Solid corrosion products

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 ... 

Electrolytic corrosion occurs in regions I and IV with the formation of soluble iron ions. Solid corrosion products which can have a protective effect are formed in region II. This is the region of surface film passivity. Certain corrosive sub-... 

In this type of corrosion, metal ions arising as a result of the process in Eq. (2-21) migrate into the medium. Solid corrosion products formed in subsequent reactions have little effect on the corrosion rate. The anodic partial current-density-potential curve is a constant straight line (see Fig. 2.4). 

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). 

The solid corrosion products in carbon dioxide and carbon monoxide are uranium dioxide, uranium carbides and carbon. The major reaction with carbon dioxide results in the formation of carbon monoxide ... 

Where a solid corrosion product is formed, meaningful results are only obtained after a conditioning period for a new measured element. Even so, the conditions under which the scale is laid down may not be the same as that for the original equipment. This objection applies equally to coupons or spools, and points to one of the basic objections of using anything other than the plant itself to monitor corrosion rates. 

Metals are subject to electrochemical corrosion in the presence of water Metal atoms lose electrons to become positively charged metal ions that go into solution. These then react with other chemical species in the soil ground-water to form solid corrosion products (e.g., metal oxides, hydroxides, sulfates). It is these solid corrosion products that often form a colored matrix with soil particles around the corroding object (Cronyn 1990). The initial formation of the metal ions takes place at a site on the metal known as the anode, whereas the electrons produced consumed by another reaction with an electron acceptor (the cathode). Due to the electrical conductivity of metals the location of the anode and cathode can be at different locations on the metal surface. In the presence of water and oxygen the cathodic reaction is... 

The combination of wear or abrasion and corrosion results in more severe attack than with either mechanical or chemical corrosive action alone. Metal is removed from the surface as dissolved ions, as particles of solid corrosion products, or as elemental metal. The spectrum of erosion corrosion ranges from primarily erosive attack, such as sandblasting, filing, or grinding of a metal surface, to primarily corrosion failures, devoid of mechanical action. 

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. 

Duration of Test Although the duration of any test will be determined by the nature and purpose of the test, an excellent procedure for evaluating the effect of time on corrosion of the metal and also on the corrosiveness of the environment in laboratory tests has been presented by Wachter and Treseder [Chem. Eng. Prog., 315-326 (June 1947)]. This technique is called the planned-interval test. Other procedures that require the removiJ of solid corrosion products between exposure periods will not measure accurately the normal changes of corrosion with time. 

Fig. 20M Schematic representation of early stages in the formation of a pit. (a) the reactions taking place in and around a pit. (6) formation of a deposit of corrosion product, partially blocking the e.rit of the pit. (c) and (d) propagation of the pit, which is almost filled with solid corrosion products.

Uniform corrosion with solid corrosion product deposit. Details of the formation of oxide species are not considered at this point. 

Importance of Solid Corrosion-Product Formation Corrosion Acceleration Versus Passivation... 

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. 

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... 

A major influence on the kinetics of the corrosion processes is exerted by the solid corrosion products that can arise from the interaction between the following ... 

In the absence of solid corrosion products, the corrosion rate can be Urnited by reducing the availability of the cathodic reactant, such as in the removal of oxygen from the water for boilers substituting for the anodic reaction an external current, as in the case of impressed current cathodic protection or inducing artificially the formation of surface films, as in the case of the application of corrosion inhibitors. 

Consideration must be given not only to the effect of corrosion on the zinc, but also to the possible effects of corrosion products in noncorrosion aspects for example, solid corrosion products may prevent a valve from shutting. Also, corrosion that may be too slight to affect the zinc may significantly contaminate the chemical with which it is in contact or change the texture or appearance of a consumer product. This chapter consists largely of brief references to actual tests and an indication of the apparent suitability of the material for contact with zinc, plus lists of chemicals known to be stored satisfactorily in zinc-coated steel containers. 

---