Interface metal/corrosion product

The superficial characteristics of atmospheric corrosion products of steel depend on the type of atmosphere where the sample has been exposed. The way of adsorption of the corrosion products obtained in the coastal atmosphere is polymolecular due to a higher content of salts. This makes easier the presence of water in the metal-corrosion products interface and determines a high corrosion rate. The adsorption of water of a corrosion product formed in a rural zone obeys a Langmuir isotherm, i.e. a monomolecular adsorption takes place. It causes a lower corrosion rate. 

The hydrogen reduction conservation process was first employed in Sweden in 1964 for ferrous artefacts recovered from the Swedish warship, the Vasa. The method was further developed at Portsmouth to treat the large number of finds recovered from the Solent and land-based archaeological sites within the Wessex region. The principle of the process is to heat the artefact in an atmosphere of hydrogen in order to sublime off the volatile chlorides and at the same time reduce the oxides, hydroxides, chlorides and eventually to the metallic state. The volume change associated with the reduction of the iron compounds is sufficiently high to enable the release of deeply-buried chlorides particularly at the metal/corrosion product interface. 

Connectors are a critical part of an electronic S5rstem. Corrosion mechanisms at connector interfaces include pore corrosion, corrosion product creep, fretting, and SCC and general attack. Pore corrosion in connectors is associated with surfaces plated with gold or other precious metals. Corrosion product creep in connectors usually involves copper corrosion products originating from exposed base... 

Surface films are formed by corrosion on practically all commercial metals and consist of solid corrosion products (see area II in Fig. 2-2). It is essential for the protective action of these surface films that they be sufficiently thick and homogeneous to sustain the transport of the reaction products between metal and medium. With ferrous materials and many other metals, the surface films have a considerably higher conductivity for electrons than for ions. Thus the cathodic redox reaction according to Eq. (2-9) is considerably less restricted than it is by the transport of metal ions. The location of the cathodic partial reaction is not only the interface between the metal and the medium but also the interface between the film and medium, in which the reaction product OH is formed on the surface film and raises the pH. With most metals this reduces the solubility of the surface film (i.e., the passive state is stabilized). 

Metal/environment interface—V ne cs of metal oxidation and dissolution, kinetics of reduction of species in solution nature and location of corrosion products him growth and him dissolution, etc. 

Thus in all corrosion reactions one (or more) of the reaction products will be an oxidised form of the metal, aquo cations (e.g. Fe (aq.), Fe (aq.)), aquo anions (e.g. HFeO aq.), Fe04"(aq.)), or solid compounds (e.g. Fe(OH)2, Fej04, Fe3 04-H2 0, Fe203-H20), while the other reaction product (or products) will be the reduced form of the non-metal. Corrosion may be regarded, therefore, as a heterogeneous redox reaction at a metal/non-metal interface in which the metal is oxidised and the non-metal is reduced. In the interaction of a metal with a specific non-metal (or non-metals) under specific environmental conditions, the chemical nature of the non-metal, the chemical and physical properties of the reaction products, and the environmental conditions (temperature, pressure, velocity, viscosity, etc.) will clearly be important in determining the form, extent and rate of the reaction. 

The electrified interface between the metal and the electrolyte solution (the metal surface may be film-free or partially or completely covered with films or corrosion products). 

Oxygen from the atmosphere, dissolved in the electrolyte solution provides the cathode reactant in the corrosion process. Since the electrolyte solution is in the form of thin films or droplets, diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Moreover, convection currents within these thin films of solution may play a part in further decreasing concentration polarisation of this cathodic process . Oxygen may also oxidise soluble corrosion products to less soluble ones which form more or less protective barriers to further corrosion, e.g. the oxidation of ferrous species to the less soluble ferric forms in the rusting of iron and steel. 

As the film dissolves more oxide film is formed, i.e. the metal/oxide interface progresses into the metal, and the overall rate may be low enough to be acceptable for a particular process. In other cases, the corrosion products precipitate on the surface of the oxide and either accelerate the overall rate by enhancing diffusion of ions through the porous outer layers or, when less porous layers are formed, access of hydrogen ions to the inner oxide surface is reduced thus decreasing the rate. 

Once the plate starts to corrode, many problems appear to affect performance and durability, even serious failure, of fhe fuel cells. For example, fhe interface contact resistance between the corroded metal plates and GDL will increase to reduce the power output. The corrosion products (mainly various cations) will contaminate the catalyst and membrane and affect eir normal functions because the polymer membrane essentially is a strong cation exchanger and the catalyst is susceptible to the ion impurity. Hence, adding a corrosion-resistant coating to the metal plate will almost inevitably assure the performance and long-term durability of a sfack. 

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

The corrosion product, a mixture of oxide, sulphide at the metal interface and sulphate outside, has a weak adhesive bond to the metal surface and cannot support large deposit masses. It is therefore unusual to find excessive amounts of sintered ash deposits and fused slag in the exact localities where severe high temperature corrosion occurs. Conversely, a strongly adhering matrix of sintered ash deposit in the absence of sulphate, sulphide or chloride phases is not markedly corrosive. 

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. 

Simple but pedagogically useful theories of electrode kinetics are presented in Chapter 3. This permits discussion of models for anodic and cathodic reactions at the metal/environment interface and for diffusion of species to and from the interface. Mathematical models of these theories lead to so-called kinetic parameters whose values govern the rate of the interface reaction. The range of values that these parameters can have and some of the variables that can influence the values are emphasized since these will relate to understanding the influence of such factors as surface conditions (roughness, corrosion product films, etc.), corrosion inhibitors and accelerators, and fluid velocity on corrosion rates. This chapter also introduces electrochemical measurements to determine values of the kinetic parameters. 

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. 

Paralinear corrosion (related to dissolution of corrosion product) does not occur for all aluminum alloys in water at all high temperatures. In Figure 13 are plotted data for an alloy (Al, 1% Ni, 0.1% Ti) corroded in water at 350°C (10). The corrosion rate was low and constant, as shown better in other figures in the same publication. For some specimens in the figure 1/3 or 2/3 of the corrosion product was removed mechanically after the first exposure period. There was no discernible effect on subsequent corrosion, indicating that control of corrosion probably resided close to the metal-oxide interface. Similar experiments for the alloys and temperatures where paralinear behavior occurs showed that removing some of the product caused an increase in subsequent corrosion rate. 

Furthermore, metal-sulfide formation is also possible at the metal/oxide interface, as observed in many corrosion products on metal samples, formed by basic fluxing. Flence, the region close to the oxide surface is much more basic than the entire salt film... 

---