The Insight: Galvanic Corrosion and Fasteners
Galvanic corrosion also known as bimetallic corrosion occurs when two metals of different electric potential are brought into contact in the presence of an electrolyte, the metal with the lower potential (least noble) will form the anode while the metal with the higher potential (most noble) will form the cathode. When current flows from the anode to the cathode, a chemical reaction will take place and metal forming the anode will corrode. The higher the electric potential, the stronger the current flow and corresponding rate of corrosion. Rate of corrosion will also be influenced by the conductivity of the electrolyte i.e. fresh water, salt water etc… Galvanic corrosion however does not occur if one of the conditions is not present. If two dissimilar metals are in direct contact without the presence of an electrolyte corrosion will not occur.
All metals have an electrical potential which has been measured through research and ranked into a galvanic series chart.
When two dissimilar metals are in contact (coupled) in the presence of a conductive solution or electrolyte (i.e. Water) electric current flows from the less noble (anodic) metal to the more noble (cathodic) metal. In any couple, the less noble metal is more active and corrodes while the more noble metal is galvanically protected. A plating metal which is less noble (lower electric potential) than the base metal it is designed to protect is usually selected. When subjected to an electrochemical reaction, the plating will corrode or sacrifice while the base metal remains protected. Once the plating has been reduced significantly, the base material will then begin to corrode. If a plating metal which is more noble is selected, the base metal would begin to corrode immediately if the plating is damaged.
For carbon steel anchors and fasteners, zinc is one of the most common plating materials used because it can be applied in a broad thickness range and because it is less noble than carbon steel. Zinc may be applied by electroplating, mechanical galvanizing, or hot dip galvanizing.
Theoretically, the life expectancy of zinc plating is based on the thickness of the plating divided by the corrosion rate. These values should only be used as a guide since actual performance will vary with local conditions, Table 1 below highlights typical corrosion rates of zinc and mild carbon steel based on corrosivity categories in accordance with ISO 9223.