[jim80]

Hello,

I need to to remove (etch) 0.0001 inch per surface on an inconel part. Nevertheless the part shape is irregular which make it quite impossible for dimensional measurement.

I was thinking of using a coupon ( block shape) to be submerged in acid together with the remaining actual parts (irregular). I assume stock lost in the coupon equal to stock lost in the actual parts.

Need help to verify if my assumption is correct:
If coupon (big inconel block) and part (small inconel block) is place in the same container of acid, the thickness of inconel eroded by acid is the same ( A=B).
Please refer to attachement.

Thanks in advance for your attention
Jim80

[Arkcon]

That would depend on the term "irregular shape", do you mean it has a bumpy surface, like a little statuette, or just a block made of right angles?  And how deep?

P.S. what's your acid?  Inconel is notoriously difficult to attack, hence the name.  Back in the day, I was able to strip off the Inconel surface off of half of a first surface mirror, in a solution of ceric ammonium nitrate (conc. forgotten, srry) in 1 N nitric.  Quite an elegant solution to the problem, many people thought.  Worked in seconds, as well.

[jim80]

Hello Arkcon,

The part is actually a half cylinder made of right angle. In another words, we bend a strip with semi circular cross section. In addition we will have some chamfering done onto the part.

The part thickness is 0.13 inch ( measuring the strip thickness and not the bent neck ).

I am planning to use concoction consisting of:
Water
Sulphuric Acid
Hydrochloric Acid
Nitric Acid
Acetic Acid
Ferric Chloride

Instead of cracking my head to measure the actual parts, I was thinking of submerging some coupons ( block shape) and measure the material removed using micrometer and co-relate that with weight loss.

Hence, it is very critical that my assumption is correct. Please refer me to some related scientific principle.

Thank you so much for the reply,

Jim80

[Arkcon]

I don't know what scientific principle is at work here, but I know it's not a chemical principle.  In any case, the rate at which a square of Inconel is attacked, should apply the same to a closed cylinder, assuming the acid is stirred enough to prevent local depletion of the active. 

But your tolerances are so tight, I just can't be sure.  You can tell my application was more forgiving. 

As a side note, do you have a good reference supporting the use of your acid mix?  I had heard that some alloys may form a passivated coating when exposed to strong nitric, which prevent attack, even by other solution components.  It seems to me that Inconel would be a perfect candidate for passivating.  On the other hand, your mix may be so good at attacking Inconel components, a passive coating never forms.

[jim80]

Hello Arkcon,

Thanks for the reply.
Really appreciate your side note regarding passivating.

Need help to verify:
The rate at which a square of Inconel is attacked, should apply the same to a closed cylinder.

Question:
If different mass for square inconel and closed cylinder inconel submerged in the same acid. Is the mass loss rate proportionate to reaction rate?

In another words:
10 gram of square inconel
5 gram of closed cylinder inconel
Placed in the same acid.

Will it be:
Mass loss rate = 1 gram / hour
Hence after 1 hour, 
10 gram of square inconel would be left with 9 gram
5 gram of square inconel would be left with 4 gram

PS: the mass loss rate is hypothetical

Thanks in advance,

Jim80

[Arkcon]

Briefly,

Need help to verify:
The rate at which a square of Inconel is attacked, should apply the same to a closed cylinder.

is what I'm not sure of.  This might be an elementary concept for chemical engineers, and we do have a few on this board.  We'll have to wait and see when they jump in.