East Providence, R.I.
Electronic connectors are typically the points in a circuit most vulnerable to the effects of corrosion. Those effects can range from reduced current capacity to intermittent or permanent failure of the circuit. Consequences related to circuit failures from corrosion can range from the inconvenient to the tragic. There is general agreement that manufacturers need better ways of protecting connectors against the conditions that promote it.
Corrosion results from either oxidation or galvanization. Most engineers know oxidation arises when the metal of the connector combines with oxygen in the atmosphere to create a metallic oxide. Most oxides are not good conductors of electricity, so the oxide coating limits or blocks current flow.
But the major cause of connector failure in harsh environments is galvanic corrosion. In a galvanic reaction, dissimilar metals give up or collect electrons in the presence of an electrolyte, usually water. The ions formed by the electron transfer slowly leach from the material, dissolving it away.
Although many connectors are designed for harsh environments, too often corrosion still cuts their service lives short. Gaps and other leakage paths some of microscopic porosity in the wires, insulation, plastic housings, and pins are ingress points for water and other liquids and gases that promote corrosion.
Leak paths in materials form for many reasons. Temperature cycling expands and contracts dissimilar materials at different rates. Stress conditions, such as vibrations or repeated bending, can also open holes in otherwise sealed materials. Once opened, there's penetration by the main culprit moisture. In some cases, leak paths can function as wicks drawing in moisture.
Lubricants and coolants that keep automated assembly lines running can attack plastic insulating materials. Likewise, steam and caustic chemicals used to wash down certain food-processing equipment can wreak havoc on connector continuity. Corrosion is further accelerated by sea salt found in marine operations such as shipping and offshore oil rigs.
There are two basic methods used to minimize corrosion in connectors: plating and sealing. Some connectors use a combination of both techniques.
It was common practice for many years to plate connector contacts with a layer of tin especially in the presence of copper and aluminum. The tin plating reduces electron transfer, lowers resistance, and prevents discoloration of the bare copper. Nonplated aluminum contacts were often coated with an oxide-inhibiting compound. However, neither practice is effective in the long run against harsh environments.
Sealing closes off the leak paths that moisture and oxygen follow to get to the contacts. Silicone-based materials and epoxy-based "potting" compounds are effective sealants in many applications. Both types are typically applied by hand to the connectors; but hand application is a relatively expensive solution to the corrosion problem. In some cases the nonuniform distribution of these hand-applied sealants makes final assembly of the end product more difficult and time-consuming. Some machines in automated assembly lines shut down completely if connectors won't slide together easily.
A recently developed connector sealant protects connectors in harsh environments to a high degree. The sealant, methacrylate polymer resin, uniformly and economically seals large batches of connectors, assemblies, and wiring harnesses. The material is applied using a process known as vacuum impregnation.
Vacuum-impregnation technology has plugged leaks in porous metal castings and powdered-metal parts for decades. During the process, a liquid thermoset or anaerobic resin fills all of the voids in a casting. Heat then hardens the resin, sealing the voids to block penetration by contaminants. Anaerobic resin has proven particularly effective in connectors because it does not need air to cure.
There are four common methods of impregnation: dry vacuum and pressure, internal pressure, wet vacuum and pressure, and wet vacuum only. Leak paths and porosity in connector components are easily filled with the wet-vacuum-only approach.
Inside a vacuum chamber, connectors, assemblies, and even entire wiring harnesses go in a bath of the low-viscosity impregnating resin. Then a vacuum pump evacuates the chamber. This forces air trapped in the connectors and wires to the surface of the bath. Once all of the air is removed, the chamber is allowed to return to normal atmospheric pressure. Standard air pressure forces the resin into every leak path, plugging them.
Water-washing the product before curing removes the resin from contact surfaces to keep the sealant from interfering with conductivity. The resin then cures in a hot bath of about 140°F. Though not necessary for the curing process, ions from the copper, aluminum, or iron inside the connector function as a catalyst to assist in the curing process. These metals give up electrons to the resin as if it were a more cathodic metal.
Once cured, the resin is irreversibly cross-linked and will not reliquefy. It will withstand temperatures up to 350°F and resist solvents, Freons, steam, oil, gasoline, glycols, and printing inks. A simple air-pressure test confirms the connector assembly is thoroughly sealed against ambient moisture and salts that could otherwise cause corrosion and product failure.
Yet another benefit of these resins is their potential for flexibility. Special formulations of the resin remain flexible after curing. This is particularly helpful under conditions where thermal cycling is extreme. For example, automotive engines can swing from below zero to several hundred degrees on cold winter days. The flexibility of the cured resin allows thermal contraction and expansion of the connector and wires without opening new leak paths.
ELECTROMOTIVE SERIES OF ELEMENTS
(+) Less noble (anodic)
Zinc Chromium Iron Cobalt
Nickel Lead Copper Mercury
Silver Platinum Gold
() More noble (cathodic)
Metals at the top of the list of electromotive elements are anodic. They're more likely to give up electrons during the corrosion process to the more cathodic metals below them.
Metals and galvanic corrosion
Key to the chemistry of galvanic corrosion is the tendency of dissimilar metals to give up or collect electrons in the presence of an electrolyte, which is usually water. In the electromotive series of elements, the farther apart the names of two metals appear on the list, the greater the potential for corrosion. In contact with moisture, anodic metals sacrifice themselves to more cathodic metals. For example, aluminum gives up electrons to copper. It is not unusual to find both of these conductive metals in connectors.
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