Galvanic corrosion (also known as bimetallic corrosion or dissimilar metal corrosion) is the breakdown of metallic surfaces as a result of the difference in electrical potential of adjacent metals and the presence of an electrolyte.
Stated differently, when two dissimilar metals are in contact in a corrosive environment, one of the metals will begin to corrode. This process is the same one that occurs inside of a battery. The metal that will be corroded and the speed of this breakdown are dependent on the difference in metals and the environment.
Conditions for corrosion:
For galvanic corrosion to occur, three conditions must be met:
Difference in electrical potentials of the two metals in contact with one another
All metals have an electrical potential assigned to them, based on their nobility. Metals such as platinum, silver, and monel have lower corrosion potentials whereas metals such as copper, aluminum, and tin have much higher potentials. Any two dissimilar metals will have a galvanic mismatch and therefore a change of corrosion.
Electrical path between the two metals
In situations of EMI shielding, the electrical path is inherently created by the conductivity of the gasket, coating, or sealant.
A fluid able to break down the metals
Examples of such fluids can include atmospheric humidity or salt fog environments. As this mist or moisture condenses and collects at the flange or gasket interface, it will create the electrolyte needed to start breaking down the metals.
Preventing galvanic corrosion while EMI shielding:
For aluminum substrates that are going to be exposed to harsh environments such as military and industrial applications, chromate conversion coatings (also known as chem filming) are recommended. On top of this coating would be a conductive or non-conductive top coat. For steels and coppers, nickel or tin plating is often used.
Corrosion-resistant conductive coatings, such as CHO-SHIELD 2000 series conductive paints, are developed with stabilizers to create a very conductive and galvanically inactive surface for high-level EMI shielding in harsh environments.
Matching EMI gasket fillers to substrates
Because EMI shielding gaskets are in direct contact with structural metal substrates, the corrosion potential must be considered. Historically, conductive fillers have needed to adapt to increasing requirements of galvanic corrosion resistance. Only within the last couple of decades have filler systems such as silver-plated aluminum replaced traditional silver-plated copper or nickel-plated graphite, to dramatically improve corrosion resistance in enclosures that experience moisture and salt fog.
Despite the excellent performance of silver-plated aluminum fillers, the development of a nickel-plated aluminum filler has set the gold standard for both EMI shielding levels as well as corrosion resistance. This filler system, utilizing inherently stable compounds, exhibits the best results on chem filmed aluminum flanges relative to any other filler system, with a 20-50% reduction even compared to silver-plated aluminum.
Design guide for corrosion control:
A properly designed interface requires a moisture-sealing gasket whose thickness, shape and compression-deflection characteristics allow it to fill all gaps caused by uneven flanges, surface irregularities, bowing between fasteners and tolerance buildups. If the gasket is designed and applied correctly, it will exclude moisture and inhibit corrosion on the flange faces and inside the package.
Follow the below steps to maximize corrosion resistance in enclosures:
Where other requirements are met, select nickel-aluminum filled elastomers for best overall sealing and corrosion protection.
Use silver-aluminum gaskets as the next best alternative to nickel-aluminum filled materials.
In aircraft applications, a “seal-to-seal” design can be used with the same gasket material applied to both flange surfaces.
Use a Co-extruded or Co-molded gasket – extruded or molded in parallel, these gaskets consist of a conductive and non-conductive elastomer in one piece. The non-conductive material is placed outboard to interface with the moisture, effectively minimizing a key condition causing galvanic corrosion.
Coat surfaces with a corrosion resistant plating.
Avoid designs that create areas for moisture to pool. Use drainage holes to allow liquids to flow away from the interface.
Avoid sharp edges or protrusions such as dovetail grooves that can damage gaskets.
Select the proper protective coating and use additional environmental sealants.
Corrosion and shielding effectiveness:
In order to conduct direct comparative shielding effectiveness testing of gasket panel sets before and after environmental exposure cycling in a standardized test set-up, Parker Chomerics established CHO-TP09. This test method is based on IEEE-STD-299 and takes into account environmental aging conditions such as salt fog, humidity, and extreme temperature cycling.
Proper enclosure design and the implementation of conductive filler systems engineered to minimize galvanic corrosion are key drivers in extending the life span of electronic enclosures and lowering long term maintenance/replacement costs.