Chapter 31

2. Chapter 31 Corrosion Corrosion • Corrosion Prevention • Forms of Corrosion

3. Corrosion consists of anode and cathode reactions that take place at different locations on a metal surface.

4. The type of corrosion is influenced by the location of the anode reaction.

5. Anodes and cathodes develop half-cell, or redox, potentials, which may be measured with reference to the standard hydrogen electrode (SHE).

6. When corrosion occurs, the anode and cathode are short circuited and their half-cell potentials are displaced toward each other.

7. After it has stabilized, the corrosion potential is measured by means of a high-impedance voltmeter.

8. Metal anodes display two types of polarization curves, which are active and active-passive.

9. The shape of the cathode polarization curve depends on whether an oxygen diffusion or chemical reduction reaction influences depolarization.

10. Electrochemical corrosion testing consists of plotting the variation between the corrosion current density and the potential using a potentiostat.

11. The pH scale can be an important indicator of corrosiveness.

12. The corrosion rate of steel versus temperature rises to a maximum value and then falls.

13. Corrosion is minimized by good design.

14. Coupon corrosion testing consists of exposing samples (coupons) of various materials to the corrosive environment for sufficient time to obtain a ranking of their performance.

15. Current cathodic protection is extensively used to protect underground pipelines.

16. Barrier layer inhibitors consist of cathodic and oxidizing (anodic) types.

17. More than one form of corrosion may participate in the failure of a component.

18. General corrosion of a component proceeds until failure occurs by overload of the thinned wall, or a leak develops from perforation due to excessive thinning of the wall.

19. Erosion-corrosion occurs under agitated conditions, such as high flow rate, leading to depolarization, often coupled with the removal of protective corrosion products. With active-passive metals, these conditions may help maintain the passive film.

20. Liquid impingement erosion exhibits a characteristic horseshoe pattern, with U shapes pointing in the direction of the flow.

21. Cavitation is caused by imploding vapor bubbles that set up shock waves on the surface of the component.

22. Crevice corrosion is caused by an oxygen concentration cell, which is formed between exposed and shielded portions on a metal surface.

23. Chloride pitting of stainless steels consists of spherical cavities that undercut the surface so that the extent of damage is worse than it appears.

24. Oxygen pitting of carbon steel consists of large jagged pits.

25. Corrosion fatigue eliminates the threshold stress (endurance limit) for fatigue failure.

26. Fretting in steel components appears as jagged pits filled with dark brown powder.

27. Certain corrosive environments cause stress-corrosion cracking in specific alloys.

28. The concentration of chloride ions at the top of a vertical heat exchanger tube with a tubesheetmade of austenitic stainless steel can lead to chloride cracking.

29. Carbon steels operating in caustic service above certain temperatures and concentrations must be stress relieved.

30. Hydride embrittlement occurs in alloys such as titanium by inward diffusion of hydrogen to form brittle metal hydrides.

31. The potential difference between the less noble (anode) and more noble (cathode) components indicates the tendency for galvanic corrosion to occur.

32. Sensitization of austenitic stainless steels is a function of time and temperature.

33. Knifelineattack occurs in a sharply defined zone of base metal adjacent to the fusion line of the weld.

34. Exfoliation primarily affects aluminum alloys and causes flaking and swelling.