This work aims to explain the mechanisms of weak bond formation in CFRP epoxy adhesive joints by studying aqueous NaCl mist contamination. First, a mechanism of epoxy ring consumption by Cl^－ ion was proposed to explain the reduction in covalent bond density at the adhesion interphase, as a function of pH alkaline drift. Second, double cantilever beam test (DCB test) was applied to CFRP joint samples with aqueous NaCl mist contamination at the adhesion interphase to prove a weak bond triggered by Cl^－ ion contamination. Third, contamination layer thickness was estimated by substituting the data into the drift function. Obtained nanometer-thin thickness indicates that the stirring flow of the adhesive dissipates the layer to form a standard joint, while steady flow keeps the layer to form a weak-bond joint. Therefore, 1) Cl^－ ion contamination at the adhesion interphase and 2) the handling in the adhesion process explain the statistical aspect of weak bond formation in the production line of CFRP adhesive joints.