Anodic versus Cathodic Finishes for Corrosion Protection of Aluminum Substrates in Interconnect Applications TECHNICAL WHITEPAPER
1) Corrosion resistance: Exposure to neutral salt- spray / salt fog (NSS) alone (static salt-spray), or in combination with mating cycle durability (dynamic salt-spray) for evaluation of corrosion resistance for durations of between 48 and 1,000 hours of exposure. Additional proposed testing may include a separate exposure to acidified salt-spray/fog (SO 2 ) for up to 336 hours. 2) Conductivity: Measurement of (connector) shell-to-shell or backshell to connector voltage drop, both before and after corrosion resistance testing. It should be noted that “accelerated” laboratory testing results may not adequately simulate actual field conditions that may include combinations of handling, minor physical damage, temperature extremes, or exposure to debris including ice or blowing dust/sand. Characteristics of Anodic and Cathodic Finishes Coating/plating systems can be described as either anodic (sacrificial) or cathodic (barrier) finishes. To understand the distinction, it is important to consider the materials galvanic potential in the most likely environmental medium. Table 1 ranks commonly used materials according to a measured voltage in a medium of seawater. The corrosion tendency for materials listed towards the top is considered cathodic or “noble” and they are generally slower corroding. As the voltage potential decreases towards the bottom of the table, materials become increasingly more anodic or “active” and susceptible to oxidation and faster corroding. (e.g., Gold is cathodic while Zinc is anodic.) Galvanic corrosion is one of the most pervasive and progressive types of corrosion. There are four fundamental components to form a galvanic electrochemical corrosion cell: an anode, a cathode, an electrolyte and an electrical connection between the anode and cathode for the flow of electrons. The basic concept for most methods of corrosion protection is to remove one or more of these cell components so that the pure metal or metal alloy of interest will not corrode (2) . It is important to take a closer look at the anodic or cathodic potential for materials of interest, applicable to this discussion Anodic (sacrificial) finishes are also rather non- intuitively referred to as “cathodic protection” because the anodic finish protects the more cathodic substrate by sacrificial corrosion. Cathodic protection of the substrate by sacrificial corrosion of the coating.
The following is a technical white paper from August 2022 authored for a DLA discussion on proposed new finish classes for 38999 and other connector specifications, written by Ty Geverink; Product Manager, member Senior Technical Staff, Glenair, Inc. Abstract Efforts to replace cadmium surface plating and other materials have been ongoing for over twenty years. Historically, chromate-conversion-coated-cadmium over an underplating of electroless nickel was the only available finish capable of meeting the conductivity and long-term corrosion protection requirements of aluminum alloy interconnect components – particularly the harsh-environment requirements defined in military and aerospace standards. A handful of alternatives to cadmium emerged in response to RoHS and REACH initiatives to eliminate (or at least reduce) the use of target materials—with nickel-fluorocarbon polymer and zinc-nickel alloy emerging as the most popular. In more recent years, additional alternatives have been developed that are being considered for adoption into various interconnect product specifications. This paper will examine the fundamental differences between these cadmium-alternative finishes with a focus on how the respective processes achieve corrosion protection. Current testing methodology and pass/fail criteria will also be explored to evaluate theoretical shortcomings versus real-world conditions. Background Aluminum is the most widely used substrate material in military and aerospace connector applications for its relatively light weight, strength, electrical and thermal properties, low cost, abundant supply, and ease of manufacturing. Un-protected aluminum however is highly active (anodic) making it susceptible to loss of electrical conductivity due to oxidation and severe corrosion in harsh environments. The successful application of aluminum in Electrical Wire Interconnect Systems (EWIS) is owed in large part to the ability of applied conductive coating/plating processes to enable retention of Al conductivity, and dramatically improve the material’s corrosion resistance – critical requirements for life-of-system survival in harsh application environments – such as aerospace and defense. Typical testing to validate surface finish effectiveness in meeting these electrical conductivity and corrosion performance requirements includes:
QwikConnect • April 2023
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