Abstract: A water-based ballpoint pen using a cemented carbide ball comprising tungsten carbide and a metal binder as a writing front end, wherein cobalt and/or nickel are contained in the cemented carbide ball as the metal binder in an amount of 1 to 20% by weight based on the total amount of the ball and which further includes an aqueous ink composition containing a colorant, water, a phosphate ester-based surfactant, and any one or more compounds of a thiadiazole represented by formula (1), 5-mercapto-3-phenyl-1,3,4-thiadiazole-2-thione or a salt thereof, and an oligomer of 2,5-dimercapto-1,3,4-thiadiazole: wherein X and Y each represents any substituent of CH3, NH2, and SM, and M represents any of hydrogen, an alkali metal, an ammonium, and an alkanolamine.

Abstract: To provide a water-based ballpoint pen with a good feel while writing, which hardly causes the proceeding of ball corrosion with time and can maintain an excellent lubrication performance at writing. A water-based ballpoint pen using a cemented carbide ball comprising tungsten carbide and a metal binder as a writing front end, wherein cobalt and/or nickel are contained in the cemented carbide ball as the metal binder in an amount of 1 to 20% by weight based on the total amount of the ball and which further includes an aqueous ink composition containing a colorant, water, a phosphate ester-based surfactant, and any one or more compounds of a thiadiazole represented by formula (1), 5-mercapto-3-phenyl-1,3,4-thiadiazole-2-thione or a salt thereof, and an oligomer of 2,5-dimercapto-1,3,4-thiadiazole: wherein X and Y each represents any substituent of CH3, NH2, and SM, and M represents any of hydrogen, an alkali metal, an ammonium, and an alkanolamine.