Nov 11
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Anodic Protection
Overview
Anodic Protection is a corrosion mitigation technique first introduced in the 1960′s in which the potential of an alloy is controlled relative to a stable reference potential in a range of values more positive or oxidizing than the corrosion potential that causes the corrosion rate to be depressed relative to that which would occur without the application of the potential. One of the major applications of this technique is the protection of carbon steel or 304 or 316 stainless steel in concentrated (greater than 93 wt%) sulfuric acid. Other applications have been reported in the literature. One excellent though dated reference that provides background on theory and practice is “Anodic Protection”, O. L. Riggs and C. E. Locke, Plenum Press, New York, 1981. Further information can be found in “Corrosion Engineering”, M. G. Fontana, Mc-Graw Hill Book Company, New York, 1986.
Theory
Certain alloys in certain conductive environments exhibit a characteristic in which as the potential is increased (relative to a reference electrode) in the anodic or oxidizing direction, the current increases until a potential is reached in which the current exhibits a rather abrupt decrease. Further increases in potential do not result in much increase in current until a much higher potential is reached. This characteristic is shown in this idealized potentiodynamic polarization scan:
As shown in this figure, the potential at which the current shows an abrupt decrease is known as the primary passivation potential. Chemically what is often occurring as that the oxidation reaction is changing, for example instead of iron metal reacting with the environment to form iron in a +2 valence state, iron reacts with the environment to form iron in a +3 state. The resulting oxidation product on the surface is more corrosion resistant. Controlling the potential above or anodic to this point tends to maintain the current at this low value. This current is the new rate of corrosion. Since the abscissa of the plot is the logarithm of current, the decrease in corrosion can be several orders of magnitude. Anodic protection is the technique in which the potential is controlled in this low corrosion regime. Information on how to generate these polarization scans can be found at (http://www.argentumsolutions.com/tutorials/polexpert_tutorialpg3.html). An estimate of the steady state current in this more passive regime is sometimes obtained by performing controlled potential tests lasting several days to several weeks.
Practice
Conceptually, the equipment used for anodic protection is straightforward. The following figure shows a simple arrangement of the equipment that might be used for a vessel:
The “Power Supply & Voltmeter” acts somewhat like a potentiostat in that it provides the current to the cathode so that the sensing point of the reference electrode is at the desired control potential. This potential is chosen so that the potential of the vessel surface is in the “passive” region for the alloy being protected. The current flows between the cathode(s) and the vessel. The number of cathodes and their position often depends on the geometry of the installation. The goal is to try to maintain all interior areas of equipment exposed to the liquid environment within this passive region.
Though the current requirements to maintain anodic protection tend to be low, the amount of current needed to get the alloy to the state of protection can be large. The above figure in the first section shows the reason. The potential of the alloy when it initially contacts the environment is at the corrosion potential. As the potential is increased, the current follows the curve in the above figure. The required maximum current applied to force the alloy above the primary passivation potential (the current at the primary passivation potential) can be several orders of magnitude above the current required for control. This need has implications in the design (size) of the power supply needed for anodic protection.
Anodic protection has been proposed to protect iron and stainless steels in a number of environments some of which are shown below.
- Stainless steel heat exchangers used for handling concentrated sulfuric acid
- Cast iron in boiling sulfuric acid
- Certain stainless steels in acetic acid
- Mild steel in certain types of phosphate containing fertilizers
- Certain steels and stainless steels in phosphoric acid in plants and tankers
One characteristic of these environments is that they usually do not induce pitting or other types of localized corrosion. Control of potential above the corrosion potential may induce localized corrosion if the environment contains agents (e.g. chloride) that are known to initiate localized corrosion of the alloy being protected. Further information can be found in the references in the Overview section above.