Metal/aqueous-environment reactions corrosion

Criteria for Metal/Aqueous-Environment Reactions Corrosion... 

Atmospheric corrosion results from a metal s ambient-temperature reaction, with the earth s atmosphere as the corrosive environment. Atmospheric corrosion is electrochemical in nature, but differs from corrosion in aqueous solutions in that the electrochemical reactions occur under very thin layers of electrolyte on the metal surface. This influences the amount of oxygen present on the metal surface, since diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Atmospheric corrosion rates of metals are strongly influenced by moisture, temperature and presence of contaminants (e.g., NaCl, SO2,. ..). Hence, significantly different resistances to atmospheric corrosion are observed depending on the geographical location, whether mral, urban or marine. 

Uniform corrosion is the deterioration of a metal surface that occurs uniformly across the material. It occurs primarily when the surface is in contact with an aqueous environment, which results in a chemical reaction between the metal and the service environment. Since this form of corrosion results in a relatively uniform degradation of apparatus material, it can be accounted for most readily at the time the equipment is designed, either by proper material selection, special coatings or linings, or increased wall thicknesses. 

Corrosion may be described as the undesirable reaction of a metal or alloy with its environment and it follows that control of the rate of process may be eflFected by modifying either of the reactants. In corrosion inhibition , additions of certain chemicals are made to the environment, although it should be noted that an aqueous environment can, in some cases, be made less aggressive by other methods, e.g. removal of dissolved oxygen or adjustment of pH. 

Corrosion refers to the loss or conversion into another insoluble compound of the surface layers of a solid in contact with a fluid. The solid is normally a metal, but the term corrosion is also used to refer to the dissolution of ionic crystals or semiconductors. In the majority of cases the fluid is water, but an important exception is the reaction of metallic surfaces with air at high temperature, leading to oxide formation, or, in industrial environments, to sulphides, etc. In the context of this book, corrosion of metals or semiconductors in contact with aqueous solution or humid air at normal temperatures is of predominant interest. 

Corrosion can also occur by a direct chemical reaction of a metal with its environment such as the formation of a volatile oxide or compounds, the dissolution of metals in fused metal halides. The reaction of molybdenum with oxygen and the reaction of iron or aluminum with chlorine are typical examples of metal/gas chemical reactions. In these reactions, the metal surface stays film-free and there is no transport of electrical charge.1 Fontana and Staehle2 have stated that corrosion should include the reaction of metals, glasses, ionic solids, polymeric solids and composites with environments that embrace liquid metals, gases, aqueous and other nonaqueous solutions. 

Rust of iron (the most abundant corrosion product), and white rust of zinc are examples of nonprotective oxides. Aluminum and magnesium oxides are more protective than iron and zinc oxides. Patina on copper is protective in certain atmospheres. Stainless steels are passivated and protected, especially in chloride-free aqueous environments due to a very thin passive film of Cr2C>3 on the surface of the steel. Most films having low porosities can control the corrosion rate by diffusion of reactants through the him. In certain cases of uniform general corrosion of metals in acids (e.g., aluminum in hydrochloric acid or iron in reducible acids or alkalis), a thin him of oxide is present on the metal surface. These reactions cannot be considered hlm-free although the him is not a rate-determining one.1... 

The oxygen electrode polarization is a measure of the degree of irreversibility of the electrochemical reaction. To find an effective electrocatalyst for reactions (19.24) and (19.25) is of a great interest because of their technical relevance in water electrolysis, fuel cells, metal corrosion in aqueous environments, biological processes, etc. 

Reaction 6 may involve an [H-H]+(ads) intermediate. The hydrogen electrode reactions are of interest from the standpoint of hydrogen-consuming fuel cells, competing reactions in various battery systems, the generation of hydrogen gas by water electrolysis, and the complementary cathodic reaction in metal corrosion in aqueous environments. The predominant pathway and rate-determining steps have been identified on a few metal electrode surfaces (30). 

Before examining in detail the theories of aqueous corrosion processes and the bases for making quantitative calculations of corrosion rates, it will be useful to develop qualitatively the major phenomena involved. The following sections review several general types of metal/corrosive-environment combinations, the chemical reactions involved, idealized mechanisms for the transfer of metal ions to the environment, and the electrochemical processes occurring at the interface between the metal and the aqueous environment. 

Prior to the 1960s, stress corrosion cracking and corrosion fatigue were principally under the purview of corrosion chemists and metallurgists, and the primary emphasis was on the response of materials in aqueous environments (e.g., sea/salt water), particularly for SCC because of the relative ease of experimentation. Much of the attention was devoted to the understanding of electrochemical reactions that are associated with metal dissolution, crack nucleation, and time-to-failure under a... 

Corrosion can be represented in terms of a simple electrochemical cell as illustrated in Figure 1 for aqueous corrosion. In the electrochemical cell, the electrical circuit is completed by the transport of charge through the aqueous electrolyte. The source of potential may be, for example, a difference in electrode composition. For metals exposed to a corrosive environment, the anodic and cathodic reactions can occur at adjacent or widely separated sites on a single metal, or on different metals that are electrically connected. As in the electrochemical cell, the electrical circuit is typically completed by an aqueous electrolyte sources of potential include differences in electrolyte concentration and in electrode composition. 

Corrosion refers to the degradation of a metal by electrochemical reaction with the environment. At room temperature, the most important corrosion reactions involve water, and the process is known as aqueous corrosion. (Corrosion at high temperatures in dry air, called oxidation tarnishing, or direct corrosion, is considered in Section 8.5.) Aqueous corrosion involves a set of complex electrochemical reactions in which the metal reverts to a more stable condition, usually an oxide or mixture of oxides and hydroxides (Figure 9.15). In many cases the products are not crystalline and are frequently mixtures of compounds. Aside from the loss of metal, the corrosion products may be voluminous. In this case, they force overlying protective layers away from the metal and so allow corrosion to proceed unchecked, which exacerbates the damage. 

The corrosion of a material may be defined as the irreversible reaction of the material with its environment, usually resulting in degradation of the material and its properties. In this chapter, we will be concerned only with the corrosion of metals in aqueous environments under near-ambient... 

L.L. Shreir, Corrosion in Aqueous Solution in Corrosion, Vol. 1, Metal/Environment Reactions, Edited by L.L. Shreir, R.A. Jarman, and G.T. Burstein, Butterworth-Heinemann, Boston, (1994). 

Corrosion in usual usage is the wet-corrosion which occurs in aqueous environments. Corrosion is defined as the deterioration of a substance (usually a metal) or its properties because of chemical or electrochemical reaction with its environment. Thermodynamically the chemically stable form of aluminum in a neutral environment is aluminum oxide. This means that oxidation of aluminum (corrosion) will always occur. Despite this the occurrence of corrosion problems for aluminum materials in realistic applications will be infrequent, because the aluminum surface oxide Aim is strong enough to hinder further corrosion. 

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