Black phosphating pre-blackening solution and preparation method thereof

The invention discloses a black phosphating pre-blackening solution and a preparation method thereof. The black phosphating pre-blackening solution comprises the following components in concentration by volume: 0.01-1g/L of bismuth nitrate, 5-25mL/L of aqueous solution of molybdate and triethanolamine, 8-10mL/L of 65% nitric acid, and the balance of water, wherein in the aqueous solution of molybdate and triethanolamine, the mass ratio of molybdate to triethanolamine to water is 1: 8: 11. The preparation method comprises the steps of dissolving bismuth nitrate into a small amount of water at normal temperature; then slowly adding a mixture of triethanolamine and molybdate, fully stirring, and adding 65% nitric acid; stirring uniformly and then adding water to 1L. The pre-blackening solution does not fade in subsequent phosphating through chemosmosis and can be treated at normal temperature, the treatment time is short, and the treatment effect is good.