Corrosion Resistance of Additively Manufactured 625 Inconel Alloy in Seawater
Authors: Baturina Olga A , Roeper Donald F. , Olig Scott 
Published: February 2020

Additive manufacturing (AM) of metals and metal alloys offers advantages of fast and flexible design of parts of various shapes, and low-waste operation. AM manufacturing of metals/alloys involves building materials layer-by-layer from a metal/alloy powder. The powder in each layer is melted by a high intensity laser or electron beam, and individual particles within the layer and between consecutive layers are fused together during this process. Due to peculiarities of manufacturing process, microstructure of AM manufactured alloys are significantly different from their cast or wrought counterparts [1-5]. Previous research has shown that depending on AM and post-processing parameters the difference in microstructure may be detrimental [5] or beneficial [3] for corrosion resistance of AM-manufactured alloys. For example, corrosion resistance of AlSi10Mg alloy made by selective laser melting (SLM) and subsequently heat-treated for 2 h at 300oC was found to be similar to that of the gravity cast alloy, while its corrosion fatigue endurance turned out to be superior to that of the cast alloy [3].
Here, we compare corrosion behavior of 625 Inconel alloy manufactured by SLM to its wrought counterpart in the marine environment. We find that the initial polarization curves look almost identical for 625 AM and 625 wrought specimens both in 0.6 M NaCl and ASTM seawater (Fig. 1). However, the difference in open circuit potential (OCP) and microstructure of two specimens suggest different composition of surface oxides and potentially different rates of their dissolution. In order to understand how variations in microstructure and surface composition affect alloys’ long-term corrosion resistance, we perform ex-situ surface characterization and in-situ analysis of alloy dissolution while holding electrode potential at -1, -0.5, 0.5 and 1V vs Ag/AgCl reference electrode.
Electrochemical experiments were conducted in three-electrode flat electrochemical cells (PAR, Inc). Freshly polished coupons of 625 AM or 625 wrought Inconel alloys served as working electrodes (WEs), while platinum mesh and Ag/AgCl in 3 M NaCl (BioLogic, Inc) were used as a counter and reference electrodes, respectively. Initial corrosion properties of the alloy electrodes were characterized by linear polarization resistance and potentiodynamic measurements. Alloy dissolution experiments were conducted in potentiostatic conditions, and concentrations of dissolved metal species were determined by ICP-MS. X-Ray Photoelectron Spectroscopy was used to determine chemical speciation of the samples after exposure to OCP and carrying out alloy dissolution experiments.
References
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[2] A. Leon, E. Aghion, Materials Characterization, 131 (2017) 188.
[3] A. Leon, A. Shirizly, E. Aghion, Metals, 6 (2016).
[4] R.I. Revilla, J. Liang, S. Godet, I. De Graeve, Journal of the Electrochemical Society, 164 (2017) C27.
[5] J.J. Yang, H.H. Yang, H.C. Yu, Z.M. Wang, X.Y. Zeng, Metallurgical and Materials Transactions a-Physical Metallurgy and Materials Science, 48A (2017) 3583.

Figure 1