Connector Corrosion Protection

Connectors are the most vulnerablepoints in a circuit. Corrosion, from either oxidation or galvanization, reducescurrent-carrying capacity and results in intermittent, and ultimatelypermanent, failure of the circuit. In harsh environments, the major cause ofconnector failure is galvanic corrosion, a process in which dissimilar metalsgive up or collect electrons in the presence of an electrolyte, usually water.

Although many connectors are designedfor use in harsh environments, too often their service lives are limited bycorrosion due to gaps and other leak paths (even microscopic porosity) in thewires, insulation, plastic housing and pins. Prior to complete failure, acorroding connector may cause its circuit to fail intermittently, causingdowntime and maintenance expense while frustrated mechanics search for thesource of the problem.

Temperature cycling causes thedissimilar materials in connectors to expand and contract at different rates. Exacerbatedby this and other stressful conditions, such as vibration or repeated bending,leak paths allow penetration by the main culprit - moisture. In some cases,leak paths even function as wicks drawing in moisture.

Harsh environments where connectors arecommonly used include automobile engine compartments; military, aircraft and aerospaceequipment; outdoor devices; and a range of industrial facilities. Lubricantsand coolants used to keep automated assembly lines running can attack plasticinsulating materials, as can the steam and caustic chemicals regularly used towash down certain food-processing equipment. And in marine applications such asshipping and offshore oil rigs, corrosion is further accelerated by sea salt.

Withthese applications in mind, there are two basic ways to minimize corrosion toconsider when designing a system and specifying the connectors that will beused in it: plating and sealing.

Formany years, plating of an electronic connector's contacts with a layer of tinhas been a common practice, especially where copper and aluminum are bothpresent. This not only reduces electron transfer, but also lowers resistanceand prevents the discoloration of bare copper. Aluminum contacts, if notplated, are often coated with an oxide-inhibiting compound. However, neitherpractice is effective against harsh environments in the long run.

Sealing,if done properly, closes off the leak paths that moisture and oxygen can follow.Sometimes, sealing is done in addition to plating. Silicone-based sealants areeffective in many applications, as are epoxy-based potting compounds. Bothtypes are typically applied by hand to the connectors, but this is a relativelyexpensive solution to the corrosion problem. In some cases, these hand-appliedsealants may make final assembly of the end product more difficult andtime-consuming, since they are usually not uniformly applied. Another factoraffecting sealed connector is that, in some automated assembly machines,connectors that don't slide together easily can shut down the whole assemblyline.

New Resin Option
Arecent advance in connector sealants, methacrylate polymer resin, providesmaximum protection for connectors in harsh environments. Instead of beingpainted or brushed onto individual connectors one by one, these nonconductiveresins can uniformly and economically seal a large batch of connectors,assemblies, and wiring harnesses all at once, in a process called vacuumimpregnation.

Vacuumimpregnation technology has been employed for decades in the complete sealingof leaks resulting from porosity in metal castings and powdered-metal parts. Inliquid form, the resins-either thermoset or anaerobic-are forced into all voidsin a casting, usually via vacuum and pressure. The material is then washed andcured, completely sealing porosity and leaving the part leak-free, even underpressure.

Thereare four common methods of impregnation: dry vacuum and pressure, internal pressure,wet vacuum and pressure and wet vacuum only. The leak paths and porosity inconnector components are easily filled with the wet vacuum only approach. Insidea vacuum chamber, connectors, assemblies, or entire wiring harnesses are placedin a bath of low-viscosity impregnating resin and air is entirely evacuatedfrom the chamber. All air in the connectors, wires, etc. rises to the surfaceof the bath and is removed from the chamber. Once the vacuum is drawn, thereturn to normal atmospheric pressure is enough to drive the resin into allleak paths.

Atthis point, the product is water-washed prior to curing, to prevent the sealantfrom interfering with conductivity. Next, the resin is cured in a hot bath -usually at about 140F. Though not necessary for the curing process, ions fromcopper, aluminum, or iron inside connectors can actually function as catalysts,assisting in the curing process. These metals give up electrons to the resin asif it were a more cathodic metal. Anaerobic resin has proven particularlyeffective for sealing connectors because it does not require air to cure.

Oncecured, the resin is irreversibly cross-linked and will not reliquify. It willwithstand temperatures up to 350F and resist solvents, Freon, steam, oil,gasoline, glycols and printing inks.

Yetanother benefit of these resins is their potential for flexibility. Whennecessary for the anticipated application, the resin can be formulated to cureto a flexible state.

Untilrecently, there has been a drawback to most impregnated thermoset resins - theytend to ignite when exposed to fire. Fortunately, fire-resistant resins are nowon the market.

Afterthe impregnation process, a simple air-pressure test can be performed to provethe connector assembly is thoroughly sealed against ambient moisture and saltsthat could otherwise cause corrosion and product failure.

Extreme EnvironmentApplication

Thedurability of connectors sealed in this manner can be especially helpful ininstallations with prolonged conditions of thermal cycling, vibration andmoisture changes, such as those encountered in many military/aerospaceapplications.

Intanks and other land vehicles, vibration and frequent temperature changes callfor rugged, tightly sealed connections. Humidity, rain and washdowns supplyplenty of moisture, which is of course more abundant for ships, boats andamphibious vehicles.

TheU.S. Air Force Research Laboratory has concluded that corrosion at thejunctions of connectors carrying signals is one of the most frequent causes ofintermittent failure and other erratic performance in aircraft electricalsystems.

High-altitudepressure drops can draw trapped air out of a connector, possibly opening newpathways for corrosion-causing moisture. Moisture can then quickly enter suchpathways in wet weather or during washdown/de-icing operations. Condensation orwater vapor in humid air could be forced into the pathways as the aircraftdescends and air pressure rises. A connector totally sealed using vacuumimpregnation has no air pockets, since they would be filled with resin.

For more information, visit:http://www.impco-inc.com.