I assume that it is not located at the connecting point of the gating and/or risers - if so and in short this is caused by improper gating & risering removal - over entuziastic use of the grinder.
Another potential root cause is the inmold atmosphere exposure - air entrapment, combined w...
Very interesting answers you got here, the bushings that you currently use are they carbide and if so what kind, also high chrome ( Ni-Hard ) cast irons are only as good as the foundry producing them - w. other words the size of the microstructure components - if it is oversized ( poor foundry...
You should completely exclude the idea of formulas - unless you are individually heath treating using an automatized system induction and/or tube furnace type, in this case you maybe able to develop some empirical data and/or formulations - in real life there are multiple factors affecting...
Interesting how the depletion is so great at the flows stated...could it be that the sea H2O is aerated somewhere in the system and we are also dealing with an aggressive environment where the normal effects related to the difference of potential are enhanced by the excess oxidating agent...