Solid corrosion products product layer

Metal corrosion is a superposition of metal dissolution or the formation of solid corrosion products and a compensating cathodic reaction. Both processes have their own thermodynamic data and kinetics including a possible transport control. Furthermore, metals are generally not chemically and physically homogeneous so that localized corrosion phenomena, local elements, mechanical stress, surface layers, etc. may play a decisive role. Therefore, one approach is the detailed analysis of all contributing reactions and their mechanisms, which however does not always give a conclusive answer for an existing corrosion in practice. 

Although most metals display an active or activation controlled region, when polarised anodically from the equilibrium potential, many metals and perhaps even more so alloys developed for engineering applications, produce a solid corrosion product. In many examples the solid is an oxide that is the stable phase rather than the ion in solution. If this solid product is formed at the metal surface and has good intimate contact with the metal, and features low ion-conductivity, the dissolution rate of the metal is limited to the rate at which metal ions can migrate through the film. The layer of corrosion product acts as a barrier to further ion movement across the interface. The resistance afforded by this corrosion layer is generally referred to as the passivity. Alloys such as the stainless steels, nickel alloys and metals like titanium owe their corrosion resistance to this passive layer. 

The mechanism for metal corrosion depends, first of all, on the type of hostile medium. Gas corrosion occurs in metal contact with an active gas. A layer of solid corrosion products (scale) is formed on the metalware surface in dry oxidative gases at elevated temperatures. Metal corrosion in electrolyte solutions, even if the solution is in the form of a thin film on a metal surface, follows the reaction... 

The final type of boundary layer results from chemical reactions of metal surfaces with the slurry chemicals. During metal CMP, the slurry chemicals react with the metal surface and form either solid or ionic products or both. Such reactions are considered corrosion reactions. Formation of solid species occurs on the metal surface, forming a surface layer which, to varying de-... 

The life-limiting increase of resistance and decrease of capacity of cycled cells is usually attributed to the deteriorating effects of corrosion of the positive current collector— the cell container in the case of sodium core cells or a rod in the case of sulfur core cells. Apart from consuming the active material, corrosion may lead to the deposition of poorly conductive layers at the current collector surface, thus interrupting the contact of the inert electrode fibers with the current collector. Corrosion products may also deposit and block both the solid electrolyte and the electrode surface. The thermodynamic instability of metals in polysulfide melts severely limits the choice of materials interfacing the sulfur electrode.A fully satisfactory solution has not yet been reported. 

Dissolution of the chlorides from the corrosion products is an essential part of the conservation process. It is essential that the artefact is immersed in an electrolyte that will not corrode the metal any further, while this dissolution is taking place. Corrosion scientists have developed redox potential - pH diagrams from thermodynamics in order to predict the most stable form of the metal. These diagrams are divided into three zones. Where metal ions are the most stable phase, this is classed as a zone of corrosion. If the metal itself is the most stable species, this is said to be the zone of immunity. The third zone is where solid metal compounds such as oxides, hydroxides, etc, are the most stable and may form a protective layer over the metal surface. This zone is termed passivity and the metal will not corrode as long as this film forms a protective barrier. The thickness of this passive layer may only be approximately 10 nm thick but as long as it covers the entire metal surface, it will prevent further corrosion. 

The removal of the chloride ion is the most essential, but the conservator must consider the other three aims when considering what method to use. For example, there are several proprietary solutions on the market or ones that can be made up in a laboratory, which will dissolve all the corrosion products but not the underlying metal. This method is not suitable if the artefact has thick layers of corrosion products on the surface as the shape will be lost. Even worse, if the artefact was just composed of solid-corrosion products (completely mineralised), there would be nothing left after immersion in these solutions This method is only suitable for those artefacts recovered with thin layers of corrosion products on their surface. 

Erosion-corrosion is a combination of corrosion and mechanical wear effects. Many metals and alloys depend on surface films or corrosion products for corrosion resistance. If these surface layers are removed, active and rapid corrosion occurs. Erosion-corrosion is often encountered under conditions of high velocity, turbulence, impinganent, and solids in suspension. Valves, pumps, agitators, and heat exchanger tubes often fail because of erosion-corrosion. 

In normal state (100 kPa), the reaction of a metal M with an oxidizing gas X2 produces a solid reaction product MX, which usually forms a layer on the surface (MIMXIM X2). The free enthalpy of such a reaction is negative for practically all metal oxides because oxides are stable in atmospheres containing oxygen. Metals therefore are not stable, and oxidation or corrosion always occurs, although it is not visible in many cases because of the low reaction rate. 

If the product layer is nearly free of pores, then the anodic dissolution of metal will practically cease. The metal is then said to be passivated . The thickness of the compact product layer will reach a stationary value. For oxide products which are essentially electronic conductors, this stationary thickness will be determined by the very low ionic conductivity in the oxide on the one hand, and by the rate of dissolution of the oxide in the electrolyte on the other. However, in many cases the oxide layers are porous, so that the electrolyte can continue to attack the metal, independently of the transport of ions and electrons in the oxide. From the above discussion it can be seen that corrosion reactions in aqueous ionic solutions in which a solid product layer is formed on a metal are among the most complicated of all heterogeneous solid state reactions. The reasons for this are the electrochemical nature of these reactions, the great number of possible elementary steps which can occur at the various phase boundaries, and electrical space charge phenomena which occur in the reaction product. 

The anodic partial reaction also involves a charge transfer at the interface because a metal atom loses electrons. It then dissolves in the solution as a hydrated or complexed ion and diffuses towards the bulk. In the vicinity of the metal surface, the concentration generated by dissolution therefore often exceeds that of the bulk electrolyte. Once the solubility threshold is reached, solid reaction products begin to precipitate and form a porous film. Alternatively, under certain conditions, metal ions do not dissolve at all but form a thin compact oxide layer, called passive film. The properties of the passive film then determine the rate of corrosion of the underlying metal (Chapter 6). 

The amount of solid reaction products forming a rust film increases gradually as corrosion progresses. Figures 8.19(b) and (c) represent a steel surface covered with a porous rust layer, exposed to wet (b) and dry (c) conditions, respectively. 

In the Cora code, the corrosion product layers outside the reactor core are rather arbitrarily subdivided into two layers, a transient one and a permanently deposited one. Supply to the transient layer occurs via deposition of suspended particles from the coolant, release from it includes erosion of particles back to the coolant as well as transport into the permanently deposited layer and partial conversion into dissolved species. In a comparable manner, the supply of nuclides to the permanent layer is assumed to result from transfer from the transient layer and the exchange equilibrium with the dissolved species present in the coolant. The deposition coefficients of suspended solids can be calculated on the basis of particle size and flow characteristics. The coefficients of relevance for the permanently deposited layer, including ionic transfer mechanisms between liquid and solid phases, can be derived from theoretical considerations as well as from laboratory studies of corrosion product solubilities. Finally, diffusion rates of nuclides at the interphase layers are needed, from the oxide layer to the coolant as well as in the reverse direction. These data can be obtained in part by theoretical considerations and by measurements at the plants. 

As with metals the corrosion of ceramics can take place by one or a combination of mechanisms. In general, a corrodent will attack a ceramic and form a corrosion product. Whether the reaction product is a gas or a solid will determine if the product remains on the surface or is fugitive. Reaction products may be gas, liquid, solid, or any combination thereof. If the reaction product formed is a solid it may form a protective layer against additional corrosion. When the reaction product is a combination of a solid and a liquid, the reaction layer may be removed. 

When a metal is oxidized at elevated temperatures, a stable scale of oxides or other compounds may buildup to cover and protect the exposed metal surface from further corrosion. The corrosion product layer may therefore act as a barrier between the underlying metal and the corrosive environment (air, flue gas, molten salt, or any other corrosive agent). The compound may be a solid, a liquid, or a gas. For instance, if chromium, vanadium, and molybdenum were exposed to air at 1100°C ... 

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