often formed at the matrix-particle interface. The harmful extent of the secondary phases depends on the kinds and amounts of the particles. It is important elec-trochemically to know the potential of the microstructural particle phases. The potential difference between aluminum matrix and secondary phase is of primary importance in the corrosion behavior of aluminum and its alloys. The potentials of solid solu-... 

In realistic applications it is necessary as a third step to check the corrosion behavior experimentally under the various electrochemical conditions expected. Environmental changes might include adhesion of corrosive materials metallurgical changes might include selective dissolution or deposition of noble metals. It is essential to inspect the corrosion behavior of aluminum and its alloys in a field test. 

Materials such as metals, alloys, steels and plastics form the theme of the fourth chapter. The behavior and use of cast irons, low alloy carbon steels and their application in atmospheric corrosion, fresh waters, seawater and soils are presented. This is followed by a discussion of stainless steels, martensitic steels and duplex steels and their behavior in various media. Aluminum and its alloys and their corrosion behavior in acids, fresh water, seawater, outdoor atmospheres and soils, copper and its alloys and their corrosion resistance in various media, nickel and its alloys and their corrosion behavior in various industrial environments, titanium and its alloys and their performance in various chemical environments, cobalt alloys and their applications, corrosion behavior of lead and its alloys, magnesium and its alloys together with their corrosion behavior, zinc and its alloys, along with their corrosion behavior, zirconium, its alloys and their corrosion behavior, tin and tin plate with their applications in atmospheric corrosion are discussed. The final part of the chapter concerns refractories and ceramics and polymeric materials and their application in various corrosive media. 

Chemically, the film is a hydrated form of aluminum oxide. The corrosion resistance of aluminum depends upon this protective oxide film, which is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired. The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it. The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode. 

Some of the voluminous literature on the oxidation and corrosion of aluminum and its alloys has a direct bearing on DMO. Pure aluminum is normally covered by an amorphous native oxide film which is partially converted to 7-alumina at the interface between the parent metal and the amorphous oxide when heated to 450°C in dry air [7-9]. This unusual behavior is explained by growth of the amorphous phase, through outward cation migration, while thickening of the 7-alumina is by epitaxial growth on the parent metal, controlled by inward oxygen diffusion. Termination... 

Alloying can increase passivity by incorporation of some components which stabilize the oxide as in the case of aluminum. Improvements in corrosion resistance have been found to correlate with an increase in the concentration of the alloying element or its oxide in the passive film and upgrade the passive behavior of Mg and its alloys (Shaw and Wolfe, 2005). 

Initial attempts to use aluminum for automotive trim were unsuccessful due to the corrosion behavior of the metal. It is therefore anodized for automotive trim applications to provide a protective oxide surface which acts as a barrier coating for corrosion pro tec tion. > 2 Aluminum and its alloys are susceptible to pitting and crevice corrosion in chloride containing environments. The corrosion resistance of anodized aluminum is therefore highly dependent on the quality of the anodized surface and the absence of scratches and other damage sites. 

It is well known that aluminum as such is fairly passive, because a very dense and uniform aluminum oxide AI2O3 layer is formed onto the metal to protect the metal from corrosion. Highly ductile light weight aluminum alloys that are passed through specific heat treatments can, however, make aluminum susceptible to corrosion. These materials may contain alloying elements such as magnesium and/or copper, which alter and complicate the corrosion behavior of aluminum. Typical forms of corrosion for the alloys are localized and pit corrosion. Due to the dense structure of the aluminum oxide layer, the corrosion rate of aluminum alloys is, however, substantially slower compared with corrosion/dissolution of CRS or HDG steel [15]. 

Song-mei, L., Hong-rui, Z. Jian-hua, L. (2007). Corrosion behavior of aluminum alloy 2024-T3 by 8-hydroxy-quinoline and its derivative in 3.5% chloride solution. Transactions ofNonf ous Metals Society of China 17 318-325. 

Aluminum, in its many forms is exceeded only by steel in tonnage directly exposed to the elements. It is produced in the form of wrought products, extrusions, and castings with a large variety of alloying elements to impart desired mechanical properties. Before anodizing, the atmospheric corrosion behavior of aluminum products fits into some fairly well-defined patterns that are related to composition. 

The present study is aimed to investigate the oxidation and corrosion behaviors of ternary Ni-Fe-Cr alloy as well as its reaction scale in light of the requirements for inert anodes in aluminum electrolysis. In particular, the influence of temperature on the oxidation and corrosion behaviors of Ni-Fe-Cr alloy was studied in oxygen and molten electrolyte at 700 °C - 950 C. 

Passivity — An active metal is one that undergoes oxidation (-> corrosion) when exposed to electrolyte containing an oxidant such as O2 or H+, common examples being iron, aluminum, and their alloys. The metal becomes passive (i.e., exhibits passivity) if it resists corrosion under conditions in which the bare metal should react significantly. This behavior is due to the formation of an oxide or hydroxide film of limited ionic conductivity (a passive film) that separates the metal from the corrosive environment. Such films often form spontaneously from the metal itself and from components of the environment (e.g., oxygen or water) or may be formed by an anodization process in which the anodic current is supplied by a power supply (see -> passivation). For example, A1 forms a passive oxide film by the reaction... 

The thin films responsible for passivity are often amorphous, and since the extent of solid solubility is dependent on the crystal structure, the rigid compositions associated with the crystalline state are not necessarily operative within these thin films. It seems possible, therefore, that with films that are predominantly oxide a certain concentration of hydroxyl ions could be present, and likewise, films that are predominantly hydroxide could contain a certain proportion of oxygen ions. This view is supported by the corrosion behavior of such metals as aluminum and the stainless steels, where different degrees of passivity are obtained by alloying or by slight changes of concentration of the corroding solution. 

Composite materials obtained by solidification of alloys have made remarkable progress in their development and applications in automotive and aerospace industries in recent decades. Among them the most current applications are the zinc and aluminum base composite materials (Long et al., 1991 Rohatgi, 1991). It is well-known that the corrosion behavior of MMCs is based on many factors such as the composition of the alloy used, the type of reinforcement particles used, the reinforcement particle sizes and their distribution in the matrix, the technique used for the manufacture, and the nature of the interface between the matrix and reinforcement. A very slight change in any of these factors can seriously affect the corrosion behavior of the material. 

Chang, J.K., Chen, S.Y., Tsai, W.T. et al. (2007) Electrodeposition of aluminum on magnesium alloy in aluminum chloride (AlClj)- -ethyl-3-methylimidazolium chloride (EMIC) ionic liquid and its corrosion behavior. Electrvchem. Commun,9, 1602-1606. 

Further work saw the use of SECM-AFM in conjunction with in situ AFM and Kelvin probe microscopy to examine and interpret the localized corrosion behavior of two aluminum alloys, EN AW-3003 and Al-Mn-Sr-Zr, which are commonly used as heat exchangers [55]. It was found that the Al-Mn-Sr-Zr alloy contained a smaller number of intermetallics with larger Volta differences, compared to the EN AW-3003 alloy. This correlated well with the SECM-AFM images that showed that the EN AW-3003 alloy was significantly more corrosion active, resulting in higher material loss during dissolution. 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

Ah MR, Nishikata A, Tsuru T (1997) Electrodeposition of aluminum-chromium alloys from AlQj-BPC melt and its corrosion and high temperatrrre oxidation behaviors. Electrochim Acta 42 2347-2351... 

The presence of trace additives is a change in environment that can be good or bad for certain metal systems. It is necessary to know the behavior of the metal in the system toward various additives to establish whether this is a positive or negative effect. For example, the presence of copper ions in solution improves the corrosion resistance of stainless steels in certain environments, while the presence of copper ions in solution will cause localized pitting and attack of aluminum alloys in other solutions. There is yet the wider option of adding inhibitors that suppress the corrosiveness of the main species in the solntion. 

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