Abstract
The aim of this work is to compare the corrosion resistance of nickel-base Alloy 625 (UNS N06625) produced by laser powder bed fusion with that obtained via conventional casting and hot working. Cyclic potentiodynamic polarization and potentiostatic tests were performed in order to evaluate the corrosion resistance of the differently manufactured alloys according to ASTM G5, and in NaCl 0.6 M solution at pH 7 and pH 3, at 40°C. The electrochemical characterization was carried out on the as-produced alloy and after annealing at 980°C for 32 minutes (according to ASTM B446). This heat treatment was also performed on the commercial hot worked alloy. Two surface conditions, namely as-built and polished surfaces, were investigated on the additive manufactured specimens. The alloy produced by laser powder bed fusion was not susceptible to pitting in the considered environments and had a good localized corrosion resistance, slightly higher than that of traditional wrought material. However, as predicted, the corrosion resistance of the as-built surfaces increased after mechanical polishing. The correlation between the corrosion performance and microstructure is also discussed.
Highlights
Alloy 625 is widely used in aerospace applications,[1] marine environments including the manufacture of heat exchangers,[2] in the Oil and Gas industry[3,4,5] and in nuclear plants[6,7,8] thanks to its high mechanical properties, good corrosion resistance and great performance at high temperatures
The Ecorr values were always higher than the equilibrium potentials of hydrogen (Figure 1), confirming that despite the nitrogen bubbling, oxygen was still present; this was confirmed by measurements using WTW FDO 925 optical oxygen sensor which indicated about 1 ppm present during the tests
- The potentiostatic tests showed evidence of possible crevice corrosion in neutral 0.6 M NaCl solution at 40°C, mainly when the sample was polarized at high anodic potential (+500 mV vs. Saturated Calomel Electrode (SCE))
Summary
Alloy 625 is widely used in aerospace applications,[1] marine environments including the manufacture of heat exchangers,[2] in the Oil and Gas industry[3,4,5] and in nuclear plants[6,7,8] thanks to its high mechanical properties, good corrosion resistance and great performance at high temperatures. It can suffer from localized corrosion, such as crevice, pitting, intergranular corrosion and stress corrosion cracking (SCC) in very aggressive conditions.[9,10,11] The manufacturing of complex shaped components out of Alloy 625 via traditional processing techniques is technically challenging and very expensive For these reasons, additive manufacturing (AM), which is the processes for the fabrication of near net-shape components via the progressive addition of material, has been widely studied for this alloy in recent years.[12] The advantages of AM include the possibility of creating components of complicated geometries avoiding complex mechanical machining as well as reducing the time-to-market and the machining wastage.[13] it is possible to obtain very fine microstructures with mechanical properties higher than those of traditional cast alloys.[14,15,16] On the other hand, AM components are often associated with porosity and the high surface roughness, and these particular microstructures can be more susceptible to different forms of corrosion than those materials obtained with more traditional techniques.[17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33] Amongst AM techniques, Laser Powder Bed Fusion (LPBF) is the most widespread technology for metals nowadays. Whist building orientations were not shown to play an important effect on the corrosion behavior,[37] improved corrosion resistance with
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