Hidden corrosion

For instance, the buildup of scale within storage tanks and piping may result in situations where corrosive agents can accumulate and accelerate a very vicious hidden corrosion process. One particular example of such conditions that became extremely important after the energy crisis of the 1970s is what is called corrosion under insulation (CUl). However, the same problem exist in maybe a less severe fashion for structural details used in the construction of a system including lap joints, gaskets, and enclosed places that cannot be accessed. 

Rose, J.L., Soley, L.E., Hay, T. and Agarwala, V.S., Ultrasonic guided waves for hidden corrosion detection in Naval aircraft. CORROSION NACExpo 2000, 55th Annual Conference and Exposition, March 26-31,2000, Orlando, FL. 

Scanned pulsed eddy current. This technique for application of eddy-current technology uses analysis of the peak amplitude and zero crossover of the response to an input pulse to characterize the loss of material. This technology has been shown to measure material loss on the bottom of a top layer, the top of a bottom layer, and the bottom of a bottom layer in two-layer samples. Material loss is displayed according to a color scheme to an accuracy of about 5 percent. A mechanical bond is not necessary, as it is with ultrasonic testing. The instrument and scanner are rugged and portable, using conventional coils and commercial probes. The technique is sensitive to hidden corrosion and provides a quantitative determination of metal loss. 

Otto, J., Pannert, W., and Hald, M. (2003) Detection of Hidden Corrosion under Paint, in Proceedings of the 11th International Symposium, Berlin 2002 (eds R.E. Green, B.B. Djordevic, and M.P. Hentschel), Springer, Berlin, p. 297. 

External corrosion can occur under insulation, especially if the weather barrier is not maintained or if the tank is operating at conditions at which condensation is likely. This form of attack is hidden and may be unnoticed for a long time. Inspection holes and plugs should... 

Explain why soiling and corrosion are hidden costs of air pollution. 

The spray is based upon either a natural plate-like material, such as vermiculite bound together with cement, or mineral fibers. Application is fast but not precise or clean, and is generally only suitable for areas where the steel will be hidden (by a false ceiling, for example). Sprays for external applications are available. However, the steel must first be provided with a compatible corrosion protection system. 

This does not, of course, mean that improvements should not be attempted. It is important, however, that corrosion scientists and technologists appreciate that significant hidden costs may arise when a novel solution is considered. A considerable effort is needed to ensure that the information needed by the design engineer is available when it is needed, and that all the parties concerned understand what is required. The documentation needed can be large, and requires an input from a number of disciplines even when it has been produced, much more effort from experienced staff is likely to be needed than if a standard solution were used. 

In general, interior steelwork is exposed to less severe conditions than exterior, but in some chemical factories the reverse is true and here special types of paint are needed. Much structural steel is encased in concrete it is therefore hidden from view and is given some protection while the concrete remains alkaline. Where the concrete is thick, corrosion may be delayed, but as the concrete becomes carbonated and particularly if it is penetrated by acidic rain water, the metal will corrode. In general it is advisable that steel which is to be encased in concrete, especially for industrial plants, should... 

Special attention is required for the following to ensure that excessive hidden localized corrosion does not occur beneath the fire-resistive insulation due to infiltrated water ... 

If one part of a metallic object is protected from the atmosphere so that there is insufficient O2 to build or maintain the oxide film, this protected region will often be the site at which corrosion is most active. The fact that such sites are usually hidden from view accounts for much of the difficulty in detecting and controlling corrosion. 

Almost every hazardous material has more than one hazard. As an emergency responder, you must be familiar with other potential, and often hidden, hazards that chemicals may present. Figure 1.8 shows the potential hidden hazards of the nine hazard classes. Across the top of the chart are all the potential hazards a chemical could have that would affect emergency responders. Down the left side are all of the colors representing the UN/DOT hazard classes. An X is used to identify the DOT hazard designated for the material and the color of the placard, which will be on the shipment. An asterisk ( ) is used to identify aU of the other potential hazards of the materials. That does not mean that a particular chemical has all of the hazards, but until you are able to obtain additional information, precautions must be taken for each. For example, some corrosive materials are classified as oxidizers, such as perchloric acid above 50% concentration. Perchloric acid above 50% concentration... 

As previously mentioned, all of the DOT hazard classes identify only the most severe hazard of that material. All of the classes have hidden hazards that are both chemical and physical in natnre. Flammability is not the only hazard associated with Class 3 flammable liqnids. They may also be poisonous or corrosive. For general purposes, there are no UN/DOT subclasses of flammable liquids. Emergency... 

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