Pyrazine derivatives are efficient environmentally–safe chemicals for protecting metallic materials against corrosion during industrial acid cleaning. However, they have been mostly investigated for steel–based materials, which limits their versatility. Therefore, assessing their efficiency for protecting other important industrial alloys, like the Cu–Ni alloys employed in industrial heat exchangers, is also necessary. Herein, we investigated 2,3-pyrazine dicarboxylic acid (PDA), singly and mixed with potassium iodide (KI), as a promising inhibitor against the corrosion of 90Cu–10Ni and 70Cu–30Ni alloys in 2% HCl. Based on short–term weight loss, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) measurements, an optimum PDA concentration (1500 ppm) impacted average of 39% and 55% on 90Cu–10Ni and 70Cu–30Ni, respectively. These values, however, depreciated slightly after 72 h. Through cooperative synergism, the addition of 5 mM KI significantly boosted PDA efficiency to 73% (90Cu–10Ni)/92% (70Cu–30Ni) at room temperature, and 51% (90Cu–10Ni)/63% (70Cu–30Ni) at 60 °C. PDA is a mixed–type corrosion inhibitor with slightly more anodic impact on 70Cu–30Ni but cathodic effect on 90Cu–10Ni. This mechanism translates into more efficient surface microstructural protection for 70Cu–30Ni than 90Cu–10Ni, based on SEM characterization.