Additive Manufacturing

Corrosion Properties of Selective Laser Melted (SLM) Stainless Steel

Corrosion of AM

Sponsor: 

Graduate Students Involved: Duane Macatangay 

The recent growth in additive manufacturing (AM) has fueled the motivation to understand the properties and performance of conventional alloys that have been synthesized through these methods. Laser powder bed fusion (LPBF) is a form of AM that uses high energy lasers to construct bulk materials from the melting and rapid solidification of powder on a layer-by-layer basis. While there has been considerable work done to optimize LPBF processing parameters for various metallic alloys and characterize the resulting microstructure and mechanical properties, little work has been performed to characterize the corrosion properties of these 3-D printed alloys.

316L is an austenitic stainless steel that is utilized in various applications due to its excellent mechanical properties and corrosion resistance. Although wrought 316L has a signature microstructure that is nominally single phase and made up of equiaxed grains, the additive process introduces a non-equilibrium microstructure. This leads to the introduction of compositional heterogeneities via melt pool boundaries and cellular structures and provide an origin for selective corrosion, leading to potential issues involving the longevity of these alloys in the as-printed state.

Sensitization occurs in stainless steels through the intergranular precipitation of chromium carbides at grain boundaries when the material is exposed to temperature ranges of 450 – 800 °C. This leads to the depletion of chromium at regions adjacent to grain boundaries and may lead to intergranular corrosion and intergranular stress corrosion cracking. The utilization of post-processing methods such as hot isotactic pressing induces temperatures within this range and may initiate this solid-solid phase transformation. The observation of accelerated sensitization in LPBF 316L provides motivation to characterize and understand this sensitization behavior.

This study explores corrosion in LPBF 316L with attention to melt pool boundary dissolution and intergranular corrosion. This study also acknowledges the effect of processing parameters on the resulting microstructure and corrosion behavior. Galvanostatic etching and potentiodynamic polarization are performed to quantify and understand the electrochemical behavior of these alloys. Characterization methods such as scanning electron microscopy and white light interferometry are used to probe the microstructure and corrosion morphology of these alloys in comparison to the conventionally wrought counterpart.

Publications:

• Macatangay, D. A., Thomas, S., Birbilis, N., and Kelly, R. G., "Unexpected Interface Corrosion and Sensitization Susceptibility in Additively Manufactured Austenitic Stainless Steel", Corrosion, Vol. 74, 2018, p. 153–157