Examining the Relationship Between Post-Build Microstructure and the Corrosion Resistance of Additively Manufactured 17-4PH Stainless Steel

The performance of wrought and post-build heat treated, additively manufactured SS17-4PH stainless steel was evaluated to determine whether additive processing can produce material with a similar corrosion resistance to the alloy in wrought form, and to establish whether the additive manufacturing (AM) processing conditions alter the electrochemical conditions required for corrosion attack. Samples built by laser powder bed fusion (LPBF) from nitrogen-atomized and argon-atomized powder were heat treated to achieve a homogenized, solutionized martensitic microstructure. A wrought sample was also solutionized to minimize the influence of Cu precipitates. Multicomponent E-pH diagrams were constructed for the three compositions in a chloride containing environment using equilibrium thermodynamic data to elucidate the chemical reactions that can occur in the SS17-4PH alloy and to indicate the regions where variations in passivity would occur. Electrochemical measurements compared the behaviors under free corrosion conditions and the pitting resistances of the three compositions in chloride environments with differing solution pH levels. The results revealed that the nitrogen retained from atomization does enhance the corrosion resistance of additively-processed SS17-4PH and that the benefit was strongest in a neutral pH environment. The results also revealed that the dissolution of the nitrogen containing matrix suppressed the activity of CuCl3−2 in the solution and enhanced the activity of NH+4 ions at electrode potentials near Epit. This signifies that the ionic phases at the surface of the electrode are influenced by both the alloy composition and the processing conditions.