EN plating for 25 year seawater service

Theoretically, the corrosion of 10-1/2% phosphorous EN is almost zero immersed in sea water, but 25 years is a lot. I don't know of any lab test or experiment that can predict over such a long term. Besides corrosion, there will be erosion issues to deal with and those depend on water velocity, presence of sand, adhesion of foreign unknown matter just to name a few.

Oil field and subsea components are typically plated with 75 microns of a high phosphorous type (>11%P) EN. However, it is unlikely that you will obtain a "perfect" coating without any pores, that will provide complete protection for 25 years, especially on free machining steel. You should expect that small pores in the coating will result in some rust spots. Usually, in seawater the pores become plugged with rust and stop growing after a few months. Electroless nickel coated components have been used successfully for periods greater than 10 years in seawater.

Wow, you are asking for what I would think was close to impossible and then you want to make it out of the cheapest material possible. Leaded steel is begging for a plating problem and occasional failure! To stand any chance of lasting 25 years it would probably be a high P EN with a minimum of 0.002 per side. I suspect that you are also going to have a galvanic couple which means that something is going to corrode over a long period. More gloom, there is no accelerated testing that will have any direct relationship to how long it will last in your use. Accelerated testing is a quality control test and not a functional use test.