A comparative study on corrosion resistance of 316 L stainless‐steel welds in nutrient fertilizer solutions such as Ca(NO₃)₂, KNO₃, KH₂PO₄

Electrochemical polarization and electrochemical impedance spectroscopy (EIS) tests were performed to evaluate corrosion resistance of 316 L weld metal in different nutrient solutions at the temperature of 20°C. Both hot‐rolled 316 L plates with dimensions of 200 mm × 100 mm × 8 mm were welded together by manual metal arc welding with an E316L‐16(A022) electrode. Welding was performed using direct current electrode negative (DCEN). A welding current of 88 ~ 115A, arc voltage of 22 ~ 23 V, and welding speed of 120 ~ 160 mm/min were used to manufacture specimens, while inter‐pass temperatures were maintained below 60°C. Potentiodynamic polarization measurements were taken at a scan rate of 10 mV/s at a potential initiated at −600 to +1600 mV. EIS measurements were performed at corrosion potential value by employing a signal with amplitude of 10 mV in a frequency range of 100 mHz–100 kHz. Observation of the optical microstructure of parent material revealed the equiaxed austenite grain, delta ferrite stringers, and micro‐carbides distributed along cold rolling in the inner region. Weld metal zones consisted of an austenite/skeletal and spherical delta ferrite structure. The polarization test showed that corrosion potentials of the weld metal (−0.377 V, −0.268 V, −0.350 V, respectively, in 45 wt.% KH 2 PO 4 , 45 wt.% KNO 3 , and 45 wt.% Ca(NO 3 ) 2 ) were clearly lower than those of the parent material (−0.233, −0.199, and −0.015 V) due to the heterogeneity of the microstructure in the welds. EIS data indicate that the corrosion mechanism is under diffusion control in the 45 wt.% KNO 3 solution, and under charge transfer control in the 45 wt.% KH 2 PO 4 and 45 wt.% Ca(NO 3 ) 2 solutions. The results showed that there was need for corrosion protection by inhibitors in the 45 wt.% KH 2 PO 4 and 45 wt.% Ca(NO 3 ) 2 solutions.