Hi Poolownerinnyc!
Small world! I'm originally from NYC (born in Manhattan but, lived most of the time in Queens) also and currently reside in Pittsburgh, PA because I needed a liver transplant over six years ago and have been living here since...
Anywho, you're getting some good advice here from Bill A about the corrosion problems you're experiencing... As Bill A mentioned, Cathodic protection is definitely the way to go with your application if you're main concern is to minimize the amount of maintenance required for your pools. I would also agree with Bill A's recommendation to use a 309 or a 312 stainless filler for the welds... The complexity of the variables involved with your situation, does'nt enable you to come up with a simple fix to your problem and I can only guess that you're looking for a one time permanent solution to this yearly maintenance problem...
After re-reading your situation I've got a few questions but I'll ask this one first; Is the rest of the underside (also making contact with the ground?) mild steel surface of the pool uncoated completely (I hope not!) or is it uncoated just within the immediate area where the drain is located? Is there a moisture barrier of some sort that you are aware of underneath the pool also? I only ask this because, I want to make sure that you do'nt have a bigger problem on the horizon and eliminate that possibility in my head...
Also, may I ask where in NYC are these pools located?
I'm going to visit the "Big Apple" soon so if you'd like, I could take a closer look at your situation when I'm in town... You can find my e-mail by clicking my blue colored forum name: "ssbn727" and then I could give you more info on how to contact me by phone. I look foward to hearing from you soon.
"REMEMBER 9-11-01 MOST OF ALL - NEVER FORGET!!!"
Respectfully,
SSBN727 Run Silent... Run Deep!!!