Corrosion inhibitors galvanic measurements

The concepts in Chapters 2 and 3 are used in Chapter 4 to discuss the corrosion of so-called active metals. Chapter 5 continues with application to active/passive type alloys. Initial emphasis in Chapter 4 is placed on how the coupling of cathodic and anodic reactions establishes a mixed electrode or surface of corrosion cells. Emphasis is placed on how the corrosion rate is established by the kinetic parameters associated with both the anodic and cathodic reactions and by the physical variables such as anode/cathode area ratios, surface films, and fluid velocity. Polarization curves are used extensively to show how these variables determine the corrosion current density and corrosion potential and, conversely, to show how electrochemical measurements can provide information on the nature of a given corroding system. Polarization curves are also used to illustrate how corrosion rates are influenced by inhibitors, galvanic coupling, and external currents. 

The effect of the phosphate layer as a cathodic inhibitor under atmospheric conditions is illustrated in Fig. 12. In this experiment, the Volta potential of a galvanized steel surface is measured as a function of time during a transition from air to Ar atmosphere, as indicated [51]. The measurement is performed with a Kelvin probe, and the Volta potential of the corroding surface is directly proportional to the corrosion potential with appropriate calibration [58]. The potential jump induced by the presence of air is a measure of the sensitivity of the surface to the oxygen reduction reaction. Here, we see that the galvanized steel surface shows a very large potential jump, on the order of 200 mV. However, the phosphated surface shows only... 

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