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 Technical Bulletins >> Corrosion Protection

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Electrochemistry of Zinc and Carbon Steel

Corrosion is an electrochemical process which occurs when four elements are present; an anode which gives up electrons, a cathode which receives electrons, an electrolyte (which is usually an aqueous solution of acids, bases, or salts) and a metallic current path. The rate at which corrosion occurs depends on the electric potential between the anodic and cathodic areas, the pH of the electrolyte, the temperature, and the water and oxygen available for chemical reactions.

Figure 1 indicates how corrosion damages carbon steel. Note that the pitted area to the right is anodic and gives up electrons while the cathodic area to the left (where water and oxygen from the air are present) is where rust appears. The pitted area where the carbon steel is weakened is not where the rust appears.

Zinc has a greater tendency to give up electrons than carbon steel, so when both are present, zinc becomes the anode and protects the carbon steel. Figure 2 indicates corrosion with the zinc giving up the electrons and becoming pitted while the carbon steel remains undamaged. From this we see that a zinc coating will protect carbon steel by “sacrificing” itself until the zinc is depleted. The rate of zinc depletion is relatively slow when the pH of the electrolyte is between 4 and 13, which covers many industrial environments.

Hot dip galvanizing has two advantages over a simple zinc coating. During galvanizing, the molten zinc reacts with the carbon steel to form layers of zinc/iron alloys. Figure 3 shows a galvanized surface with 5 layers, the top layer is 100% zinc and the bottom layer is carbon steel. The alloy layers between have increasing hardness to provide mechanical (barrier) protection and because of their zinc content they are also anodic relative to carbon steel. The hardness of these alloy layers provides much more protection from scratches than paint can provide. This is important for most pipe supports applications.

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