Corrosion Behavior and Contact Resistance of Electroplated γ-ZnNi with Passivation Layers
Authors: Volz Steven Michael , Claypool James B , O'Keefe Matthew , Fahrenholtz William 
Published: February 2020

Low contact resistance, environmentally friendly coatings are needed to replace toxic hexavalent chromium from the passivation layers used in Department of Defense (DoD) electrical system components. Currently, electrical connectors use an electroplated cadmium coating and a hexavalent chromium based conversion coating (CrCC) to provide corrosion protection while maintaining a contact resistance of <5 mΩ before corrosion and <10 mΩ after ASTM B117 salt spray testing[1]. The cadmium can be replaced by a low hydrogen embrittlement (LHE) alkaline ZnNi coating. However, there are no commercial non-hexavalent chromium passivation coatings for LHE ZnNi that can consistently provide a contact resistance below the values given in DoD specifications. The goal of this study is to study the use of alternative passivation coatings as a potential replacement for CrCCs on LHE ZnNi coatings.

The corrosion behavior and contact resistance for five different surface passivation conditions were studied for steel substrates with an electroplated LHE ZnNi coating:  1) as-deposited LHE ZnNi; 2) CrCC; 3) trivalent chromium passivation (TCP); 4) a cobalt-free trivalent chromium passivation (Co-Free); and 5) a cerium-based passivation coating. The LHE ZnNi coatings were deposited by a commercial vendor.  The CrCC, TCP and Co-Free coatings were deposited onto the LHE ZnNi by commercial vendors.  The final surface treatment, the cerium-based passivation coating, was deposited using a procedure developed in previous projects [2,3].  The cerium-based coatings were deposited from an aqueous solution of cerium salt that contained H2O2 and an organic gelatin.  Surfaces were prepared by initially cleaning with a degreasing solution and then activated with either an acidic or alkaline solution.  Coatings were deposited by immersion of the substrate into the aqueous solution for 15 to 1800 seconds.  After deposition, the cerium-based coatings were post-treated in an aqueous solution of NaH2PO4 heated to 85°C for 5 minutes. 

To characterize the corrosion resistance of the coatings, electrochemical tests were performed for all surface treatments using electrochemical impedance spectroscopy (EIS) and cyclic potentiodynamic scans (CPDS). The results from CPDS showed that all of the coatings had self-healing properties, becoming nobler after corrosion, which were dependent on the LHE ZnNi coating. In addition, CPDS also showed that the coatings did not exhibit pitting corrosion. Initial results from ASTM B117 salt spray testing confirmed that all of the panels resisted pitting corrosion for at least twenty-four hours. Corrosion behavior will be studied in salt spray for times up to at least 500 hours. Results indicated that the cerium-based passivation coatings had an initial impedance of about 400 Ω before salt spray, which was similar to those of the TCP and CrCC (Figure 1). Depending on the processing parameters used for deposition of the cerium-based passivation coatings, a wide range of impedance values were measured. For example, cerium coating 2 used a pH of 4.72, 15 ml of H2O2, and 60 seconds deposition time, whereas cerium coating 3 used a pH of 2.07, 10 ml of H2O2, and 90 seconds deposition time (Figure 2). After panels were exposed to ASTM B117 salt spray for twenty-four hours the impedance values of the cerium-based coatings were lower than the initial values. These values were in the same range as the as-deposited LHE ZnNi, TCP and Co-Free coatings after similar exposure times. Impedance values will be measured for all of the surface treatments for salt spray exposure times of up to at least 500 hours.

The contact resistance was measured for each surface treatment before and after salt spray exposure. Cerium coatings 1, 2, 3, Co-Free, TCP and CrCC had contact resistance values of 0.6, 0.6, 1.0, 0.5, 0.7 and 2.7 mΩ, respectively. Even though the initial contact resistances for the cerium-based passivations were higher, the values were below the 5 mΩ requirement for electrical connectors. After exposure to ASTM B117 salt spray for twenty-four hours, each type of surface treatment had samples with measured values <10 mΩ. Additional testing will determine if the contact resistance can continue to meet the requirement for exposure times of at least 500 hours.

The research that will be described in the presentation will focus on determining the mechanisms by which the alternative passivations coatings degrade during salt spray exposure.

Figure 1