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Broken Drill Bit in Inconel Tube

Broken Drill Bit in Inconel Tube

We've got a small piece of high speed steel drill bit twisted off inside an Inconel 600 thermowell (a horizontally oriented blind hole about 1/4" diameter and 8" deep, with the drill bit between about 4" and 4.5" from the open end).  The geometry, location and size of the well and bit etc. are such that removing the bit by EDM or drilling out the bit are pretty much infeasible.  The well is welded into a vessel and removal/replacement of the well is a major deal.

I was thinking of corroding the broken piece of bit out of the well over a period of several weeks, which is feasible.  I was thinking that nitric acid, say 7% concentration, would be a good choice.  Is my choice of corrodent a good one?  Any other suggestions for speedier corrodents with no additional risk to the Inconel?  Anything else to watch out for?

I'll be doing a beaker test before I attempt the real thing.

RE: Broken Drill Bit in Inconel Tube


The well is welded into a vessel and removal/replacement of the well is a major deal.

Why? Normally, thermowells are either threaded and seal welded to a boss (fitting) or are threaded into the vessel wall and seal welded or are partial penetration welded into the vessel wall. We have replaced a number of these over the years w/o problems.

RE: Broken Drill Bit in Inconel Tube

Moltenmetal,  I,m not a materials guy so dont rely on this comment but we once set up a  rig test - - Two blocks of steel in a bucket of brine with a 6 volt battery charger wired to the blocks - - within a week the -ve block had almost vanished    Possibly we had done serious damage to the +ve block but can you accelerate the destruction of your drill bit by anything similar.
Let us know how you finally solve the problem.

RE: Broken Drill Bit in Inconel Tube

Here is an idea for you.  Take the rest of the broken bit that you have, grind a point on it so it looks like a single sided chisel.  Heat the thermowell up and run the modified bit into the hole  running counter clock wise to try to disloge it.


RE: Broken Drill Bit in Inconel Tube

The well is full-pen welded through the wall of the vessel.  So yes, removing the well is a big deal, especially with the vessel in place.

The drill bit is 1/4", so tanged drill extractors etc are not feasible.  Drilling has proven a risky waste of time.  

My beaker test with 10% nitric is showing promise- attack on the bit with no attack on the inconel to speak of.  But it'll be slow if we go this route.  Surprisingly (to me at least) the attack on the bit seems far faster with 10% than with 35% acid, and the green colour of the corrosion film suggests Fe(II) rather than Fe(III).  But I suspect dilute sulphuric would be faster still.  Anybody got NACE tables at hand for dilute sulphuric corrosion rates on Inconel 600?  Or any other faster corrodents for high speed steel at room temperature which will leave Inconel unscathed?

RE: Broken Drill Bit in Inconel Tube

According to corrosion data from Special Metals for Alloy 600;

in dilute (5%) sulfuric acid, test duration 100 hrs, at 65 deg F, 2.4 mpy
in dilute (10%) " ", NR, test duration 24 hrs, 4.2 mpy

RE: Broken Drill Bit in Inconel Tube

Thanks metengr: might be worth another beaker test.

RE: Broken Drill Bit in Inconel Tube

As the acid reacts with the drill bit it create s an inert product on the drill bit.   If you can keep removing this product so that you can react fresh acid with fresh steel the process will go faster.   

Sorry, I don’t know much about this specific situation but when I have to corrode material I find that a combination of acids can work well.  E.G. Sulfuric and Nitric

You might try posting in the Practical Machinist forums.   Lots of on hands experience there.


Thomas J. Walz
Carbide Processors, Inc.

RE: Broken Drill Bit in Inconel Tube

tomwalz:  yep, there's a loose non-adherent greenish film that forms on the bit, probably containing Fe(II) by its colour.  As I said, I'm surprised that nitrate doesn't oxidize Fe(II) to Fe(III), but you learn new things every day!  Merely swishing the acid around in the beaker removes it.  In our situation we'd flush with fresh dilute acid daily and let it sit overnight.

Mixed nitric/sulphuric fumes with NO2, doesn't it?  Definitely an aggressive corrodent but VERY nasty to work with!

RE: Broken Drill Bit in Inconel Tube

Iron salts are not green. Nickel salts are green.

RE: Broken Drill Bit in Inconel Tube


Good catch.

Mostly I etch ceramics so I can braze them with a torch and standard alloys.  I guess I have gotten in the habit of thinking of HF as dangerous and anything else as less dangerous.  (Probably not a good habit.)

Not sure how relevant the following is but it might help.    Occasionally we have to remove braze alloy (50% Silver, Copper, Zinc, with maybe 5% Nickel) from a batch of parts.   We use about 1/3 Nitric, 1/3 Sulfuric and 1/3 water.     

Initially the reaction is vigorous with the nasty green smoke.  After a while that slows down because a layer has formed on the parts.   This seems to be a relatively soft layer because we can reach into the solution with a glass rod, scrape the surface and see the reaction accelerate.   

After a while the scraping doesn’t seem to work.  Ina batch of 500 parts there may be 100 parts that only have the braze alloy partially removed.  Putting in fresh acid doesn’t seem to do anything.  We usually end up grinding these last few parts.  No idea why the original reaction quits working.     

This is a rework process for us that we use only when we really need the parts and the replacement time is too long so we never really studied it much.

Thomas J. Walz
Carbide Processors, Inc.

RE: Broken Drill Bit in Inconel Tube

compositepro:  beg to differ:  iron (II) salts are usually pale green, whereas iron (III) salts are usually yellow to brown.  Nickel(II) salts and their solutions are often a screaming grass green too, but there is no evidence of attack on my Inconel (72% Ni) components in my beaer tests. The NACE data I've seen would suggest the same- I600 should be very resistant to room temperature nitric acid up to about 70% concentration.

tomwalz:  I'm pleased to hear that your carbide parts stand up to nitrating acid (the mixture you mentioned).  Are your carbides nickel-cemented or cobalt-cemented?

RE: Broken Drill Bit in Inconel Tube

Results after 3 days testing:  10% nitric acid, agitated once daily:  drill bit mass loss 20%, Inconel tube mass loss 0%.

RE: Broken Drill Bit in Inconel Tube

think it would go faster if you applied a bit of constant vibration? (If thats really necessary -- at 1/4" dia, and once you loose 20% of the mass you may be able to use a neodyium magnet on a stick)

RE: Broken Drill Bit in Inconel Tube

NickE:  with my luck, I'd be stuck with a glass-hard, non-corrodable neodynium magnet stuck in my thermowell!  Nope- I'll corrode it until I can flush it out.

RE: Broken Drill Bit in Inconel Tube


Congratulations on successful etching.  

RE: Carbide and corrosion

Right now I think there are eleven factors that contribute to carbide wear.  We deal with a lot of WC / Co carbide.  However the focus is more and more on advanced grades.  

We work with TiCN (and similar) with NiCr binders.  We also work with Alumina for saws and tools.  

Dr. Rudy of Sintex (Pacific Hardmetals) did some great things with nickel matrix materials but, since his passing several years ago, they are no longer reliably available.    

We now use several techniques to make advanced carbide grades that are much more corrosion resistant.  Nano grains seem to be much more resistant to corrosion simply because they make much less binder material available for attack.  Binders are much more complex than simple elemental Co.   We eliminate a great deal of micro fracturing and macro fracturing through better mixing so we eliminate or reduce binder pools, lakes, etc.  In brazed applications we have the opportunity to use braze alloys that provide a suspension like effect as well as simply joining which greatly reduce or eliminates breakage.  


Thomas J. Walz
Carbide Processors, Inc.

RE: Broken Drill Bit in Inconel Tube

Inconel 600 has just barely enough chromium (14-17%) to protect it from nitric acid. Dissolved iron does become Fe(+3) in nitric, which accelerates the reaction. The green color is at least partly Ni(+2), but that should decrease with fresh solution as the Inconel passivates to a Cr2O3-rich surface.

moltenmetal, how about a final report on this?

RE: Broken Drill Bit in Inconel Tube

kenvlach:  we've done the beaker test and the results are in my previous post.  We had no mass loss on the inconel tube in the 7% nitric with 20% mass loss on the drill bit.  We're' just setting up to do this for real over the next week or so.

RE: Broken Drill Bit in Inconel Tube

How did it go?  This is fascinating.

RE: Broken Drill Bit in Inconel Tube


Turns out that etching in the beaker was far more effective than in the thermowell itself- three weeks of exposure didn't even loosen it perceptibly.  It seems we DID get an adherent scale after a time which inhibited further corrosion, and I was too chicken to go the nitrating acid route.  So we picked away at it with carbide tooling until the pieces were small enough to blow out.

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