We moved into our home just over 3 years ago. The other night we had a large rainstorm (10 inches on 24 hours). I noticed a leak in one room. Climb up on the roof and notice the lead vent collar around the PVC pipe was disintegrating allow water to get down. I think it is a good thing we didn't seal it as the problem would have probably moved to the outer OSB or leaked into the foam. We had a roofer come out and replace the 2 on the roof with a plastic/rubber version that should cure the problem. The roofer said he has never seem the lead collars disintegrate like that. The only thing he could guess was there was some chemical reaction caused by the foam.

Anybody have any experience with that?

Thanks,
Bruce
Tyler, TX

Bruce,
By your posting, I am quessing you have a SIPS roof system.
You did not mention how long the collars had been in place.
At any rate, it is most unlikely that the lead failure was due to anything associated with the structure of your roof, and certainly not the foam in the SIPS. (Do you think a piece of lead would dissolve in a styrofoam coffee cup?)
The two far more likely causes would be 1: Contaminates in the lead that the boot was made of. Or 2: Some type of environmental contaminate (read: acid rain). If you have no major air pollution to deal with, the most likely cause would be poor quality lead.
Also, be aware that the lower quality rubber collars can start to crack out in 5-10 years, so routine inspections are a good thing.

Bruce;

I doubt there was any reaction to the foam , a properly installed lead boot should not be in contact with the foam anyway.

You probably just got a bad boot from the get-go or it was damages during the install, I have been using lead boots for 40 years on conventional and SIP buildings and prefer them to the rubber that are known to degrade  from exposure to UV rays

Wes;

had not considered the acid rain;  you may be right about that

Yes SIPs roof panels. The lead collars were installed originally. So they are about 3.5 years old. The foam doesn't touch the collar but the foam isn't sealed completely around the vent and hole thru OSB. Roofer has many years experience and never seen thus issue in this area, so I discounted any enviornmental factors. There has to be something specific to our roof. Only difference between our roof and the norm around here is the foam. There does appear to be some chemical reaction. The lead collar is literally crumbling and in 3 years that is just unacceptable. I didn't want to replace them every couple of years.

Yes SIPs roof panels. The lead collars were installed originally. So they are about 3.5 years old. The foam doesn't touch the collar but the foam isn't sealed completely around the vent and hole thru OSB. Roofer has many years experience and never seen thus issue in this area, so I discounted any enviornmental factors. There has to be something specific to our roof. Only difference between our roof and the norm around here is the foam. There does appear to be some chemical reaction. The lead collar is literally crumbling and in 3 years that is just unacceptable. I didn't want to replace them every couple of years.

What is the roofing material made of ? shingles or metal?

SQUIRRELS!!!!

Elk shingles.

Roofer mentioned squirrels but he said there weren't bite marks. Definitely disintegrating.

If I could figure out how to post a pic I would.

I have Elk on my roof too with lead boots (9 years old) squirrels have not eaten the lead boots yet ( I have seen it on other jobs) but they have eaten everything else, ridge cap, rakes edge, steel flashings and other shingles right down to the SIP deck trying to find a way in

We have lots of squirrels but never seen them on my roof.

Guess it has nothing to do with the SIPs. And I'll have to watch the new caps closely as the 2 vents are the only holes in the SIPs.

We don't know any specifics about the SIPS panels.
The foam itself definitely would not be a source of the problem.
HOWEVER, some manufacturors treat their panels with various chemicals to protect against mold, etc. I wonder if your panels were treated with some chemical that could have outgassed enough to cause a reaction with the lead.  The borate solution that is, I think, the most common mold inhibitor used in SIPS panels, does have sodium ions in it.  Sodium will react with lead, however the borate solution is very stable, and unlikely to release sodium ions unless there is substantial quantities of water vapor available to react with the solution.   If possible, check the underside of the SIPS panels at the penetrations, make sure that these penetrations are sealed with expanding foam to stop any warm, moist air from migrating up into the panels. 
One more point to consider: lead is very reactive with ammonium nitrate (fertilizer). If they were exposed to this prior to or after their installation, that could be the reason for their deterioration.

There is no way that the lead collars are disintegrating due to environmental factors. Lead lasts a hundred years at least even with acid rain and such. Your problem is an animal of some sort. It is often very difficult to see bite marks and the damage can look like corrosion but lead has a sweet taste and many small animals love to consume it. Rubber seals can still be eaten away but they get attacked by the sun as well. Lead is much better. To protect it from critters you can surround the flashing with some chicken wire.

I agree that it is an animal, besides squirrels ............................ rats/ mice, oppossum and racoon can cause havoc with roofing and flashings

I have had EXACTLY the same issue. Lead has disintegrated into a white almost chaulk powder. Only refence I have come across is "lead bloom" which was noted in lead ship models enclosed in glass for muesum displays. the displays used a wood (particle board) base. There is reason to believe that wood of the SIPS panels may have outgassed and caused a reaction with the lead. I believe what occured in my situation was doing the final construction we got hit by a major thuderstorm right before the house was roofed.. The SIPS got good and wet and then we sealed the house up. Now that the house (12 years old) has had time to breathe and normalize the humidity , I believe the situation is no longer an issue. There needs to be more research on this. P. Ry