Galvanic Corrosion Potential

Yes, there would probably be severe corrosion because you have a fairly large voltage between the metals, and a conductive "electrolyte". Since you can't change the electrolyte, you should try to electrically insulate the 2 metals. If you can't do that, then apply a good epoxy coating to the high-alloy side, near the connection. You will then greatly reduce the galvanic couple between them, because you are coating the cathode (protected) side. Any defects in the coating won't mean very much, whereas if you coat the iron side (anode) any pinholes will be severely attacked.

Many thanks for the help....one further question...the components being utilised will be installed into a water injection well where the electrolyte will be de-oxygenated seawater. I presume that with the reduction of oxygen there should be a corresponding reduction in the corrosion rate?..........

You cannot insulate the carbon (highly noble) from the iron in the cast iron. I've seen cast iron reduced to honeycombed graphite in marine use. Consider cathodic protection.

Lack of oxygen will help reduce the corr. rate, and possibly reduce it to the point the compoments will last a long time. It certainly is worth running some tests to find out, if you have the time.

I meant to use the word "isolate" rather than "insulate", but I think you understood. Stevenal has a good suggestion about cathodic protection. The more wetted surface area you can coat, the lower the required amperage for protection will be.

Also, be on the lookout for MIC caused by anerobic bacteria, esp. SRB's. They are REAL nasty.

I've always heard that the conductivity of stainless steels was such that stainless didn't generate galvanic couples very readily as the noble material. Was this an oversimplification, or does the added alloy content of the superduplex material render it more of a problem?

The current is way too small to be affected by low metal cond.

What counts is the surface condition-active or passive. Passive surface SS (a thin Cr oxide layer) is VERY noble to grey cast iron-at least to the iron in it.

moltenmetal,
Based on practical experience S/S can and does hookup with any thing in seawater. It maight take a little longer to start but it will.