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#068 Electrolytic Corrosion Protect Measures - 1

Category : Corrosion - Corrosion Protection
January28, 2011

We have discussed so far on metal corrosion types, causes, and countermeasures. From this point on, let us look at some corrosion prevention measures.

The corrosion prevention basics are: (1) Use of corrosion resistant metals, and (2) Corrosive environment improvements. Details are as follows.
(1) Make the metals electro-chemically more noble by alloying and/or plating and coating.
(2) Convert corrosive environments into non-corrosive environments by adding soil neutralizers for objects embedded in corrosive soils, and add inhibitors (corrosion inhibiting chemicals) for water submerged objects.
Additionally as related to (1), take the corrosion issues in considerations for the designing and constructions processes and eliminate crevice corrosions, concentration differential cell phenomena, uneven fluid flow, and localized heating.
Electro-chemical Corrosion protection measures are widely applied for large ships, Bay/Port structures, and chemical plant equipment. Typically called electrolytic Corrosion protection measures, there are Cathodic and Anodic methods.
[Fig.1] Corrosion Prevention of Ship Hull with Zinc

(1) Cathodic Corrosion protection Method

Ship hulls constantly being submerged in corrosive sea water suffers corrosions due to the hull steel being electrochemically non-noble where a cell is formed between copper alloy screws and hull steel material. Customarily, zinc ingots have been placed in stern areas to protect the hull corrosions. The zinc ingots were used as sacrificial metal where the zinc dissolves instead of the hull. The principle is the same as zinc plated roof plates. The zinc in this case is called Sacrificial Anode.
There are two cathodic Corrosion protect methods. One method is to use an external DC source to polarize the metal to be protected as a cathode, and the other is to connect less noble metals. The former is called External Power Supply Method or Powered Method, and the latter case the zinc ingot method mentioned above is called Galvanic Anode Method or Sacrificial Anode Method.

1) For the sacrificial anode methods, zinc, magnesium, and aluminum can be used. However, aluminum cannot be used in fresh water since likely to become more noble than steel in fresh water, though can be used as non-noble metal in sea water since the passive surface layer will be destroyed.
2) For the external power supply methods, direct current, sufficient to overcome the corrosion current flowing out from the metals to the environment, is used. As anodes, non-dissolving materials such as graphite and ferrite.
[Fig.2] External Power Supply Method