account for consequences from aggressive handling or damage; however, the anodic, sacrificial class W finish has proven to be satisfactorily immune from detrimental effects due to handling and damage. The preferred condition for a harsh environment rated finish is where the outer finish is sacrificial to any undercoating(s) and if possible, to the base material. When considering aluminum as the base material, since it is highly reactive (anodic), an additional strategy is to more effectively seal the finish (to eliminate pores and/or micro-cracks) as a means of protecting the aluminum (7) . This may be accomplished by providing a durable cathodic barrier undercoating followed by an anodic sacrificial outer finish that is sacrificial to, or at least galvanically compatible within 0.15V to the Aluminum substrate. Anodic and Cathodic Characteristics of Cadmium Alternative Finishes Cadmium-alternative finishes first introduced in the early 2000s, that remain in active use today include Nickel-fluorocarbon polymer and black Zinc-Nickel alloy. More recently appearing in some interconnect product specifications is Tin-Zinc alloy. Intermateability testing between these alternatives and class W Cadmium finish is documented in AIR5919 (6) . Pass/ fail criteria to establish compatibility requires the different intermated connectors to meet the required performance testing. This is not necessarily indicative of actual galvanic compatibility; only that any galvanic potential between the finishes is not sufficient to cause a failure. These finishes have demonstrated the capability to meet requirements which do not include any tests that simulate damage to expose the Aluminum substrate which could be disastrous for a cathodic finish. Nickel-fluorocarbon polymer, defined as finish class T in MIL-DTL-38999 (4) and finish class X in AS85049 (5) . The outer layer coating of this finish is a co-deposit of electroless Nickel-Phosphorous alloy, and a fluorocarbon Polymer (PTFE). Nickel-Phosphorous alloy is quite cathodic compared to Aluminum with a galvanic potential differential of over 0.50V. This characteristic establishes Nickel-fluorocarbon polymer as a cathodic barrier finish. Black zinc-nickel alloy, defined as class Z in both MIL- DTL-38999 (4) and AS85049 (5) , provides an electroless nickel undercoating with an outer layer of chromated conversion coated zinc-nickel alloy, which has a galvanic potential within 0.15V of both aluminum and cadmium, meeting the criteria of an anodic sacrificial finish. Similarly, Tin-Zinc alloy recently defined as class V in AS85049 (5) and class codes L and M (classes T and TJ) in MIL-DTL-28840 (8) , consists of an outer layer
of chromated conversion coated Tin-Zinc alloy, with a galvanic potential within 0.15V of both aluminum and cadmium, also meets the criteria of an anodic
sacrificial finish. Conclusion
On aluminum substrates, anodic, sacrificial style finish systems, like cadmium, zinc-nickel, and tin-zinc are preferable for harsh environment applications. When damaged, aluminum material that is exposed to the corrosive atmosphere can receive continued protection from the surrounding galvanically-similar outer finish layer. Cathodic barrier style finish systems such as electroless nickel or nickel-fluorocarbon polymer can deliver significant corrosion protection provided these finishes are not damaged. When damaged, exposed aluminum material becomes sacrificial to the cathodic outer finish layer. The cathodic finish remains protected, instead of the aluminum substrate, and galvanic corrosion will penetrate the aluminum propagating laterally beneath the coating , potentially leading to catastrophic failure. For this reason, cathodic barrier finish systems are better suited for protected environments, or harsh environments where the possibility of damage from rough handling is essentially eliminated. Acknowledgements The author would like to specifically acknowledge Mr. Greg Brown, Mr. Mehrdad “Mike” Ghara and Mr. Narongphon “Boyd” Changkaochai in support of this paper. Background Mr. Ty Geverink is Accessories Product Manager and Senior Technical Staff member at Glenair, Inc., located in Glendale, CA. Mr. Geverink has a BSCS (Associated Technical College) and has worked in the interconnect industry since 1981. References: 1) MIL-STD-14072: Finishes for Ground Based Electronic Equipment 2) ASM HANDBOOK VOLUME 13A: Corrosion: Fundamentals, Testing, and Protection 3) https://www.corrosionpedia.com/definition/226/cathodic- coating - September 1, 2020 4) MIL-DTL-38999: Connectors, Electrical, Circular, Miniature, High Density, Quick Disconnect (Bayonet, Threaded or Breech Coupling), Environment Resistant with Crimp Removable Contacts or Hermetically Sealed with Fixed, Solderable Contacts, General Specification for 5) SAE International; AS85049: Connector Accessories, Electrical, General Specification for 6) SAE International; AIR5919: Alternatives to Cadmium Plating 7) SAE International; AEISS 2009 Paper: Cad-Free Finishes for Interconnect Applications: Performance, Processing and Other Considerations by Greg Brown; VP Engineering, Glenair, Inc. 8) MIL-DTL-28840: Connectors, Electrical, Circular, Threaded, High Shock, High Density, Shipboard, Class D, General Specification For 9) MIL-PRF-28876: Connectors, Fiber Optic, Circular, Plug and Receptacle Style, Multiple Removable Termini, General Specification for.
QwikConnect • April 2023
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