The primary method for determining the diffusible hydrogen in a test sample involves allowing hydrogen to evolve by diffusion at room temperature. The hydrogen is collected over a liquid in which it will not dissolve, usually mercury. The recommended practice is to leave the sample until the volume of hydrogen collected does not increase on successive days. The hydrogen evolved, together with the test piece weight, are used to calculate a volume of diffusible hydrogen (ml at standard temperature and pressure) per 100g deposited metal (ml/100g). Sometimes a value of hydrogen in fused metal is calculated.

Collection of diffusible hydrogen over mercury until there is no more hydrogen evolved can take a few weeks, so rapid methods have been developed to determine the diffusible hydrogen in less time. Such methods inevitably involve the evolution of hydrogen from the test sample at a temperature above room temperature, and using a different measurement method, as mercury cannot be used at temperatures above 50°C. The use of mercury is increasingly becoming restricted, and so the primary method is currently under review.

Rapid methods

Rapid methods for determining the diffusible hydrogen use temperatures from 45°C to 400°C, and methods such as vacuum hot extraction, carrier gas methods and gas chromatography are used. Although some less strongly trapped hydrogen is driven off at elevated temperature, methods employing temperatures up to 400°C have been shown to give results very close to values determined by the primary (mercury) method, for a number of ferritic steel weldmetals.

• Vacuum hot extraction
  Vacuum hot extraction involves heating the specimen, in a vacuum-pumped system, to the required temperature, to allow hydrogen to evolve quickly. The evolved hydrogen is transferred to the analysis volume, along with other gases emitted from the sample or furnace tube. Condensable gases are removed via cold traps, and the pressure of the gases is monitored until hydrogen evolution stops. The hydrogen is removed from the analysis volume through a palladium/silver osmosis tube, and the pressure of the residual gases is measured. The difference in pressure is the partial pressure of the hydrogen extracted from the sample. Typical times at varying temperatures for the evolution of diffusible hydrogen are as follows:

• Vacuum hot extraction is also used to determine the residual hydrogen, or the total hydrogen of the sample. The method is the same as for diffusible hydrogen measurement, but the temperature is 650°C, to free all trapped hydrogen. The residual hydrogen measurements are taken when the diffusible hydrogen has been determined by another method, or at a lower temperature. Total hydrogen is determined when 650°C is used throughout testing, and takes approximately 30 minutes.
• Carrier gas extraction
  Carrier gas extraction uses an inert carrier gas (such as Argon), and a detector. The detector output traces a peak on a recorder. The area of the peak is then proportional to the volume of hydrogen that has evolved from the sample.
• Gas chromatography
  Gas chromatography generally uses dedicated equipment, and hydrogen analysing-only units are available. The output can be in a graphical or numerical print out, and the diffusible hydrogen content can be determined.

Reference: TWI

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