3 Experimental Procedure and Analytical Techniques

3 Experimental Procedure and Analytical Techniques

3.1 Material and Sample Preparation

The various procedures described in this section were imperative for the preparation of the samples. An extensive amount of surface finishing was undertaken to ensure that where necessary, any possible mechanical damages present on the surface did not interfere with the composition or structure of the oxides formed. Scratch and deformation free surfaces are essential in most materialographic specimen preparations. With modern high technological materials the requirements for puirty, cleanliness and equal distribution of alloying elemenets and phases are very important. For this particular investigation, the effect of deformities or a difference in chemical composition on the surface - due to incorrect preparation, presents serious issues such as it may impact the PWSCC characteristics and oxidation during testing.

3.1.1 Solution Annealing

The samples were subjected to thermal treatment in a furnace heated to 1100 °C for a time period of 30 minutes, before being water quenched. This caused the dissolution of carbides along with the redistribution of chromium – in order to obtain a supersaturated carbon solid solution which served to facilitate the internal oxidation investigation at the later stages.

3.1.2 Mechanical Grinding & Polishing

The samples were cut into squares with a specified size of 20mm x 20mm x 2mm, in order to ensure precision - the cutting process was carried out using an abrasive wheel cutting machine. 

The cut samples were mounted in a bakelite resin using a hydraulically operated press. Once mounted, the sample surfaces were prepared through a grinding and polishing process. The grinding phase involved the samples being progressively grinded with silicon carbide papers at 240, 360, 600, 800, 1200, 2400, 4000 grit. Before moving to the next paper, samples were examined then rotated at a 90 degrees angle and grinded again. This was done to more accurately determine the removal of previous mechanical damage and scratches. Grinding took an approximation of 2-3 minutes per paper and the samples were washed with water before the commencement of grinding with finer papers. The P600 samples were grinded up until the 600 grit paper was reached- whereas the OPS samples were grinded to completion at 4000 grit. The reason for the difference in surface finishing was to provide a distinction between a deformed layer and a stress-free surface during testing.

After the samples had been sufficiently grinded they were then polished using a rotary polisher and diamond paste of 3um and 1 um. After each polishing step the specimens were carefully washed with soap, rinsed with water and ethanol and then put under a hot air dryer. The specimens were then observed under an optical light microscope, to confirm that scratches of the appropriate length were removed – this process was repeated until the desired surface was achieved.

3.1.3 OPS Polishing

The final polishing step involved the use of active oxide polishing to provide the best possible surface finish (scratch and deformation free). OPS is a collodial silica suspension (white) with a pH of 9.8 and a grain size of around 0.04um, it was diluted with water at a 1:4 ratio – to prevent the surface of the alloy from being affected by its alkalinity. OPS increases chemical activity hence shortening polishing times, they also have excellent stability making them highly economical. Each specimen was polished for at least half an hour and then washed with soap, water and then ethanol before being dried. The use of OPS was neccesssary because mechanical grinding as a stand alone, is not sufficient to create a completely deformation free surface – OPS works through both chemical and mechanical action [1]. Once the OPS polishing was concluded the samples were then removed from the bakelite resin and then ultrasonically cleaned in deonized water to remove possible collodial contaminants.  

 3.2 Oxidation Tests

The samples were subjected to oxidation tests using a horizontal oxidation rig, for which the schematics are shown in Figure 1 below. The rig was contained in a fume cupboard during periods of operation as a safety measure. The conditions in the rig – with temperatures of 480°C and pressures of 1 bar, were setup to create a similar environment to that found in the primary circuit of a PWR. The oxidising potential of the system was controlled by using valves to alter the flow rate of hydrogen and steam. The samples were held in a double-walled reaction tube made out of quartz situated inside the furnace.

The rig operates using deionized water which gets pumped with a HLPC peristaltic pump into a pre-heated area, where water gets vaporised into steam which then mixes with hydrogen to form hydrogenated steam. The hydrogen and steam mixture then travels into the double walled reaction tube where it undergoes heating. The heated hydrogenated steam was then directed into the inner tube where the samples were located. The hydrogenated steam coming out of the tube was then condensed, thus allowing the hydrogen gas to escape through a pipe to be released into the fume cupboard.  Hydrogen flux was controlled using a flow meter, whilst the flow of steam was calculated through steam charts and knowing the amount of water pumped into the system by the peristaltic pump. Being able to vary hydrogen flux and also control the pump settings made it possible to adjust the ratio of hydrogen present in the steam – allowing the partial pressure of oxygen to be determined.

[pic 1]

3.2.1 Start-up and shut down process

The steps below serve to outline the testing procedures used for the horizontal rig:

Starting conditions and checks:

• First, the samples were put in a quartz boat which was then loaded into a reaction tube – with the surfaces of interest facing up, and placed in the furnace.
• All the joints were tightened with spanners to in order to prevent leakages, as an additional measure the system was purged with hydrogen for 1 hour.
• The furnace was then heated to the desired temperature of 480°C to prevent pre-oxidation
• The hydrogen flux was set to the appropriate levels using the flow-meter, whilst the peristaltic pump was turned on and the water inlet valves opened to control the water flow.
• The water from the pump (which was heated to steam), was then mixed with the hydrogen and inserted into the reaction tube – beginning the eperiment
• Once the exposure time had been reached, the furnace and the pump were turned off, however the hydrogen flux was increased to prevent any further oxidation processes to occur during cooling. The samples were left inside the tube for 24 hours before being extracted for observation.

3.2.2 Technical Limitations

The water was introduced to the systems by the peristaltic pump through drops - meaning its flow rate oscillated with time. This had an effect on hydrogenated steam, as the prominence of hydrogen would have varied throughout the testing. As a result of this the oxygen partial pressure would also have oscillated, so the values of partial pressure used are simply the averages over the duration of the experiment.