Corrosion electrolytic theory

From the standpoint of the electrolytic theory, the explanation of the corrosion of iron is not complicated, and so far has been found in accordance with all the facts. Briefly stated, the explanation is as follows Iron has a certain solution tension, even when the iron is chemically pure and the solvent pure water. The solution tension is modified by impurities or additional substances contained in the metal and in the solvent. The effect of the slightest segregation in the metal, or even unequal stresses and strains in the surface, will throw the surface out of equilibrium, and the solution tension will be greater at some points than at others. 

These sort of problems make it difficult to obtain reliable high temperature data on the aqueous chemistry of transition metal ions. Unfortunately the necessary timescales for even the simpler experimental studies are frequently too long for a Ph.D. student to make reasonable progress in 3 years from scratch or for industrial researchers to make much reportable progress before the patience of those supporting the work is exhausted. Results can be reported far more rapidly from, for example, corrosion experiments and since corrosion theories are in general of so little predictive value, each relevant alloy/electrolyte combination needs its own study. In such circumstances it is hardly surprising that thermodynamic studies have been (with a few notable exceptions) relatively poorly supported, while corrosion data continue to be amassed without any reliable thermodynamic framework within which to understand them. 

ACIDS AND BASES. The conventional definition of an acid is that it is an electrolyte that furnishes protons, i.e.. hydrogen ions. H+. An acid is sour to the taste and usually quite corrosive. A base is an electrolyte that furnishes hydroxyl ions, OH . A base is bitter to die taste and also usually quite corrosive. These definitions were formulated in terms of water solutions and. consequently, do not embrace situations where some ionizing medium other than water may be involved. In the definition of Lowry and Brnsted, an acid is a proton donor and a base is a proton acceptor. Acid-base theory is described later. 

The Mechanism of Corrosion.—An attractive theory of the mechanism of corrosion has been outlined by Aitchison.2 Compact iron, when examined under the microscope (see Part III.), is seen to consist of crystals of ferrite separated from each other by an amorphous cement. It is reasonable to suppose that the solution pressure of this cement differs from that of the ferrite, for differences of this kind invariably occur between amorphous and crystalline varieties of substances. Upon immersion in an electrolyte, therefore, such as ordinary tap water or aqueous solutions of inorganic salts, a difference of potential exists leading to corrosion. If the cement is positive to the ferrite, it is the cement that will oxidise away and vice versa. In a perfectly annealed specimen, in which there is but little mechanical strain, the action will, in the main, be confined to that between the cement and ferrite. If, however, there is any appreciable potential difference between the crystals of ferrite themselves, this will increase the effect, the total observed corrosion being the sum of the two actions. 

When two dissimilar metals are immersed in an electrolyte they usually develop different potentials in accordance with the theory already presented. If the metals are in contact the potential difference provides the driving force for corrosion. Severe corrosion often occurs as a result of the contact between two metals. In shell and tube heat exchangers where the tubes are fabricated from a corrosion resistant alloy, and the shell is made from mild steel for instance to reduce the capital cost, corrosion is very likely unless adequate protection is made. The less resistant of the two metals is caused to corrode, or to corrode more rapidly, while the resistant metal or alloy corrodes much less or may be even completely protected. The basis for galvanic corrosion is illustrated on Fig. 10.6. Metal A has a lower electrode potential than metal B. Ions migrate in the conducting solution while electrons flow across the junction of the two metals, as a result metal A is corroded at C. 

Halide ions, according to the adsorption theory of passivity, tend to break down passivity by competing with the passivator for adsorption sites on the metal surface. Should a halide ion find a vacant site and closely approach the surface, hydration and dissolution of metal ions are favored, and the anodic reaction can proceed with low activation energy, in contrast to the high activation energy required when a passivator is adsorbed. The anode reaction, if it persists, is confined to localized areas where the competitive process first succeeds, because surrounding metal immediately becomes cathode of an electrolytic cell, and is protected by flow of current from further anode activity, a process called cathodic protection. This attack at specific sites leads to corrosion pitting typical of metals otherwise passive that are actually corroded by their environment. 

Today, these theories of solutions and of electrolytic solutions are used in the analysis of the solvent development of exposed resists, lithographic mask degradation due to corrosion, electromigration of chromium ions, etc. 

Cathodic protection (CP) is defined as the reduction or elimination of corrosion by making the metal a cathode by means of impressed current or sacrificial anode (usually magnesimn, aluminum, or zinc) [11]. This method uses cathodic polarization to control electrode kinetics occurring on the metal-electrolyte interface. The principle of cathodic protection can be explained by the Wagner-Traud mixed potential theory [12]. 

As mentioned in Sect. II.C.2, a small amount of electrolyte (as a water-soluble magnesium salt) was added to the emulsion as a corrosion inhibitor. This electrolyte could in theory balance the osmotic gradient and inhibit aging. However, the dehydration and desalting process was difficult to control and often the heavy crude oil ended up with larger amounts of brine that required larger amounts of electrolyte to offset the osmotic gradient... 

Polarisation methods involve changing the potential of the WE and monitoring the current which is produced as a function of time or potential. One of the most relevant physical quantities measured by DC polarisation methods is linear polarisation resistance (LPR). Its definition is based on the mixed potential theory proposed by Wagner and Traud [4], that explains the corrosion reactions by assuming that cathodic and anodic partial reactions occur at the metal-electrolyte interface at a certain corrosion (or mixed ) potential,... 

Useful atomic and subatomic scale information on hydroxylated oxide surfaces and their interaction with aggressive ions (e.g., Cl ) can be provided by theoretical chemistry, whose application to corrosion-related issues has been developed in the context of the metal/liquid interfaces [34 9]. The application of ah initio density functional theory (DFT) and other atomistic methods to the problem of passivity breakdown is, however, limited by the complexity of the systems that must include three phases, metal(alloy)/oxide/electrolyte, then-interfaces, electric field, and temperature effects for a realistic description. Besides, the description of the oxide layer must take into account its orientation, the presence of surface defects and bulk point defects, and that of nanostructural defects that are key actors for the reactivity. Nevertheless, these methods can be applied to test mechanistic hypotheses. 

The study of diffusion processes of electrolytes and non-electrolytes in aqueous solutions is important for fundamental reasons, helping to imderstand the nature of aqueous electrolyte stmcture, for practical applications in fields such as corrosion, and provide transport data necessary to model diffusion in pharmaceutical applications. Although no theory on diffusion in electrolyte or non-electrolyte solutions is capable of giving generally reliable data onO, there are, however, estimating pmposes, whose data, when compared with the experimental values, will allow us to take off conclusions on the nature of the system. 

According to mixed-potential theory, any overall electrochemical reaction can be algebraically divided into half-cell oxidation and reduction reactions, and there can be no net electrical charge accumulation [J7], For open-circuit corrosion in the absence of an applied potential, the oxidation of the metal and the reduction of some species in solution occur simultaneously at the metal/electrolyte interface, as described by Eq 14, Under these circumstances, the net measurable current density, t pp, is zero. However, a finite rate of corrosion defined by t con. occurs at anodic sites on the metal surface, as indicated in Fig. 1. When the corrosion potential, Eco ., is located at a potential that is distincdy different from the reversible electrode potentials (E dox) of either the corroding metal or the species in solution that is cathodically reduced, the oxidation of cathodic reactants or the reduction of any metallic ions in solution becomes negligible. Because the magnitude of at E is the quantity of interest in the corroding system, this parameter must be determined independendy of the oxidation reaction rates of other adsorbed or dissolved reactants. 

In order to be able to properly design and interpret the results from galvanic corrosion tests, it is necessary to have some appreciation of the electrochemical theory behind galvanic corrosion. Metal corrosion consists of at least two reactions. The first is the metal going into solution in the electrolyte... 

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