Aluminum oxides, corrosion, behavior

Keywords Aluminum electrolysis, inert anode, oxidation, corrosion behaviors. 

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. 

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. 

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 corrosion resistance of stainless steels and nickel-based alloys in aqueous solutions can often be increased by addition of chromium or aluminum. " Chromium protects the base metal from corrosion by forming an oxide layer at the surface. Chromium is also considered to be an important alloying metal for steels in MCFC applications. Chromium containing stainless steel, however, leads to the induced loss of electrolyte. Previous studies done to characterize the corrosion behavior of chromium in MCFC conditions have shown the formation of several lithium chromium oxides by reaction with the electrolyte. This corrosion process also results in increased ohmic loss because of the formation of scales on the steel. Aluminum additions similarly have a positive effect on corrosion resistance. " However, corrosion scales formed in aluminum containing alloys show low conductivity leading to a significant ohmic polarization loss. 

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. 

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. 

A puzzling feature about corrosion is the difference between iron (or steel) and other metals in their behavior toward corrosion. Aluminum, for example, has a greater tendency to corrode than iron, but the corrosion of aluminum is not problematic. Why not As we will see, the answer lies in the nature of the products of the corrosion reaction. What differences are there between the corrosive formation of aluminum oxide and iron(III) oxide How can we picture corrosion at an atomic level to account for these differences ... 

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]. 

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. 

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-... 

Silver exhibits a corrosion behavior which hardly resembles that of any of the other metals described. Its unique behavior is to a large extent governed by the existence of Ag upon dissolution of silver into the aqueous layer. Ag2S is the most abundant component of the corrosion products formed. AgCl can form in environments with high chloride content, whereas no oxides, nitrates, sulfates, or carbonates have been reported in coimection with atmospheric exposure. Silver exhibits corrosion rates comparable to those of aluminum, lower than those of zinc, and much lower than those of iron. 

High-Temperature Oxidation and Corrosion Behaviors of Ni-Fe-Cr Alloy for Inert Anode Materials in Aluminum Electrolysis... 

High-temperature oxidation and corrosion behaviors of Ni-Fe-Cr alloy as inert anodes for aluminum electrolysis have been studied in oxygen and molten electrolyte. The oxidation and corrosion scales on the anodes tested were analyzed using XRD and SEM-EDS. The oxidation rate is found to increase with increasing temperature from 700 °C to 950 °C, which can be approximately described by an inverse power rate function. The oxidation scales at 750 °C, 920 °C and 950 °C contain Cr-rich phase along with FeCr204 and (Eeo.eCro.4)203. The corrosion extent of Ni-Fe-Cr anodes in electrolyte is dominated by temperature, which can make the scales thickness double from 700 °C to 750 °C or from 920 °C to 950 °C. Cr and Fe in the scales on the anode in electrolysis corrode preferentially into the molten electrolyte, while the nickel oxides could better sustain the corrosive environment in electrolysis. The results can be useful for developing inert anode material for potential application in aluminum electrolysis. 

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. 

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