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Conversion coatings are used to inhibit corrosion on aluminum structures while maintaining electrical conductivity. The most common type of conversion coatings in aerospace applications (MIL-DTL-5541 Type I), contain hexavalent chromium compounds as the corrosion-inhibiting additive. These Type I conversion coatings have a long pedigree and are highly effective in preventing corrosion; however, the hexavalent chromium compounds in these coatings are carcinogenic and water-soluble. Therefore, the use of these compounds is highly regulated in order to protect both workers and the public leading to high cost in both use and disposal. In addition to these regulations, use of these materials on new designs for DOD is prohibited by DFARS 48 CFR Parts 223 and 252, and is scheduled to be prohibited in Europe in September 2017 by REACH regulations. In response, new more environmentally friendly non-hexavalent chromium-based processes are becoming available. Coatings resulting from these types of processes are referred to as MIL-DTL-5541 Type II conversion coatings. The long term reliability and performance impacts resulting from the use of these coatings are not fully understood and there currently is an effort in the U. S. aerospace industry organized by NASA to fully define these impacts while hardware is still in the design stage. While significant work has been performed to define the corrosion performance of various Type II conversion coatings, there has been minimal work performed to quantify the impact a Type II conversion coating would have on RF electrical assemblies where plated printed wiring boards (PWBs) and aluminum structures come in intimate contact. The primary concern for these assemblies is that these junctions are inherently susceptible to galvanic corrosion; PWBs are clad with copper, which is highly cathodic while aluminum is highly anodic. In order to reduce the potential for galvanic corrosion, PWBs in DOD applications are typically plated with SnPb coating which is less cathodic than the copper. In addition, an immersion bath is used to coat the aluminum with a conversion coating that is less anodic. Changes to the conversion coating could increase the galvanic corrosion occurring at this junction. In addition, RF signals may also be negatively impacted by changes to the electrical resistivity and parasitic capacitances caused by changes to this junction. For this reason, it is highly desirable to create a RF test article that is highly sensitive to the impacts of galvanic corrosion at the junctions of passivated aluminum and plated printed wiring boards. This talk discusses a RF test article that is designed to assess the longitudinal impact of galvanic corrosion on electrical assemblies. The test article features a specialized suspended stripline/microstrip stepped impedance filter that is designed to de-tune in the presence of galvanic corrosion. The design of this filter uses a pair of machined aluminum housings to sandwich a thin two sided printed wiring board. The high-impedance sections of the filter employ cavities above and below the thin PWB to create an effective airstripline transmission line. The low-impedance sections of the filter employ a PWB ground plane to create a microstrip mode. Small aluminum feet are machined in the bottom aluminum housing to create an electrical contact between the aluminum housing and PWB ground plane. These feet are designed to function as sacrificial elements that corrode away in the presence of galvanic corrosion, creating series capacitance in the ground signal path. This talk reviews recent test results that show how the response of this specially designed filter changes in the presence of galvanic corrosion and compares these results with electrical simulations. This talk also discusses how information gained from the filter response can be used to assess the electrical impact of Type II conversion coatings. Finally, this talk will discuss the experimental design needed to quantify the impact of Type II conversion coatings with respect to the current baseline processes.

Conversion coatings are used to inhibit corrosion on aluminum structures while maintaining electrical conductivity. The most common type of conversion coatings in aerospace applications (MIL-DTL-5541 Type I), contain hexavalent chromium compounds as the corrosion-inhibiting additive. These Type I conversion coatings have a long pedigree and are highly effective in preventing corrosion; however, the hexavalent chromium compounds in these coatings are carcinogenic and water-soluble. Therefore, the use of these compounds is highly regulated in order to protect both workers and the public leading to high cost in both use and disposal. In addition to these regulations, use of these materials on new designs for DOD is prohibited by DFARS 48 CFR Parts 223 and 252, and is scheduled to be prohibited in Europe in September 2017 by REACH regulations. In response, new more environmentally friendly non-hexavalent chromium-based processes are becoming available. Coatings resulting from these types of processes are referred to as MIL-DTL-5541 Type II conversion coatings. The long term reliability and performance impacts resulting from the use of these coatings are not fully understood and there currently is an effort in the aerospace industry organized by NASA to fully define these impacts while hardware is still in the design stage. While significant work has been performed to define the corrosion performance of various type-II conversion coatings, there has been minimal work performed to quantify the impact a type-II conversion coating would have on RF electrical assemblies. Of particular interest is the impact a conversion coating can have on microwave loss at higher frequencies. Many RF electrical assemblies use aluminum radiator and waveguide structures to transfer energy between components and radiate into freespace. If microwave losses increase due to a change in conversion coating, there could be negative impacts to key performance parameters such as system sensitivity, dynamic range, noise figure, and radiated power. Understanding this impact is critical in determining whether the design change impact is isolated only to the conversion coating or whether it propagates to other subcomponents to compensate for the loss in performance. The standard way to quantify the electrical resistance of conversion coatings is defined by MIL-DTL-81706B. The test involves collecting a DC resistance measurement on a processed panel using a two-probe measurement with 200 psi of pressure applied to the probes. The resulting value is averaged from 10 samples of data collected across the panel. While this test in MIL-DTL-81706B is well defined, it has significant limitations that caused this research to seek another way to quantify this value. First, the repeatability of the two-point probe is not consistent across the panel. Some of the conversion coatings can be brittle and can easily be disturbed by the force applied by the probes. The poor repeatability is exacerbated when the test articles are environmentally exposed, leaving a non-uniform surface. Finally, this test methodology is performed at DC, which does not directly quantify the impact of the coating at microwave frequencies due to phenomena such as skin effect and potential plasmonic response. This talk discusses an experiment performed to assess the impact of the use of type-II conversion coatings on microwave loss. In order to assess this impact, a set of precision machined waveguide structures were used as test articles in the experiment. The advantage of using this waveguide-based approach is that it provides a distributed surface to assess the average impact of conversion coatings on surface resistivity. This average resistivity more closely maps to the RF losses seen by microwave systems. In addition, testing the waveguide test article provides a very repeatable test methodology; waveguide technology is very mature from a manufacturing perspective. Also, the waveguide flanges provide a repeatable way to connect to the test article so long as they are masked or cleaned after any potential environmental exposure. Finally, the rectangular shape of the waveguide can be canonically described by a closed form expression, improving understanding of the specific mechanisms leading to the loss. This talk discusses an experiment where multiple 3-foot pieces of WR-28 were used as test articles. The WR-28 test articles were chosen to assess the impact to performance at Ka-band. The 3-foot sections are convenient articles because they can easily be measured on a workbench while at the same time being electrically long at Ka-band (on the order of 100 wavelengths). This talk discusses three different populations of test articles, each coated with a different type of conversion coating. This talk also discusses how an initial measurement of these test articles before environmental exposure showed little difference between these populations. Finally, this talk will discuss plans for environmental testing and in-process RF measurements to be captured during these tests.