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#069 Electrolytic Corrosion Protect Measures - 2

Category : Corrosion - Corrosion Protection
February 4, 2011
(3)Comparison of Sacrificial Anode Method and External Power Supply Method

The advantages of the External Power Supply Method are: (1) voltage and current are easily adjusted, (2) therefore, is adoptable to changing corrosive conditions, (3) semi-permanent installation is possible if the anodes are sufficiently dissolvable, therefore, is economical.
The Sacrificial Anode Methods, on the other hand, are (1) installations are simple and require no maintenance for certain periods of time, (2) installable where no power source is available or on small equipment, and economical. The shortcomings of the External Power Supply Method are: (1) requires cumbersome current adjustments (costs are high for automatic adjustment systems), (2) the anodes may become inoperative if not sufficiently strong or dissolvable, (3) high initial costs, etc
The shortcomings of the Sacrificial Anode Methods are: (1) Corrosion protect effects may not be sufficient if anode placements in environments are improper, (2) long term corrosion protection cannot be expected without replacing the anodes, typically requiring replacements in a few years intervals.

(2) Problems of Anodic Corrosion protect Methods

There are following problems associated with Anodic Corrosion protect methods.

(1) Electrical current required for corrosion protection

The electrical current required for corrosion protection varies by corrosive environments. For instance, electrical resistance of fresh water is 100 times or more higher than that of sea water. The current required for corrosion resistance depends on the amount of diffused oxygen and does not vary whether fresh or sea water.
Therefore, the electrical power required is the same but the voltage requirement is much higher due to high electrical resistance of the fresh water (same for soil).

(2) Excessive Corrosion protect effects

Excessive cathode polarization results in waste of electrical power as well as many other problems. The hydrogen generated by cathode polarization may cause hydrogen embrittlement fractures on metals that are prone to occlude hydrogen. Additionally, amphoteric metals corrode due to raised cathode pH values.

(3) Corrosion protect measures for inside of pipes

For cases where electrical current flow and corrosion protection distance may be limited such as insides of pipes, corresponding electrode placements are required. Any precipitated deposits of cathode polarization would accelerate the corrosions.

(4) Impingements on adjacent metals

When an electrical current is applied to a target object, electrical corrosion may occur on adjacent metals. This is more of an issue for high electrical resistance environments such as in the ground than sea water. Since it is often not clear what embedded objects are near by in the ground, the installation must follow a thorough investigation.