Corrosion tunnels

Swann, P. R., Mechanism of Corrosion Tunnelling with Special Reference to CujAu , Corrosion, 25, 147 (1969)... 

A. McLeish, Bucharest metro dealing with stray current corrosion . Tunnel Construction and Piling Conference 99, September 8-10, London, 1999. 

Corrosion tunneling Plasticity Tarnish Film-induced mpture cleavage... 

Cu-Ni-Fe and Ni-Fe-Co ternary alloys have been studied as candidate materials system for the inert anodes. It was found that Cu-Ni-Fe alloy had a two- phase microstructure of Cu-rich phase and Fe-Ni-rich phase and such chemical non-homogeneity showed a negative impact on their corrosion resistance. This is because the Fe-rich phase can be preferentially corroded during aluminum electrolysis, thus providing iron-fluoride corrosion tunnels within the anode corrosion scales For Ni-Fe-Co ternary alloy, the additions of 10 wt.% and 30 wt.% Co to the Ni-Fe alloy system were found to significantly improve the anodic wear resistance and oxidation resistance due to the suppression of FexO formation. Flowever, the reaction scales had excessive porosity, non-homogeneity and poor surface adherence, which could not provide sufficient protection to the metal anodes... 

Figure 2.21 "Worm-like" filiform corrosion tunnels. (Reproduced with permission from Daubert Cromwell.)...

Figure 2.21 shows worm-hke filiform corrosion tunnels forming under a coating at the Atmospheric Test Site. 

These materials are developed from the polyetherimides introduced by General Electric (see also Section 18.14.2). At the time of writing one grade, Ultem Siltem STM 1500, is being offered. It is of particular interest as a material for wire and cable insulation, as it not only has excellent flame resistance coupled with low smoke generation but also avoids possible toxic and corrosion hazards of halogenated polymers. This can be of importance where there are possible escape problems in the event of a fire, such as in tunnels, aircraft and marine (particularly submarine) vessels. 

The very new techniques of scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) have yet to establish themselves in the field of corrosion science. These techniques are capable of revealing surface structure to atomic resolution, and are totally undamaging to the surface. They can be used in principle in any environment in situ, even under polarization within an electrolyte. Their application to date has been chiefly to clean metal surfaces and surfaces carrying single monolayers of adsorbed material, rendering examination of the adsorption of inhibitors possible. They will indubitably find use in passive film analysis. 

This appears as a random non-branching white tunnel of corrosion product either on the surface of non-protected metal or beneath thin surface coatings. It is a structurally insensitive form of corrosion which is more often detrimental to appearance than strength, although thin foil may be perforated and attack of thin clad sheet (as used in aircraft construction) may expose the less corrosion resistant aluminium alloy core. Filiform corrosion is not commonly experienced with aluminium, as reflected by the insignificance afforded it in reviews on the phenomena (Section 1.6). 

Oppenheim IC, Trevor DJ, Chidsey CED, Trevor PL, Sieradzki K. 1991. In situ scanning tunneling microscopy of corrosion of silver-gold alloys. Science 254 687-689. 

Intake water tunnels are generally made from concrete, and absorption of water by concrete is the main reason for corrosion in reinforcement. In intake structures the problems are due to concrete failure from salts penetrating into the concrete and corroding the rebar. Hard, dense concrete with ample cover to reinforcement and without cracks and porosity has good resistance to corrosion against seawater. In Indian nuclear power plants, the experience with concrete intake tunnels with respect to corrosion behavior is not bad except that special care is required for protection against algae, clams, mussels, etc. which attach to the tunnel surface. 

Most of the present uses of the scanning tunneling microscope involve studies of surface chemistry. Processes such as the deposition of monomolecular layers on smooth surfaces can be studied, the nature of industrial catalysts can be probed, and metal corrosion can be examined. The possibility also exists that complex biological structures can be determined with the STM. 

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