Corrosion with salt films

The oxidation products are almost insoluble and lead to the formation of protective films. They promote aeration cells if these products do not cover the metal surface uniformly. Ions of soluble salts play an important role in these cells. In the schematic diagram in Fig. 4-1 it is assumed that from the start the two corrosion partial reactions are taking place at two entirely separate locations. This process must quickly come to a complete standstill if soluble salts are absent, because otherwise the ions produced according to Eqs. (2-21) and (2-17) would form a local space charge. Corrosion in salt-free water is only possible if the two partial reactions are not spatially separated, but occur at the same place with equivalent current densities. The reaction products then react according to Eq. (4-2) and in the subsequent reactions (4-3a) and (4-3b) to form protective films. Similar behavior occurs in salt-free sandy soils. 

In usual polar aprotic solvents, all active metals (e.g., Li, Mg, Ca, and Al) are covered with surface films due to the reduction of solution components by the active metal and the consequent precipitation of insoluble species, but in the above molten salts these corrosion/passivation phenomena may be much less pronounced. Thus, deposition of divalent metals such as magnesium and calcium, which is not possible in usual polar aprotic systems because of the surface film barrier, may be feasible in electrolyte systems based on these molten salts. 

The EHD method with a RDE has been applied to the characterisation of porous layers of corrosion products formed on carbon steel [110], for the characterisation of salt films formed on copper [90, 111, 112] and iron [113], and for biofilms developed in natural seawater [114]. Corrosion inhibition films formed by an organic surfactant acting on the surface of pure iron have been characterised in this way, too [115]. An effect of a... 

Nevertheless, there were reliable reports of corrosion occurring with molybdenum disulphide in films and in greases. Several such reports arose from the US Army , originating with a salt fog test of a missile launcher in which all parts coated with solid film lubricant rusted badly. Subsequent reports described galvanic corrosion of various metals with molybdenum disulphide in moist atmospheres. 

In corrosion systems, a salt film may cover an electrode that is itself covered by a porous oxide layer. If two different layers are superimposed, the geometrical analysis shows that the equivalent circuit corresponds to that described in Section 9.3.1 with an additional series Rti a. circuit to take into account the effect of the second porous layer. The circuit shown in Figure 9.5 is approximate because it assumes that the botmdary between the inner and outer layers can be considered to be an equipotential plane. This plane will, however, be influenced by the presence of pores. The circuit shown in Figure 9.5 will provide a good representation for systems with an outer layer that is much thicker than the inner layer and with an inner layer that has relatively few pores. 

It is now very well established that the pitting corrosion of a number of metals and alloys is accompanied by the formation of metal salt films [172-181]. In particular, Beck and Alkire [181] predicted that the current densities resulting from pit initiation could be large enough to produce a supersaturated metal salt solution in the vicinity of the metal surface with subsequent, rapid precipitation of a salt film. For some metals, the precursor to the re-establishment of passivity is the precipitation of a metal salt film [172—... 

Corrosion mechanisms in molten sulfates consist of a sequence of chemical reactions and transport processes including oxide dissolution, transport of dissolved species through the salt film, and subsequent precipitation of oxide within the salt film in contact with the gas atmosphere. 

Corrosion under paint films and atmospheric corrosion. The loss of metal due to corrosion can be observed with absorption contrast, and any changes in the paint film (e.g., cracks, swelling with water) could be observed with phase contrast imaging. If suitable ions are introduced, their distribution could be tracked with energy difference imaging, described above. For atmospheric corrosion, salt droplets and crystallization of salts and corrosion products could be observed and correlated with pit development. 

---