Passivated metal surfaces

Corrosion inhibitor/lubricity improvement additives are used panicularly in militai y fuel for the dual puiyiose of passivating metal surfaces and improving the lubricating properties of the fuel in equipment such as fuel pumps. The militai y also specifies the use of a fuel system icing inhibitor as an additive to prevent filter blocking by ice crystal formation, because militai y aircraft tend not to use fuel line filter heaters, which are standard equipment on civil aircraft. 

The arbitrary division of behaviour has been made because of the extreme behaviour of some chemicals that initiate small areas of attack on a well-passivated metal surface. The form of attack may manifest itself as stress-corrosion cracking, crevice attack or pitting. At certain temperatures and pressures, minute quantities of certain chemicals can result in this form of attack. Chloride ions, in particular, are responsible for many of the failures observed, and it can be present as an impurity in a large number of raw materials. This has led to the development of metals and alloys that can withstand pitting and crevice corrosion, but on the whole these are comparatively expensive. It has become important, therefore, to be able to predict the conditions where more conventional materials may be used. The effect of an increase in concentration on pitting corrosion follows a similar relationship to the Freundlich equation where... 

Pitting (pitting corrosion) the formation of small holes in an otherwise passive metal surface as a consequence of locally accelerated corrosion. 

New boilers, heat exchangers, and other equipment must be provided with a PCC program to remove oil, grease, mill scale, pipe-threading compounds, drawing compounds, and other detritus that otherwise prevent the formation of clean, passivated metal surfaces and encourage... 

The various cleaning procedures available also promote the formation of properly passivated metal surfaces as part of the program start-up process. 

The short-term application of an increased dose rate of chemical inhibitor enhances the corrosion inhibiting potential of an already passivated metal surface. Nevertheless, in order to change the metal surface from an active state to a passive state, the electrode potential must be raised to a level above that of the passivation potential. Typically this is achieved by the use of chromate, nitrite, and/or orthophosphate in the presence of oxygen. 

For instationary conditions of the passive layer, i.e. if layer formation occurs and 02 will be formed, Acp2 3 will deviate from its equilibrium value by an overpotential t]2 3. These kinetic parameters of a complicated system as the passivated metal surface will be discussed in the next section. 

Passivity breakdown appears to occur preferentially at local heterogeneities, such as inclusions, grain boundaries, dislocations, and flaws on the passive metal surface. In the case of stainless steels, the passivity breakdown and pit initiation occur almost exclusively at sites of MnS inclusions, and the pitting potential was observed to decrease linearly with the increasing size of MnS inclusions [44]. With metals containing no apparent defects, however, passivity breakdown is likely to occur in the presence of sufficient concentrations of film breaking ions. It is worth noting that any of the localized phenomena is nondeterministic but somehow stochastic. For stainless steels in chloride solution, the passivity breakdown was found to obey a stochastic distribution [45]. 

DEHA. [WJl. Grace/D nbom] Diethyl-hydroxylandne passivates metal surfaces, scavenges dissolved oxygen, volatilizes for cmnpleie boiler imtec-tion. 

A number of papers have been published on the merits and application of ozone in industrial cooling water systems. Claims made in a patent [Humphrey et al 1979] are that ozone in concentrations of 2 - 20 mg/l prevents scale formation and removes existing scale, passivates metal surfaces to inhibit further corrosion and disinfects the water to prevent algae growth. Edwards [1983, 1987] and Edwards and Sellers [1991] have published valuable reviews of ozone technology and cooling water. As well as biocidal activity the benefits of using ozone include ... 

The chemical process leading to the passivated metal surface is not yet fully understood. According to Our hypothesis, the metal is stoichiometri-cally oxidised (after a first etching step) with the... 

Others are the reduction of Fe + and [Fe(CN)6] in solution. These systems are often used for chemical corrosion tests. Pitted metals expose a small area of a few intensively dissolving corrosion pits that are not protected by a passive layer and a large cathode of the passive metal surface. Because of the large size of the cathode, a much smaller cathodic current density is required for the compensating reduction of the redox system in comparison to the active metal dissolution within the pits. However, electronic conduction is still required across the passive layer. Figure 3 depicts the existing sections of a pitted metal surface with the related electrode reactions, the very small metal dissolution /pass, and the redox reaction ha,pass via the protecting oxide film and... 

Influence of Adsorbed Layers on Cathodic Reactions In a similar way, adsorbates may retard the cathodic reactions on bare or passivated metal surfaces. It... 

The application of atomistic modeling, mostly using DFT but also using reactive MD, to oxide surfaces, characteristic of passivated metallic surfaces, and their interaction with water and aggressive ions has been presented in this chapter and discussed in the context of passivity breakdown. The modeling of oxide surfaces and the reviewed DFT data on MgO and Ti02 surfaces and their hydroxylation can be summarized as follows ... 

The electron-transfer reaction (6.38) is of particular importance in corrosion and electrochemical technology. It can take place on active as well as passive metal surfaces. 

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