Thermogalvanic corrosion

The driving force of a thermogalvanic corrosion cell is therefore the e.m.f. attributable to these four effects, but modified by anodic and cathodic polarisation of the metal electrodes as a result of local action corrosion processes. 

Thermogalvanic corrosion Crevice corrosion concentration cells... 

While ordinary galvanic corrosion (two-metal corrosion) can be identified by a visual inspection alone, for thermogalvanic corrosion we need also to know a little about the service conditions. 

Thermogalvanic corrosion is prevented by appropriate design and measures to avoid uneven heating/cooling and forming of hot spots. For heat-insulated pipes and equipment is important that the insulation is continuous. The corrosion form may under certain conditions be prevented by cathodic protection or coatings. 

Avoid hot as well as cold spots. Heat exchangers and other equipment where heat transport occurs should be so designed that the surface temperature varies as little as possible. On superheated spots, increased (possibly thermogalvanic) corrosion will occur. For systems containing vapour, cold spots leading to local condensation and corrosion should be avoided (Figure 10.8). 

Insulate legs on hot tanks to prevent thermogalvanic corrosion at welds. 

While concentration and oxygen cells are responsible for perhaps 90 percent of the corrosion in soils and natural waters, other cells, such as the temperature cell shown in Fig. 7.28, may be still quite damaging when they get established. In such cells, the two electrodes are of the same metal, but one is maintained at a higher temperature than the other by some external means. In most cases, the electrode at the higher temperature becomes the anode to cause what has been called thermogalvanic corrosion. 

Bell GEC, Schiff MJ, Wilson DF Field observations and laboratory investigations of thermogalvanic corrosion of copper tubing. CORROSION 97, Paper 568. Houston, Tex. NACE International, 1997. 

Denickelification. Denickelification of 70-30 alloys (i.e., the selective leaching of nickel out of an alloy matrix) has been encountered occasionally in refinery overhead condenser service, where hydrocarbon streams condense at temperatures above 150°C. This appears to be due to thermogalvanic effects resulting from the occurrence of local hot spots. The solution has been to remove deposits that lead to the hot spots, either by more frequent cleaning or by increasing flow rates. Denickelification was also observed recently in modern warship heat exchangers where some 70-30 copper-nickel tubes suffered severe hot spots corrosion. To prevent this problem from recurring. 

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