Thanks every one.

Yes John, I had already built a basic model (with SUS only load) in both programmes and then carried out the manual calcs as well. CAESAR II and the other programme give quite different results when the model is run to B31.3 with the corrosion allowance, (although if I change the wall thickness in my CAESAR II model to the corroded thickness and then have 0 corrosion allowance then the results do agree.)

I know that the corrosion allowance is to be considered for B31.3 SUS stresses and I understand how both programmes calculate the Zeff value, so in the end it looks like it is coming down to interpretation of the code.