National Physical Laboratory

Validation of oxygen purge techniques for stress corrosion cracking tests.

Hesketh, J, Cooling, P, Hinds, G
NPL Report MAT 71, January 2015
NPL Doc. Ref:
PDB: 7451 | DDB: 7122
Document Type:
NPL Report

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The effectiveness of standard laboratory oxygen purge techniques used to prepare deaerated solutions for stress corrosion cracking tests has been assessed under a range of purging conditions. The rate of decrease of dissolved oxygen concentration in deionised water was observed to depend more strongly on the depth of the sparging outlet below the waterline than on the volume of the void space or the area of the solution/gas phase interface. This is ascribed to faster gas exchange at the bubble/solution interface than at the waterline/gas phase interface. Furthermore, in concentrated NaCl solution the deaeration rate was found to be retarded by around 20% compared to that in deionised water. The standard NPL pre-test procedure for overnight deaeration of 10 L and 20 L test solutions was demonstrated to be fully effective in reducing dissolved oxygen levels to below 10 ppb, the value specified by EFC 17 for the testing of corrosion resistant alloys in the presence of hydrogen sulphide. In contrast, the widely accepted procedure of sparging a solution with nitrogen at 0.1 L/min for 1 hour per litre of solution was shown to be inadequate (i) in concentrated NaCl solutions and (ii) in vessels with relatively low solution height, including standard culture vessels of up to 5 L in volume and vessels that are only partially filled. Validation of individual deaeration procedures via measurement of dissolved oxygen concentration is recommended to ensure best practice.

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