Chapter 3 Section (6) Diffusible Hydrogen: Low Hydrogen Electrodes and Weld Metal Hydrogen Content

Diffusible hydrogen designation: The addition of the AC indicates compliance with diffusible hydrogen test requirements for weld metal from low hydrogen electrodes (as received or conditioned), with an average value not exceeding $z\,\text{ml}$ of $H_2$ per $100\,\text{grams}$ of deposited metal, where $z$ is $2$ , $4$ , $8$ , or $16$ .
Suffix designators (e.g., e fifty five eighteen, e eighty eighteen twenty h c r):
- 20: Optional designator for a moisture-resistant electrode.
- HC: Designates the amount of diffusible hydrogen in $\text{ml} / 100\,\text{grams}$ of deposited metal.
- R letter-number combination: Indicates the chemical composition of the deposited weld metal.
Coating thickness depends on the specific type of electrode.

Copper fixture: Used according to AWSA 4.3 to hold the test piece during welding to determine weld metal diffusible hydrogen.
Gas chromatography: The modern testing method used to eliminate the inherent hazards of mercury ( $Hg$ ).
Testing process: Four test samples are cleaned individually and placed into a tube under an inert atmosphere, typically argon gas, at high temperature for a determined time.

Moisture impact: Humidity resulting from the manufacturing process has a more harmful effect on diffusible hydrogen levels than absorbed humidity.
Atmospheric moisture: Moisture in the air at the time of welding directly influences diffusible hydrogen levels, even when using very dry covered electrodes.
Handling containers: Plastic containers equipped with a rubber gasket seal reduce the rate of moisture absorption during handling, but are unheated and do not serve as a substitute for heated storage.