Surface Treatment Effects on the Corrosion Characteristics of Additively-Manufactured Metals
Authors: Stull Jamie , Clark Courtney , Johnson Donald R , Dervishi Enkeleda , Edwards Randall L , McBride Michael , Hooks Daniel E 
Published: May 2020

Metal Additive Manufacturing (MAM) powder-bed technology has recently become a widespread manufacturing option for, medical, aerospace, and auto industries. While this clearly demonstrates the impact of this new production path for manufacturing, MAM products must still meet many traditional requirements to interface with other components for maximum performance and reliability. These requirements include surface specifications, such as finish (roughness), coatings, and corrosion resistance. However, initial studies indicate that these requirements may not directly transfer to MAM materials, with complications including degraded corrosion resistance, rougher finishes in hidden surfaces, and inability to achieve specified plated surface requirements. Currently, there is a knowledge gap with respect to how MAM components will perform to these specifications, and testing cycles can exceed several years.

The focus of our work was to identify and understand the effects of production surface treatments and how they affect the corrosive properties on MAM materials in corrosive environments. This includes understanding the effects of heat treatments applied to MAM components in post-processing, a step which can significantly affect results. This work will help identify appropriate post-processing treatments to ensure MAM components meet the expected requirements for surface finish and corrosion resistance.

Surface treatments that are typically used in manufacturing include chemical cleaning, polishing processes, and heat treatments. While these processes have been optimized for wrought type materials the unique surface finish and grain structure will require more tailored approaches. Heat-treatments applied to additive manufactured (AM) stainless steel have been shown to improve the corrosion properties of stainless steel that could be linked to chromium segregation in the grain boundaries, leading to intergranular corrosion and intergranular stress corrosion cracking, due to the elevated temperatures used during manufacturing. While most heat-treatments have been optimized for wrought material, augmented heat-treatments could result in an improvement on the corrosion properties of MAM materials.

Furthermore, polishing treatments such a mechanical, machining, or electropolishing can be used to improve the surface finish of MAM materials. Changes in the surface finish, such as roughness, will affect the corrosion properties of metals in corrosive environments. Electropolishing is attractive treatment since it is a non-contact surface processes method. Unlike physical methods, such as mechanical polishing or machining, it can more evenly polish difficult to reach surfaces. It has also been shown to modify the protective passivation layer that is found on the surface of AM stainless steels, which can improve the corrosion resistance of the materials. We have explored the effects of tradition DC electropolishing in strong acids, but also pulse electropolishing which can be performed in less toxic solutions. We present how various electropolishing parameters effect the corrosion performance of MAM materials