TAILIEUCHUNG - Asea Brown Boveri Pocket Book P2

If different metals are joined together in a manner permitting conduction, and both are wetted by a liquid such as water, acids, etc., an electrolytic cell is formed which gives rise to corrosion. The amount of corrosion increases with the differences in potential. If such conducting joints cannot be avoided, the two metals must be insulated from each other by protective coatings or by constructional means. | 1 Physical chemical and technical values Electrochemical series If different metals are joined together in a manner permitting conduction and both are wetted by a liquid such as water acids etc. an electrolytic cell is formed which gives rise to corrosion. The amount of corrosion increases with the differences in potential. If such conducting joints cannot be avoided the two metals must be insulated from each other by protective coatings or by constructional means. In outdoor installations therefore aluminium copper connectors or washers of copper-plated aluminium sheet are used to join aluminium and copper while in dry indoor installations aluminium and copper may be joined without the need for special protective measures. Table 1-8 Electrochemical series normal potentials against hydrogen in volts. 1. Lithium approx. 10. Zinc approx. - 19. Hydrogen approx. 2. Potassium approx. 11. Chromium approx. 20. Antimony approx. 3. Barium approx. 12. Iron approx. - 21. Bismuth approx. 4. Sodium approx. 13. Cadmium approx. - 22. Arsenic approx. 5. Strontium approx. 14. Thallium approx. - 23. Copper approx. 6. Calcium approx. 15. Cobalt approx. - 24. Silver approx. 7. Magnesium approx. 16. Nickel approx. - 25. Mercury approx. 8. Aluminium approx. 17. Tin approx. - 26. Platinum approx. 9. Manganese approx. - 18. Lead approx. - 27. Gold approx. If two metals included in this table come into contact the metal mentioned first will corrode. The less noble metal becomes the anode and the more noble acts as the cathode. As a result the less noble metal corrodes and the more noble metal is protected. Metallic oxides are always less strongly electronegative i. e. nobler in the electrolytic sense than the pure metals. Electrolytic potential differences can therefore also occur between metal surfaces which to the engineer appear very little .

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