430FR stainless from Carpenter has a range of 0.25 - 0.40% Sulfur, which greatly improves machinability, but also severely impairs corrosion resistance.  The addition of Molybdenum up to the 0.50% maximum will somewhat offset the effect of Sulfur on corrosion resistance.  You should confirm that you are actually getting the Molybdenum addition, as it is optional, not required.

Carpenter has another free machining grade called Project 70 Stainless Type 182-FM, which has 1.5 - 2.5% Mo, and it is ferritic, so it will still be magnetic.  I would recommend evaluating this grade, and restrict the Sulfur content to be at the minimum of the specified range.

Now, if you do not require machinability, or do not mind reduced machinability, then you may consider other alloys like Type 434 or Carpenter Chrome Core® 29 Solenoid Quality Stainless.  You can find out more information on Carpenter's excellent website http://www.cartech.com .  In order to view all of the technical information, you must register, but it is free.

TVP gives good advice here. You may, however, want to not be saddled with Cartech's proprietary grades when this is a fairly straightforward situation.
Compared to a sulfur-impaired 430 ( i.e.430FR), try straight 430. The absense of added sulfur will help. Next go to a titanium-stabilized 430, i.e. 439. In this grade a pinch ( about 0.35%) of titaium ties up all the carbon, nitrogen and sulfur dramatically improving corrosion resistance. It performs nearly as well as 304 and about the same as 434 without the added cost of molybdenum.

mcguire,

Hey, what's the big idea, giving good advice that doesn't include using more expensive material (Mo-containing alloys) or more expensive material (Carpenter's proprietary alloys)?  

In all seriousness, I figured I would continue with the Carpenter stuff since artlaf mentioned their specific alloy.  Also, the relatively short time it took for corrosion to appear using 430FR made me err on the side of conservatism.  I knew you would have some excellent advice on this one.  

Thanks for the kudos.
   Mike McGuire

More info.  The material is used in the pressure section of a solenoid valve.  This  is a 3-piece assembly which is EB or laser and TIG welded.  I realize that free machining grades are not recommended for welding but we have been successfully doing this for better than 10 years and proof pressure check the welded assembly 100%.  Also, the parts require round bar stock and a search on grade 439 seems to indicate that it is available in flat or strip only since it appears to be primarily used on exhaust systems.  While searching I came across Sea-Cure (type 446 Stabilized with Columbium plus Titanium) which is supposed to have much improve corrosion resistance over 430 but I'm not sure about its machinability, cost, or availability.

The reason you're not supposed to weld 430, free machining or not,is that the cooling from austenitizing temperature
sensitizes the material and it must be annealed to  restore its already marginal corrosion resistance. Is this part of your corrosion problem?
 Forget Sea-Cure; it's very expensive and available only in flat stock.