PSI - Issue 68

Tsanka Dikova et al. / Procedia Structural Integrity 68 (2025) 99–105 Tsanka Dikova & Natalina Panova / Structural Integrity Procedia 00 (2025) 000–000

101

of the additively manufactured AISI 304 steel in aqueous solution of 25% H 2 SO 4 , 5% CuSO4.5H 2 O, and 12.5% NaF. They also found that the corrosion resistance depends on the δ-ferrite content in the microstructure: the higher δ-ferrite content, the lower the corrosion resistance due to the inefficient chromium oxide passivation. Large varieties of laser technological processes are used nowadays for production of biomedical devices for general and dental medicine. The laser assisted AT are very promising for manufacturing of personalized implants. In these processes, the corrosion resistance of the final product depends on the morphology, microstructure and composition of the surface layer, which is determined by the technological parameters such as the laser power, scanning speed, laser spot diameter, distance between the scanning traces etc. Last years, studies on the corrosion resistance of the SLM and DLD fabricated AISI 316 L stainless steel are mostly performed. The aim of the present study is to investigate the failure of laser melted surfaces of AISI 321 steel under electrochemical corrosion in two types of biological fluids: artificial saliva with different acidity (pH 5,6 and 6,5) and Ringer’s solution. 2. Materials and methods Prismatic samples with dimensions 10 mm x 30 mm x 100 mm were milled of austenite stainless steel AISI 321 (EN X6CrNiTi 18-10) with chemical composition in wt. %: 0.075% C; 18.20% Cr; 10.85% Ni; 0.98 Si; 1.82% Mn; 0.042% P; 0.012% S; 0.52% Ti and Fe the rest. After milling, the samples surface was ground and treated by laser. The surface treatment was realized by continuous wave (CW) CO 2 laser (initial power N=1.2 kW, wave length λ=10.6 μm). Single pass was performed in the middle of the sample’s 30mm side. Laser treatment was done with regimes shown in Table 1. They ensured melting of the surface layer to different depth by varying of the laser spot diameter d [cm] and scanning speed V [cm/s]. The technological parameters power density Ns [W/cm 2 ] and volume energy density Ev [J/cm 3 ] were calculated using formulas given in the works of Dikova et al. (2014) and (2015). The laser melted surfaces were ground and polished to different roughness grade. Prismatic samples with dimensions 10 mm x 30 mm x 10 mm were cut for electrochemical corrosion test. Before the test, all samples were polished for 15 min, washed and degreased. Electrochemical tests were performed at temperature 37°C in three types of corrosion media: Ringer’s solution, AS (Fusayama, Meyer) and AS with high acidity. A Ringer’s solution (pH 6.4) contained 9g/l NaCl, 0,42g/l KCl, 0,48g/l CaCl 2, 0,2g/l NaHCO 3 . A Fusayama – Meyer AS (pH 6.5 ) was with composition KCl (0,4 g/l), NaCl (0,4 g/l), CaCl 2 .2H 2 O (0,906 g/l), NaH 2 PO 4 .2H 2 O (0,690 g/l) Na 2 S.9H 2 O (0,005) g/),Urea (1 g/l), as acetic acid was added for decreasing pH to 5.6. Two tests were performed for each corrosion medium: measurement of open circuit potentials (free potentials) Ef until reaching steady state potentials Ess and potentiodynamic anodic polarization. The external anodic polarization was carried out using a RADELKIS OH-405 potentiostat, to which a standard three-electrode cell was connected. The studied specimen was a working electrode, a saturated calomel electrode was a reference electrode, and a platinum electrode was used as a counter electrode. The potential was changed from -550 mV to +1250 mV at a rate of 1 mV/s in tests with Ringer’s solution and from -500 mV to +1000 mV at a rate of 1 mV/s in the tests with artificial saliva. All potential values were calculated against a normal hydrogen electrode (NHE). More detailed information can be found in the works of Dikova et al. (2014) and (2015). Surface morphology of all specimens before and after galvanic corrosion test was observed using optical microscopes (OM) Olympus SZ51 and XJL-17A equipped with digital camera No. ТР6080000В. The samples’ surface and chemical composition were examined by a scanning electron microscope (SEM/FIB LYRA I XMU, TESCAN), equipped with EDX detector (Quantax 200, Bruker). The investigation of the microstructure and in-depth Table 1. Surface treatment of the samples. Sample’s number Regime parameters in treatment by CW CO 2 laser Surface finish d ( cm ) V ( cm/s ) Ns*10 3 ( W/cm 2 ) Ev*10 3 ( J/cm 3 ) 0 1 4 6 - - - - grinded grinded grinded Polished 0,4 0,3 0,3 0,3 0,5 0,6 9,5 31,7 34,0 28,3 17,0 17,0