Amidine inhibitors for brass in 1 M HNO3 were synthesized, and the electrochemical behaviours were investigated by different electrochemical techniques under flow conditions. This type of inhibitor can effectively overcome the fluid impact in flowing HNO3 solutions for at least 24 h. The amidine inhibitors caused mainly cathodic curves to low current density and significantly strengthened the impedance under flow conditions. The relationship between concentration, flow conditions, and inhibition effects was investigated by weak electrochemical polarization curves, electrochemical impedance spectroscopy, and scanning electron microscopy. Adsorption thermodynamics indicated that all of the inhibitors function via chemisorption processes with Gibbs free energies of approximately -40 kJ/mol. A modified adsorption model was constructed based on the results of electrochemical experiments to explain why higher concentrations resulted in worsened inhibition effects due to fluid impact. The adsorption mechanism was identified by X-ray photoelectron spectroscopy, ATR-FTIR spectroscopy, and quantum chemical calculations. All of the chemistry analyses indicated that the amidine and heteroatoms contributed to surface adsorption.