Figure 2 Corrosion Prevention of Barrier Protection (2)
Figure 1 Corrosion Prevention of Sacrificial Protection (2)
The corrosion penetration occurs parallel with the substrate. This protection remains effective even when voids, pores or defects are present in the coating. Cathodic (barrier) finishes provide corrosion protection by providing a more corrosion-resistant cathodic barrier coating between the environment and the less corrosion-resistant anodic substrate. The success of this system in providing corrosion protection relies on effectively sealing the substrate so only the slower-corroding cathodic coating is exposed to the environment, and no galvanic action between the coating and substrate can occur. To accomplish this, the coating must be free of porosity or other defects. In corrosive environments accelerated corrosion of the substrate occurs if cathodic coating fails to protect the substrate (3) . The corrosion mechanics of a breached barrier system can be seen in the example of a nickel coating over a steel substrate shown in Figure 2 . The reaction in the process of this system includes the steel substrate giving up electrons to the more noble Nickel coating which passes these on to hydrogen, and results in steel leaving the matrix as an ion. As a result, the noble coating is protected, instead of the steel substrate and the corrosion penetration occurs perpendicular to the substrate and propagates laterally below the coating. Unlike sacrificial protection, any defects or damage introduced to the coating can result in catastrophic failure. Application Considerations Historically, interconnect product specifications have defined finish “classes” for aluminum material and corrosion performance requirements specific to those classes. In general, these performance requirements are representative of anticipated environments that include two categories: indoors or “protected environment” applications, and outdoors or “harsh environment” applications. “Protected environment” applications include the interior of aircraft or Naval vessels which are typically not exposed to weather and associated corrosive
elements. For these applications, the predominant finish has been electroless Nickel, defined as finish class F in MIL-DTL-38999 (4) and finish class N in AS85049 (5) . In this application environment, the cathodic barrier electroless Nickel finish protects the aluminum substrate from natural oxidation that could reduce electrical performance and provides resistance to 48 hours of static salt-spray testing to ensure limited corrosion protection. Consequences from aggressive handling or damage are not considered due to the generally limited exposure to electrolytes necessary for galvanic corrosion to occur. “Harsh Environment” applications include locations such as the weather decks of Naval Vessels, external surfaces of military ground vehicles and equipment, and exposed (SWAMP zone) areas of aircraft. These frequently wet areas often include salt, chloride-sands, and other corrosive elements including exhaust gasses that can precipitate as acids. Harsh environments also anticipate rough treatment of interconnect components throughout the service life. For this reason, some military connector specifications for outdoors applications such as MIL- DTL-28840(8) or MIL-DTL-28876(9) have imposed testing for “Impact” among the qualification tests required for these connectors. Repeated exposure to impacts can chips or crack finishes, exposing the aluminum substrate. Far more corrosion protection is required to provide satisfactory performance over the anticipated service-life expectancy. The predominant finish for this environment has been chromate-conversion-coated-cadmium over an underplating of electroless nickel, defined as finish class W in both MIL-DTL-38999 (4) and AS85049 (5) . In this finish class, the aluminum substrate is first sealed with a cathodic undercoating of electroless nickel, followed by an outer layer finish of anodic, sacrificial, and Aluminum-compatible cadmium which is then passivated and sealed with a thin chromate- conversion coating. This system provides continued electrical performance and provides resistance to 500 hours of dynamic salt-spray testing that includes mating durability cycles to ensure continued corrosion resistance. The testing sequence currently does not
QwikConnect • April 2023
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