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As the carburised layer is not visible after polishing, the samples have to undergo an electrochemical etching process for the layer to be analysed. The electrochemical etching corrodes the surface of the samples, with the grain boundaries corroding faster than the grains; thus making the grain boundaries stand out. The chemical composition of the material also affects the rate of corrosion so the carburised layer will corrode at a different rate to the substrate material and the nickel plate layer. This allows the carburised layer to stand out and be analysed.

 

The electrochemical etching process begins with filling a large beaker with enough deionised water to submerge the sample. It is important to use deionised water as the ions and bacteria in tap water could contaminate the material. Oxalic acid weighing 10% of the deionised waters mass is then added to the solution. The oxalic acid is in powder form therefore the solution is heated and stirred with magnetic stirrer to aide in dissolving the acid.

 

Once the solution had turned clear, the electrochemical etching process was ready to begin. A small piece of platinum was attached to the negative pole of a direct current source and was submerged into the solution, acting as a cathode. One at a time the mounted samples were placed face up in the solution and were connected to the positive pole of the direct current source for 10 to 15 seconds at 8V. Once the samples were cleaned off, they were observed under a microscope to make sure that the grain boundaries and the carburised layer were visible. If they were not visible then the process was repeated.