**1. Introduction**

Alloy 625 (UNS N06625) is one of the most used material in chemical and oil and gas industries for its corrosion resistance in either oxidant or non-oxidant acids. It is also used in the aerospace industry thanks to the resistance to thermal oxidation [1,2]. In the oil and gas industry, high aggressiveness of the environment requires the use of materials with outstanding corrosion resistance to ensure the reliability of assets, reduce the risk of breakage, prevent consequences on the environment and guarantee the safety of people. The continuous growth of demand and the depletion of reservoir lead the oil and natural gas extraction industry to exploit deeper wells, with extreme conditions (High Pressure High Temperature Wells—HPHT). The use of traditional alloys with low both mechanical and corrosion resistance is not suitable under such hypothesis and international standards i.e., NACE MR 0175/ISO 15156 defines such environment as "sour" even in presence of few ppm of H2S in the formation gas

due to the high operating pressures. Materials resistant to sulfide stress corrosion cracking (SSCC) are required in these environments and high temperatures, high fraction of water in the formation and presence of CO2 and H2S implies the use of general and localized corrosion resistant alloys (Corrosion Resistance Alloys—CRA). Inconel 625® (Alloy 625) is a nickel-based superalloy strengthened by solid-solution hardening of Nb and Mo in a Ni-Cr matrix [3]. Alloy 625 has a combination of high yield strength, fatigue strength, and excellent corrosion resistance in aggressive environments, it has found widespread applications in the aerospace, marine, and nuclear industries where complex shapes are often required. The alloy shows general corrosion rates less than 0.5 mm/year in concentrated non-oxidizing organic and inorganic acids [4,5]. The resistance to pitting is very high, much higher than the traditional austenitic stainless steels. Thanks to the high nickel content, it is immune to chloride stress corrosion (SCC, Stress Corrosion Cracking). Alloy 625 is precipitation-hardening alloy, due to the formation of very fine phases, which strengthen the austenitic matrix, because of aging treatments between 550–750 ◦C. The formation of these phases or their decomposition modifies both the yield strength and ductility of the alloy [6]. Furthermore, the formation of carbides and secondary phases can also affect the corrosion resistance. In particular, the alloy can become susceptible to intergranular corrosion if subjected to improper solubilization treatment [7].

In the last years, a very promising additive manufacturing (AM) technique has been developed in order to find alternative processes based on Laser Powder Bed Fusion (LPBF) of metal powders—layer by layer—by using a laser following the 3D CAD model. This production method has advantages in terms of cost reduction and lead time, because it eliminates swarf—typical of traditional subtractive machining—welding operations, and assembly phases. Lot of efforts has been devoted to AM of Alloy 625 alloy [8–14]. Alloy 625 is sensitive to precipitation of intermetallic phases such as Ni3M phases, Ni2(Cr, Mo) Laves phase as well as MC primary carbides and M6C and M23C6 secondary carbides. Large amount of literature exists on the heat-treatment-induced phase evolution of wrought Alloy 625 [3,15,16]. The effect of post-weld heat treatment has been also studied by several groups [17–19]. As AM techniques are processes characterized by melting and solidification, microstructures are expected to be different due to different thermal gradients. Precipitation of phases that occurs in AM Alloy 625 are not present at all in wrought material or they only appear after tens to hundreds of hours [2].

Despite huge amount of data and scientific papers regarding the mechanical, physical and microstructural characterization of AM of nickel-based alloys, on the authors knowledge there are no works on the corrosion behavior of the alloy obtained by means of LPBF. On the contrary several works underline the effect of the different microstructures obtained by means of LPBF on the corrosion behavior of Ti6Vl4V alloy [20–22], on AISI 316L [23–25], high strength steels [26] on Cr/Co alloys [27,28] and AlSi10Mg [29–34].

The relation between microstructure and corrosion resistance is of fundamental importance for the qualification of materials selected for hostile environments. The paper is devoted to the study of susceptibility to intergranular corrosion of Alloy 625 produced by LPBF in as-built condition and after typical heat treatment usually recommended for oil and gas service. The behavior of LPBF alloy has been compared to hot-worked, heat treated commercial Alloy 625.

#### **2. Materials and Methods**

Commercial gas atomized Alloy 625 powder was used for LPBF process and hot-worked bar with 16 mm diameter was considered for comparison purposes. The bar was furnished in annealed condition, after heat treatment at 980 ◦C for 32 min and water quenching (Grade 1, according to ASTM B446 standard). The chemical composition of LPBF and hot-worked alloys as well as microstructures are shown in Figure 1. The microstructure was revealed by grinding with silicon carbides emery papers up to 4000 grit, polishing with diamond paste up to 1 μm, and then etching with Kalling's N◦2 reagen<sup>t</sup> or electrolytic oxalic acid according to ASTM A262—Practice A. As-built LPBF specimens were etched by Mixed Acid Solution (15 mL HCl, 10 mL H3COOH and 10 mL HNO3, Carlo Erba RPA reagents, Cornaredo, Milan, Italy) (Figure 1). The microstructures of LPBF specimens were observed on planes parallel or perpendicular to the building direction, whereas hot-worked specimens were only examined on transverse plane.

**Figure 1.** Chemical composition and 3D optical image composite of as-built IN625 sample showing the melt pool contours (MPCs) along the building direction (z-axis) and perpendicular to the building direction (x-y) plane; mixed acids reagen<sup>t</sup> was used.
