Aqueous corrosion surface reaction products

While acid corrosion in glass fibers is diffusion-controiied and therefore /f-kinetics are expected, the process in aqueous and aikaiine soiutions is considered much more complicated because of the many influencing factors. The reaction kinetics depends on the (local) pH value. It is conventional opinion that, with the switch from a diffusion-controlled corrosion mechanism to an interfacial-controlled mechanism, a rapid shift from ft-to t-kinetics takes place, and the process follows linear t-kinetics except for short exposure times and low temperatures. However, in the literature, dependencies on t are also found, with values for a varying between 0.5 and 1 [819]. The chemical stability of glass fibers under alkaline attack is also significantly influenced by insoluble corrosion or reaction products on the fiber surface. 

This is essentially a corrosion reaction involving anodic metal dissolution where the conjugate reaction is the hydrogen (qv) evolution process. Hence, the rate depends on temperature, concentration of acid, inhibiting agents, nature of the surface oxide film, etc. Unless the metal chloride is insoluble in aqueous solution eg, Ag or Hg ", the reaction products are removed from the metal or alloy surface by dissolution. The extent of removal is controUed by the local hydrodynamic conditions. 

Chemical/Physical. Matheson and Tratnyek (1994) studied the reaction of fine-grained iron metal in an anaerobic aqueous solution (15 °C) containing chloroform (107 pM). Initially, chloroform underwent rapid dehydrochlorination forming methylene chloride and chloride ions. As the concentration of methylene chloride increased, the rate of reaction appeared to decrease. After 140 h, no additional products were identified. The authors reported that reductive dehalogenation of chloroform and other chlorinated hydrocarbons used in this study appears to take place in conjunction with the oxidative dissolution or corrosion of the iron metal through a diffusion-limited surface reaction. 

Permeable refractories are thus generally more susceptible to corrosion due to their porosity and the consequent high surface area of bond phase exposed to the corrodent. There are very few corrodents which would significantly attack SiC by dissolution in an aqueous medium. If SiC is attacked, the general mechanism is one of oxidation of the SiC to Si02. The Si02 is usually then removed as a reaction product exposing fresh SiC surfaces to corrosion. 

These sulfonates can be used to optimize the efficacy of cationic inhibitors for protecting metals against corrosive acids. Such inhibitors form adsorption layers on the surface of a metal which will act as a penetration barrier for the acid medium, as well as for the reaction products. However, there is no continuous layer formed by the cationic surfactant because the surfaces show sites of different charges. An inhibitor consisting of a mixture of cationic and anionic surfactants leads to a compact adsorption layer and to a high effectiveness of the inhibitor system. Although the adduct of anionics and cationics is poorly soluble in aqueous acid solutions, an inhibitor concentration of ca. 5 ppm is already sufficient for the protection of the metal. 

One may now extend such detailed study of the molecular/water interfaces to shed light on processes more directly relevant to corrosion. For instance, specific details regarding the reactivity of water molecules at the metal-water interface have been modeled with varying degrees of sophistication [27, 28, 112-116]. The dissociation of H2O at the surface into products presents an important first probe reaction and bas been related to the initiation of passivity of fresh metal surfaces when first exposed to aqueous solution ... 

The key rate-controlling steps are commonly referred to as concentration control and activation control [4]. Concentration control implies that the rate determining step is the rate at which reactants or products move to or from the corroding surface. Concentration control is also referred to as mass transfer or diffusion control. Common illustrations of concentration control include the ability of a reactant, e.g., dissolved oxygen, to diffuse to a steel siuface in an aqueous electrolyte or the ability of a reaction product, e.g., hydroxide, to diffuse from a steel surface. High supply rates of reactants favor a higher corrosion rate low difriision rates of products from a surface hinder corrosion rate. 

One of the key issues of high temperature corrosion is that as a rule all metallic materials are unstable in their service environment. This means that in all cases corrosion products are formed and that corrosion itself cannot be avoided at all. In order to achieve corrosion resistance, therefore, the corrosion reaction has to be guided in such a way that the corrosion product itself offers protection against the environment. This is the case if the corrosion product forms a protective surface scale. Such a scale can be compared with passive layers in aqueous corrosion, which prevent increased attack of the underlying metal. 

Thermodynamics gives an indication of the tendency of electrode reactions to occur, whereas electrode kinetics addresses the rates of such reactions. The reactions of concern are mainly corrosion reactions, hence, it is more appropriate to call the kinetics of such reactions as corrosion kinetics. In order to understand the theory of aqueous corrosion, it is important to develop a complete understanding of the kinetics of reaction proceeding on an electrode surface in contact with an aqueous electrolyte. Methods which are used to study the rate of a reaction involve the determination of the amount of reactants remaining in products after a given time. In aqueous corrosion, it is very important to appreciate the nature of irreversible reactions which take place on the electrode surface during corrosion. 

A comprehensive list of standard potentials is found in Ref. 7. Table 2-3 gives a few values for redox reactions. Since most metal ions react with OH ions to form solid corrosion products giving protective surface films, it is appropriate to represent the corrosion behavior of metals in aqueous solutions in terms of pH and Ufj. Figure 2-2 shows a Pourbaix diagram for the system Fe/HjO. The boundary lines correspond to the equilibria ... 

The interface between two volumetric domains is designated a surface domain, and its dimensionality is one less than a volumetric domain. Concentrations of species in a surface domain have dimensions of mol/m2, for example. The four types of surface domains shown in Fig. 11.2 are A-G, the interface between the aqueous domain and the gas A-C, the interface between the aqueous domain and the corrosion-product layer C-G, the interface between the corrosion layer and the gas and C-B, the interface between the corrosion layer and the bulk copper layer. Chemical reactions of species residing in one volumetric domain with species in another volumetric domain have to occur at an interface, namely a surface... 

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... 

To control the reaction rate of the acid, retarders such as alkyl sulfonates, alkyl phosphonates and alkyl amines are used to form hydrophobic films on carbonate surfaces. These protective films act as a barrier to slow acid attack. Another method involves the use of foaming agents to stabilize the carbon dioxide foam that is created when CO2 is released as a product of the acidetching reaction. This CO2 foam acts as a barrier to slow acid attack. Yet another method for controlling the acid activity in an oil well is the use of emulsions containing kerosene or diesel as the continuous oil phase and hydrochloric acid as the dispersed aqueous phase. Acid-in-oil emulsions are most commonly used because oil separates the acid from the carbonate surface (and from machine parts, thus reducing the level of corrosion). Moreover, acid reaction rates can be further decreased by surfactant retarders that increase the wettability of the carbonate surface for oil. 

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