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Galvanic Corrosion Carbon Steel
Galvanic Corrosion of Carbon Steel occurs at structural joints where dissimilar metals are joined or where dissimilar fasteners are used.
In galvanic corrosion one metal is the anode upon which the attack occurs and the other metal is the cathode.
The behavior of a metal in a galvanic couple is dictated by the relative position of the metals in the galvanic corrosion chart or galvanic series of metals in seawater.
Carbon Steel which contains less than 0.18% carbon, is anodic to many metals in seawater, including passive stainless steels while is cathodic to materials like aluminum, zinc.
This means that galvanic corrosion on carbon steel will take place when it’s coupled to stainless steel or high nickel alloys.
Care should be exercised to insure that a paint coating does not expose small areas of steel coupled to large areas of a more noble metal in order to avoid Carbon Steel Galvanic Corrosion.
In corrosion tests of bimetallic couples in coastal and inland tropical environments in Panama the corrosion of carbon steel was increased by about 130% by 316 stainless steel 45% by Monel, and 35% by phosphor bronze.
The oxide scale on a steel that results from hot-rolling can be cathodic to the bare metal, but because the mill scale does not form a good protective coating small areas of bare metal may be exposed.
Therefore, mill scale should be completely removed from carbon steel to be exposed to an aqueous electrolyte in order to prevent carbon steel galvanic corrosion.
The mechanism of Galvanic Corrosion is used to protect Carbon Steel when immersed in water or buried in soil through the application of a Cathodic Protection.
This is achieved connecting Carbon Steel to a less noble material like Aluminum or Zinc.
In this case galvanic corrosion on carbon steel will not take place, and the carbon steel structures will be protected.
ref: Material Deterioration Prevention and Control Guide MIL-HDBK-735
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