Surface lead removal from steel 3

[kaefaux]

I have a large steel structure (the surface of concern is ASTM A572) that has been tempered in an oven at ~1200 deg F. Accidentally, a considerable amount of lead was left on the surface before the heat treatment. The lead subsequently melted and contaminated the surface of the steel (non-contact test show areas as high as 2% by weight of lead). Materials science is not my background (nor that of my coworkers) and I was hoping someone here would have experience in this area. We have tried to grind/sand the area but the numbers are still higher than we'd like.

Is there a process/service I could employ to remove the lead from the contaminated areas? Any leads? Thanks in advance.

 

[swall]

Why do you feel the need to remove the lead?

 

[kaefaux]

Requirements from our customer; we feel it's safe after our attempt at removal but they are requesting further work to be done.

 

[swall]

What type of analysis will assure the customer the lead has been removed? You need to be in complete agreement on this. Are you sure it was only lead and not a lead tin alloy, like solder? How much leeway do you have to remove surface material?

 

[kaefaux]

Thanks for the quick responses. We'll do the same non-contact test (I'm not sure what process it is, we brought in a third-party steel mill's equipment to check this). While I'm not certain what the lead contamination threshold needed to satisfy the customer is, I can say it is lower than it is now.

It was a lead weight (the kind you would use as a sinker while fishing) not solder. I can't speak to the composition of it, but my understanding is it was primarily made of lead. Quantitatively I don't know how much surface material we could remove. We would, however, want to minimize that amount as best we can.

In my preliminary research I've found hobbyists using a 50/50 mix of household hydrogen peroxide and vinegar to remove lead deposits from steel barrels of guns. They let it sit for 2-3 minutes, rinse it with water and then use a stiff brush to remove it... would this be a possible solution? My ignorance is showing, but it seems it oxidizes (?) the metal deposits, allowing them to be easily removed with the brush. It may cause small amounts of rusting on the surrounding steel surface but they may acceptable given that the lead is mostly removed. Can anyone speak to that?

 

[swall]

With lead deposits in gun barrels, you have lead that is mechanically adherent. With melted on lead, you have wetted the surface (like with soldering). With the gun barrels, the chemical probably works by finding voids in the lead deposit and then working from underneath. With a wetted layer of lead, you probably would not be able to achieve the same result. If I were in your position, I would want to know the thickness of the lead layer and if there has been any penetration into the base metal. This could be accomplished by running an experiment melting some of the same type lead weights on some of the same steel plate for the time and temperature cycle of the original stress relief. A metallurgical lab could then perform a microscopic analysis of a cross section and provide lead layer characteristics. Once you have that information, you can then evaluate mechanical and chemical removal methods or possibly electrochemical removal methods.

 

[redpicker]

You can go ahead and try the vinigar/peroxide solution, it may solve your problem. I've seen is used as 2 parts white vinegar to 1 part hydrogen peroxide (3%) for a contact time of 3 to 5 minutes, followed by fresh water rinse. It shouldn't harm the steel; but it may slightly discolor the surface.

I would also look into methods for analyzing lead content, particularly for the method used ("third party's steel-mill's equipment isn't very helpful). Some non-contact methods can be very misleading in their results. Maybe you should re-check the result using a different method?

rp

 

[TomDOT]

As an aside - the liquid waste from the vinegar/peroxide treatment should be primarily lead acetate, which is fairly toxic. Dispose properly.

 

[swall]

Following up on what TomDOT wrote--the waste from ANY lead removal process, be it blasting or chemical pickling will have to be properly handled and disposed of.

 

[kaefaux]

Thanks all. Redpicker: I was thinking the same thing about re-checking the results. We weren't thrilled with the method but weren't sure what else to do on a time crunch bearing in mind we couldn't do any destructive sample-taking.

 

[mfgenggear]

Kaefaux

I believe very important advice has been given.
Must do testing as given above. to be sure complete removal.

HTH
Mfgenggear

 

[TomDOT]

Portable XRF should be a fast, easy, accurate method. There are quite a few environmental firms who will rent them out. Just make sure they rent you one which is set up for lead measurement.

 

[mfgenggear]

Tom Dot

Very nice. Have you applied an XRF?
what is the accuracy?

Thanks
Mfgenggear

 

[TomDOT]

I have an older unit we use for screening structures for lead paint. They're pretty darn accurate. Since they rely on excitation of specific energy electrons in lead using a radioactive source: the more lead you have, the faster your result.

Results are typically mg/cm2.

 

[metman]

If you use swall's suggestion of ...running an experiment melting...

you might want to try grit blasting to remove the lead but I would not try that on the actual part without prior testing because it might just make the Lead more adherent.

Design for RELIABILITY, manufacturability, and maintainability

 

[TomDOT]

I'm dubious about mechanical removal methods because lead is so soft. I know it is very difficult to cleanly remove zinc from steel mechanically - it tends to smear around.

From a health and safety standpoint, you will need containment and collection of the dust plus supplied air for your blasters, a decon area, et cetera if you try to grit blast the lead off. I know when lead paint is blasted off of a bridge, there are a host of lead-safe practices and training required.

Unless the OP has prior experience with blasting lead, this is a rabbit hole I would not go down.

 

[Maui]

Since I haven't seen any images of this part geometry I don't know if this would work, but could you simply heat the surface of the structure locally to melt the lead that is present and position the surface so that the liquid could be easily collected or removed? Using a material that the lead would readily wet could be a way to "sponge" it up.

Maui

http://www.EngineeringMetallurgy.com