This is the XDMF solution in transient that I get.
The problem is that your mesh is not in metres (probably in millimetres). See below:
In SALOME, you need to set the dimensions of your geometry to metres otherwise, FESTIM will understand that there is 2 metres of nickel to go through 
What happened here is that with the stepsize that you gave, the mesh wasn’t refined enough. But if you increase the stepsize dramatically and also increase the final time, you will see a correct diffusion behaviour.
Consider scaling your geometry in SALOME.