Galvanic corrosion definition

Galvanic corrosion and the factors affecting it have been discussed in Chapter 20. However, a few precautionary comments are in order for aluminum, since it is anodic to most common materials of construction, with the exception of magnesium and zinc. In the presence of a good electrolyte, as little as 15 mV difference in corrosion potential of the two metals can have an effect, and if the difference is 30 mV or greater the anodic material will definitely corrode sacrificially to protect the contacting cathodic metal. A recently revised report on galvanic corrosion, with emphasis on automotive applications is available [73]. 

The experimental results suggest that the deposition of NO2 into dew accelerates the corrosion of galvanized steel however, it is difficult to establish this definitively because the concentration of dissolved NOx in the dew is small, thus its... 

Definition Cathodic protection (CP) is defined as the reduction or elimination of corrosion by making the metal a cathode using an impressed current or attachment to a sacrificial (galvanic) anode. It is a process that reduces the anodic corrosion reaction by creating an electric field at the surface of the metal so that the net flow of current is into the metal. 

Examples of metals that are passive under Definition 1, on the other hand, include chromium, nickel, molybdenum, titanium, zirconium, the stainless steels, 70%Ni-30% Cu alloys (Monel), and several other metals and alloys. Also included are metals that become passive in passivator solutions, such as iron in dissolved chromates. Metals and alloys in this category show a marked tendency to polarize anodicaUy. Pronounced anodic polarization reduces observed reaction rates, so that metals passive under Definition 1 usually conform as well to Definition 2 based on low corrosion rates. The corrosion potentials of metals passive by Definition 1 approach the open-circuit cathode potentials (e.g., the oxygen electrode) hence, as components of galvanic cells, they exhibit potentials near those of the noble metals. 

One of the key factors in any corrosion situation is the environment. The definition and characteristics of this variable can be quite complex. One can use thermodynamics, e.g., Pourbaix or -pH diagrams, to evaluate the theoretical activity of a given metal or alloy provided the chemical makeup of the environment is known. But for practical situations, it is important to realize that the environment is a variable that can change with time and conditions. It is also important to realize that the environment that actually affects a metal corresponds to the microenvironmental conditions that this metal really sees, i.e., the local environment at the surface of the metal. It is indeed the reactivity of this local environment that will determine the real corrosion damage. Thus, an experiment that investigates only the nominal environmental condition without consideration of local effects such as flow, pH cells, deposits, and galvanic effects is useless for lifetime prediction. 

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