BS ISO 22910:2020 pdf download – Corrosion of metals and alloys — Measurement of the electrochemical critical localized corrosion temperature (E-CLCT) for Ti alloys fabricated via the additive manufacturing method
7 Test solutions For the test, 250 g of reagent-grade sodium chloride (NaCl) is dissolved in 750 ml of reagent water, producing an aqueous solution containing 25 % NaCl by mass. The ratio of the volume of solution in the test cell to the specimen area shall be at least 100 ml/cm 2 .
8 Test specimens Any specimen geometry compatible with the specimen holder may be used. Two different test surfaces of the specimen are used, which are made by the layers perpendicular to or parallel to the stacking direction, depending on the stacking direction via AM. A minimum test volume of 10 mm × 10 mm × 1 mm or higher shall be used. A less than 20 % ratio of edge area to total area is desirable. 9 Procedure 9.1 Preparation of reference electrodes The difference in potential between the reference electrode and two other validation electrodes shall be measured. If the potential difference is greater than 3 mV, the test electrodes shall be rejected.
9.2 Preparation of Ti alloy specimen The specimen shall be prepared to ensure a reproducible surface finish. The specimen shall be cleaned immediately prior to immersion in the solution by degreasing, rinsing in high-purity water, followed by ethanol or a similar solvent, and air drying. After degreasing, care shall be taken not to contaminate the test surface of the specimen.
9.3 Preparation of solution The solution shall be prepared using reagent-grade chemicals and reagent-grade water purified to 20 µS/cm by distillation equivalent to type I grade (see ISO 3696).
9.4 Setting up the E-CLCT test The exposed surface area of the specimen shall be measured. Two different test surfaces of the specimen are prepared, comprising layers perpendicular to or parallel to the stacking direction, depending on the stacking direction via AM. The specimen, counter electrode and salt bridge shall be placed in the test cell. The test cell shall be filled with the solution. It is important to ensure that the specimen is immersed and exposed in the solution. The salt bridge is filled with test solution and is free of air bubbles. The solution shall be stirred continuously throughout the test by bubbling an inert gas through the solution at a controlled rate.
The electrodes shall be connected to the potentiostat and the data-recording device. The connections for temperature measurement and control shall be made. The open-circuit potential of the test specimen shall be recorded and the desired anodic potential shall be applied to the specimen. The recommended applied potential for Ti alloys (e.g. Ti-6Al-4V) in the concentrated sodium chloride (25 %) solution is 2,8 V with respect to the saturated calomel electrode (SCE) (25 °C). If uncertainty exists concerning whether 2,8 V SCE is sufficiently high to obtain the potential independent E-CLCT, a test at 2,9 V with respect to SCE (25 °C) may be performed. A significant deviation between the E-CLCT obtained at 2,8 V SCE and 2,9 V SCE indicates the need for re-evaluation and new choice of potential (for information, refer to Annex A). Following application of potential for 60 s or longer, the specimen temperature shall be increased at a controlled rate.
The current and solution temperature shall be monitored throughout the test.
The E-CLCT is defined as the temperature at which a sharp increase in current density occurs during the temperature ramp at 1 °C/min.
9.5 Ending test
The test shall be terminated when the E-CLCT is determined.
The specimen shall be removed from the solution and rinsed in water, cleaned with ethanol, rinsed with high-purity water, cleaned with ethanol or a similar solvent, and dried in air.
The specimen shall be inspected using an optical microscope to determine pitting and crevice corrosion.
The ending current density may be higher than 1 mA/cm 2 (e.g. 5 mA/cm 2 ) if the surface is evaluated after testing.
10 Evaluation of test results
The E-CLCT shall not be compared with values obtained using a different procedure. The E-CLCT of AM Ti alloys is specific to the test method used and should only be used as a comparative measure of performance.
Localized corrosion is generally of random nature, and thus the number of specimens required for experimental condition is preferably greater than three considering data scatter and analysis.