Galvanic corrosion occurs when dissimilar metals are in direct contact with each other in the presence of an electrolyte solution (like road spray that collects between trailer components or even high humidity in the atmosphere).
The process of galvanic corrosion begins when electrons from the active metal are attracted to the less active metal. If an electrolyte is present, this attraction enables the active metal particles to break apart into electrons and metallic ions.
The electrons generate a current flow going from the more active metal to the less active metal.
The metallic ions travel through the electrolyte and attach to the surface of the less active metal. This is the same principle seen in a battery. (The active metal is the anode and the less active metal is the cathode. )
Several guidelines can be employed to minimize galvanic corrosion. See the following slides for some best practices when designing and repairing trailers and truck bodies.
All metals have different electropotential. This table shows the ranking of metals in seawater (the electroyte) arranged from the most active to least active. The farther apart two metals are in the Galvanic Series the more likely corrosion will occur.
When contact is unavoidable, select components based on metals closer together in the series.
The attraction or pull that the less active metal has on the more active metal is amplified if the area of the active metal is smaller. The corrosion process accelerates in relation to the size difference.
When dissimilar metals come in contact, use a non-absorbent insulate between them. A polypropylene tape of 1.7 mils minimum thickness with a dielectric strength of 300-400 volts/mil is adequate for most trailer and body applications.
Paint or prime the two metals, even if they already have protective coatings. This extra coating is especially helpful in isolating joints. It is particularly important to coat the active metal.
Avoid threaded fittings with materials far apart in the Galvanic Series. Use washers, gaskets, and sleeves made of plastic or a compatible metal before connecting dissimilar metals.
Apply corrosion-inhibiting material (pastes, washers, compounds, etc.) under heads of screws or bolts inserted into dissimilar metal, even if they already have been treated.
If active metal components must be used, the design should allow for them to be easily replaced or make them thicker for longer life.
Where possible, use tape instead of caulk. Caulk in a joint between dissimilar metals can squeeze out and allow the two metals to contact. Closed-cell neoprene tape tends to do a better job of isolating.
Read this related article for more suggestions: "How to deal with dissimilar metals".
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