I live on the water in r.i. (salt water cove). My primary sump is in a basin that is large, under the basement's concrete floor, and comes on only when the tide is in enough to raise the water level to 4 to 5 inches below the concrete floor. This depends on the tide cylce but the pump comes on probably for 15 days a month and cycles every 5-10 minutes or so while the tide is in (a few hours per day when the tide is high). The pit enpties in 15-20 seconds until the next cycle. This system works fine and would keep the basement dry except for 5 or 10 days a year when the tide is extremely high and there is substantial rain.
I also have a second sytem of 12" perforated pipe buried outside about 10 feet underground encircling the foundation with crushed stone for a foot below and above which connects to a standpipe (12" diameter) into which I hang a second sump pump for those times that heavy rain and a high tide challenge the capacity of the sump pump in the floor.
With both pumps operating the basement has been dry for over five years.
My question is how to get the most life from the pump in the stand pipe. The brackish water rusts everything eventually and I have been getting 4-5 years performance from the standpipe sump pumps. I recently replaced a rusted, nonfunctioning standpipe pump with the cast iron Zoeller 0.3 HP pump. I have two choices: (1) hang the pump at the bottom of the stand pipe and let it get wet every day and "assist" the in-floor pump or (2) hang it 18" or so higher so that it only kicks in when the water table is very high". This would be infrequent, maybe a day or three per month. The in floor pump handles most days just fine.
Would hanging the standpipe pump higher so that it is usually hanging in air and not pumping on a daily basis extend the life of the pump with respect to rust damage? Or not. Remember the water is brackish (it tastes salty and represents sea water seeping into the water table near the coastline). I know you need water, iron and air to support rust. In the hanging position the water would see the pump only occasionally. In the "deep" location it would see water daily. Which situation, is either, would reduce the rusting process and extend the life of the hanging pump?
Seems intuitive that hanging the pump in air would slow the corrosion rate.
That said, have you thought about a sacrificial zinc anode? These are widely employed in marine applications for protection of metal continuously exposed to salt water.