Moisture in the flux, or electrode coating, is the principal source of hydrogen in weld metal produced by flux-based arc welding processes.
There is, however, no simple universal relationship between flux/electrode coating moisture content and weld metal hydrogen content. Although some American practices consider the measurement of moisture in the coating to be a direct replacement for diffusible hydrogen determination, it is important to consider the type of electrode coating/flux being used. With no regard to the type of coating, the comparison of diffusible hydrogen with moisture level results in a data set with no visible trends, as in Fig.1. For example, one typical limit of moisture content is that 0.2% moisture corresponds to Scale D (<5ml/100g) hydrogen, and it is evident from Fig.1 that this cannot be guaranteed.
Fig.1. Correlation between weld metal diffusible hydrogen and coating moisture content. (Results refer to several coating formulations and determination of moisture content utilising different drying temperatures and varying raw materials) [1]
However, the correlation between flux moisture content and weld metal diffusible hydrogen is greatly improved when the type of coating is taken into account. Furthermore, moisture levels can give a useful measure (again considering coating type) of likely hydrogen levels due to exposure to damp conditions ( Fig.2).
Fig.2. Relationship between re-absorbed moisture and hydrogen level for an E7016 and an E7028 electrode (4mm electrodes re-dried for 1 hour at 450°C then subjected to differing atmospheric conditions to achieve coating moisture level).
Reference
1.McKeown, D: 'Hydrogen and its control in weld metal' Metal Construction 1985 17 (10) 655-661
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