New high entropy alloys with good corrosion resistance in severe environment are receiving increasing attention. This work reports upon the microstructure and the corrosion resistance of the non-equiatomic Fe₃₆Mn₂₀Ni₂₀Cr₁₆Al₅Si₃ alloy in different acidic solutions. This alloy was designed by thermodynamic calculations using CALPHAD SOFTWARE, fabricated through casting, subjected to cold-rolling and solution-treatment, and compared with SS304 stainless steel. The corrosion test was performed through electrochemical behavior in 0.6 M NaCl and 0.6 M NaCl with 0.5 M H₂SO₄ and 0.6 M NaCl with 1 M H₂SO₄ solutions. Experimental results indicate that the alloy is composed of FCC phase as the main constituent besides a small amount of other BCC/B2 phases and other intermetallics. The corrosion test measurements revealed that cold-rolled Fe₃₆Mn₂₀Ni₂₀Cr₁₆Al₅Si₃ alloy is more resistant to corrosion in 0.6 M NaCl, while it is more susceptible to localized pits in H₂SO₄ to 0.6 M NaCl. Experimental results indicate that the pits are preferentially occurred in the areas of BCC/B2 phase precipitates. The solution-treated Fe₃₆Mn₂₀Ni₂₀Cr₁₆Al₅Si₃ HEA has the highest corrosion resistance compared to others with the addition of H₂SO₄ to 0.6 M NaCl. Surface morphologies of the different conditions were studied, and relevant results were reported.