This project plans to demonstrate and evaluate a pretreatment with optimal timing and coating solutions for additively manufactured (AM), non-stainless steels that provide superior corrosion protection for multiple Department of Defense (DoD) platforms. The research team will achieve this by examining the mechanism and application of non-chrome pretreatments, electrodeposition coating (“electrocoat or e-coat”), to determine ideal timing of application of nonchromium coatings during the AM workflow cycle. This will enable engineering authorities to develop AM specific pretreatment specifications and coating requirements for AM metals to expedite adoption of novel AM methods to facilitate increased mission readiness through use of on-demand production techniques.
Many existing pretreatments used in the DoD contain hexavalent chromium (Cr), which has strict regulation against its use, or contain other Cr-based which are being more severely regulated based on recent guidance. Printed steel shows an increased susceptibility to pitting, particularly in the samples with the greatest defect density. Additionally, printed steel part is much like a giant weld, so corrosion resistance is expected to be worse than traditionally manufactured steel parts, especially, because the relatively high roughness of AM parts (layer-by-layer). Spray application of pretreatment and primers is not possible with most AM parts due to their relatively high complexity and lack of line of site access to the steel surfaces. There are several Cr-free pretreatments approved for DoD use on steels and are on the Qualified Product List for Federal Specification TT-C-490: Chemical Conversion Coatings and Pretreatments for Metallic Substrates, that will provide a measure of corrosion protection.
Further improvements in field durability can only be ensured through priming of the steel. Electrocoating (e-coat) materials and process exist, that are well known and ideal for complex geometries where line of site spray application to coatings is not possible. This project will apply these technology solutions toward the AM process for non-stainless steels, with several surface treatments and finishing processes to provide an adequate base for the subsequent specified coating system. This project effort will build AM steel components, employing powder bed fusion, using lasers to melt thin layers of metal powder to build three-dimensional parts. The AF9628 steel alloy was originally developed by the U.S. Air Force for application in bombs (as bomb blaster), has shown potential applications for ground vehicle replacement parts due to its excellent strength and toughness and will be the focus of this research. AF9628 is unique since it does not contain tungsten or cobalt. E-coat is an excellent way to protect steel parts, offering superior corrosion resistance, uniformity, and an even coat. It is used significantly in the automotive industry complicated parts and already qualified according to MIL-DTL-53084 chemical agent resistant coatings specification but has not been proven for use in AM of non-stainless steels.
The DoD is working hard on developing AM capabilities that can reduce cost, production times and increase capacity to close the technological and industrial gaps. This project seeks to apply, evaluate, and demonstrate the performance of pretreatments and/or e-coat to AM AF9628 steels that is environmentally friendly with superior corrosion resistance while ensuring AM production and warfighters receive critical capabilities at the point of need. The project will apply technology solutions toward the AM process for non-stainless steels to demonstrate effective corrosion mitigation solutions. This will enable creating standards and modifying specifications to stipulate corrosion mitigation for non-stainless AM steels solution.