An Examination of Collecting Mediums for the Determination of Diffusible Hydrogen in Welded Metal

Abstract

Glycerin is generally used as a collecting medium for the measurement of diffusible hydrogen in welded metal in japan, because the glycerin is defined by JIS Z 3113. However, it has been recognized that the measured values of diffusible hydrogen are different by a sort of collecting mediums such as glycerin, liquid paraffin, some alcohols and mercury.
In order to make clear the cause of this difference, the necessary conditions as a collecting medium and the behaviors of hydrogen bubbles in the collecting medium were discussed.
(1) A comparison of the amount of diffusible hydrogen by using the glycerin and mercury methods has been made and the amount of hydrogen by glycerin method showed always less values of about 50% than the mercury method.
(2) It was observed that small sizes of bubbles could not rise to the liquid surface within the difined time and suspended in the glycerin medium. As be understood from the Stokes's equation, the rate of rising a gas bubble in mediums is mainly related to the viscosity of medium and the size of gas bubble. The difference of viscosity between glycerin and mercury is remarkable, therfore the rate of rising a bubble in the mercury is calculated as about 10⁴ times greater than that in the glycerin.
(3) An apparatus being able to collect the suspending bubbles in the glycerin medium were developed. From the use of this apparatus, it was found that the total amount of hydrogen (including the suspending bubbles) in the glycerin method agreed well with the result of mercury method. It is concluded that the glycerin method is not satisfied for the determination of diffusible hydrogen in the welded metals.