heavy etching cast iron

I am interested in a chemical that will heavily etch, and even dissolve cast iron. Spent the afternoon searching the archives, and reading about Ferric
Chloride and other options that people have posted about when mild etching was required.
I have not tried Ferric, but I have tried Nital (4% nitric with ethanol), also Muriatic with a few oz of Lye and the results were not as aggressive for what I am looking for. The goal is to remove a few pounds(!) of material from the inside of a cast iron intake manifold over the period of around 48 hours. This is all in areas where tooling / blasting cannot reach. Alternatively, electro-etching is not an option either, as the manifold has many deep internal passages and twists.
I have built plates to block off the ports and fill the manifold, and also have an outdoor, fan ventilated area... along with the necc safety apparel. Looking for someone with experience in this area for guidance. This has been called acid porting in the past, and I am not interested in anyone's opinion on the benifits regarding increased engine output, just looking for some chemistry help,
TIA,
Kev
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KevinG wrote:

I think this is going to be a disaster. I have experience with Ferric Chloride, at least, as a copper etchant. Your problem is that you may initially get just what you are looking for, and then, 2 weeks later, you find the manifold in pieces, with big hunks missing. The trouble is getting the etchant (whatever you use) to STOP when it is done. Cast iron is notoriously porous, and the etchant will hide in the pores and do mischief over time. If you do try Ferric Chloride, you will need a very aggressive neutralizer to fully inert any remaining etchant hidden in pores of the metal. Alkalis are probably going to be good for that.
My other reason to suspect problems is the grain structure of cast Iron. It has a very loose grain structure, with lost of inclusions. Most etchants will follow grain boundaries, preferentially working BETWEEN the grains. This leads to faster etching, but very UNEVEN etching! In other words, the surfaces start off somewhat smooth, and become very jagged as the etching progresses. Instead of moving a wall back while preserving the general contour, it will turn the wall into swiss cheese, or moon mountains, or something like that. Maybe this will still get you what you want.
Jon
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Muriatic acid will be more aggressive if you don't mix lye with it. 8^).
Randy

cast
Ferric
has
apparel.
for
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If you're going to remove pounds of metal, you're going to need pounds of acid. The inside surface is going to look like the moon, too, high-carbon ferrous material will pit terribly. Mixed perchloric/phosphoric acid was what we used to desolve iron for chemical analysis, didn't care about surface finish with that.
I guess my question is, why? Edelbrock too high priced, or you think you've got a better idea?
Stan
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Randy - an unscientific (but theoretically accurate) test we did found that the muriatic alone would just give the cast iron a deep cleaning. We took a 100 gram chuck of cast iron and immersed it in a container with straight muriatic. Leaving it in 12 hour steps, we would remove it, then put it in a bath of baking soda, then place it on our .01 increment digital paint scale, measuring how much lighter the chuck would get over a 48 hr period.
Now I do not remember what the actual numbers were, but when doing the same test with a similar sized cast iron chunk, the lye mixed with the muriatic deteriorated the chunk alot quicker, some 30% if memory serves.
Not saying I know anything about chemistry, but this seemed to be a pretty valid test. Where did I go wrong?
--

Jon - Agreed... on the outside this may look like a disaster, but we did
plan on immersing in with some sort of neutralizer, maybe a 15 gallon
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KevinG wrote:

Lye is sodium hydroxide (NaOH), muriatic acid is hydrochloric acid (HCl). Mixing them gives salt (NaCl) and water and a lot of heat. IIRC, also hydrogen gas. But the point is that they neutralize each other. It's a mystery to me how the mix of the 2 of them could be more powerful.
Bob
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You'll do a lot better if you abrade the surfaces while the acid is there. One problem that often happens with corrosion processes is thatt the end result isn't soluable in the liquid and thus blocks the continuing action. In addition, cast iron being a mixture of a number of elements, will not erode evenly. I'll also note that the application of an electric current will make things proceed a lot faster.
-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works evevery time it is tried!
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Disclaimer! Use at your own risk! I've never tried these and I don't know if they'll even work, but maybe it'll give you some ideas for experimentation. I usually only read this book for amusement. These etching formulas for iron come from a 1939 book "Fortunes in Formulas" (Which explains the archaic chemical names);
(all parts by weight) 1/ Pure Nitric Acid with an equal weight of water (for deep etching) 2/ 1 part Tartaric Acid by weight, 15 parts Mercuric Chloride, 420 parts water, 16 to 20 drops nitric acid if one part equals 28.5 grains. 3/ 120 parts of 80% Spirit, 8 parts pure nitric acid, 1 part Silver Nitrate. 4/ 40 parts of 30% Pure Acetic Acid, 10 parts absolute Alchohol, 10 parts pure Nitric Acid. 5/ 10 parts fuming Nitric Acid, 50 parts 30% pure Acetic Acid. 6/ A Chromic Acid solution. 7/ 1 part Bromine, 100 parts Water. 8/ 1 part Mercuric Chloride, 30 parts Water. 9/ 1 part Antimonic Chloride, 6 parts Water, 6 parts Hydrochloric Acid.
They seem to have been fond of Nitric Acid...

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