This Corrosion
Spent yesterday over at Making Space in Havant messing with electrochemical etching. I've done a moderate amount of marking of steel, but somehow have never got around to doing any deeper etches (which is daft really, given the kit I've got
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Ferric chloride is effectively formed at the anode, or (to be a bit more accurate) as the iron is etched, ferric ions and chloride ions are formed and go into solution. At the cathode, sodium cannot be released, as (if it were formed) it would react with water to give sodium hydroxide and hydrogen, so sodium hydroxide and hydrogen are what get released from the cathode in practice. As more sodium hydroxide is formed, the solution gets more alkaline. Ferric ions meet hydroxide ions and you get insoluble 'ferric hydroxide' which is of rather indefinite composition, a mixture of hydroxide and hydrated ferric oxide.....basically the same as rust!
>I wasn't entirely comfortable with the suggestion of just tipping the result down the
>sink to dispose; I'm pretty much agin tipping copper solutions too.
Given the quantities of metal actually being dissolved, I'd not worry too much. ....not, of course, that I would suggest putting chemicals down the sink, especially in a public forum, but the quantities involved are tiny!
>Do you have any pointers on the hard ground mix? I could just try stopping-out
> fluid and see about cutting through that as a possibility.
Ummm. It was a long while ago. I think I got the recipe out of Untracht. Unfortunately here in exile I don't have Untracht available (or a shed etc!) to check. As I recall it was beeswax, rosin (colophony) and bitumen. I cannot remember if I used the semi-hard bitumen (builder's yard stuff) or if I used some of the Liberon rock-hard bitumen (gilsonite). It was thinned with white spirit. The result was rather 'heavy', tarry stuff. I put it on thick. The main problem (once dried) is it's working temperature range. Too cold, and it flakes like nail varnish, too warm and it smears and does not come easily off the metal surface. It is best used under a desk lamp, to warm it up a bit. The hypodermic needle (green or white is best) can be used either like a knife (scribing outlines) or on its side to lift or scrape. I was digging through a fairly thick layer, and was etching deeply, so fine detail was neither intended nor achieved. Line broadening due to undercutting was a major problem, but I was etching of the order of 0.5 to 1mm deep.
Consider this example:-
https://8210293225661075208-a-1802744773732722657-s-sites.googlegroups.com/site/subpetran/home/cabinet/chisel.jpg
The design (a G in a circle) was etched, then I over-filled it with a mix of copper, tin, and a gold earring, then rubbed it down. Unfortunatly I rubbed too hard, and erased some of the design. The etching wasn't deep enough. I slow-rust browned the background for the hell of it.
Now consider this example:-
https://8210293225661075208-a-1802744773732722657-s-sites.googlegroups.com/site/subpetran/home/cabinet/doublesight.jpg
This (mild steel) was etched, the etching filled with lead-free solder, rubbed down flush, the steel slow-rust browned and a final ultra light coating of linseed applied, sealing the browning and giving the brownish gold look to the inlay. This time, I made sure the etch was deep enough BUT at the cost of considerable line-broadening. Note the 'GH 2005'. It was reasonably sharp when scribed, but spread so it looks like it was drawn with a very thick marker pen. Note the corners of the abstract design; they are also very rounded.
I used commercial etching ground on the Bat Cleaver.
It was from these people
http://www.lawrence.co.uk/acatalog/Etching_Grounds___Varnish.html
This stuff was thinner than the home brew, and easier to get a thin but decent coating with. It was much easier to get thin lines and sharp corners. My heavy home brew was used for stopping out. Here, however,I was simply etching an intaglio design, and it did not need to be hugely deep. I suspect (not tried!) that were this material tried for really deep etching, the it would ben more likely to lift at the edges due to undercutting than my heavy stuff.
I'd not recommend stopping-out varnish as a substute for etching ground. Its not intended to be scraped through cleanly like hard ground. The recipes for it I have seen were basically just bitumen in a solvent, and the exact properties would depend on the grade of bitumen.
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(sorry!)
So what is inhibiting the reaction over time? Obviously there's chlorine being offgassed; does the raise of pH adversely affect the cell or is it something else? And at what point do the reaction products include sodium chlorate & hypochlorite? ISTR it needing to be fairly alkaline before they are produced, but my memory may be hazy on that.
Yeah, you're right about the quantities involved going down the drain - I'm being irrationally over-cautious on that these days. The justification is usually "if everyone did it...", but since hardly anyone does these things, the argument falls flat on its face.
I should have checked Untracht before asking, shouldn't I?! In a related area - and apropos of very little - I've just had the suggestion of using molasses as a brightener in electroforming more-or-less corroborated; in Hardy's Jewelry Repair Manual, he talks about using corn syrup to improve the surface finish in plating baby shoes.
The ground I was using was applied by heating the plate, rubbing the ground on to deposit some, then using a roller to try and even it. Manky stuff - I've now pretty much given up on traditional pitch for forming, so I'm not overjoyed to be reaquainted with it, I guess. The stopping-out varnish that Cookson's sell is actually Mylands Black Polish, which as it's shellac-based, I can see being a swine to scratch through cleanly.
Some nice examples there - intrigued by the sight mount; I've never used cap & ball, so the use of a micrometer rear sight seems slightly incongruous to me - is the accuracy enough to benefit from it? With inlays, I'm more inclined to think in terms of mechanical than heat - so undercut the edges, maybe raise some stitches on the lowered area and then lock the inlay in place. Down to the tools available more than anything else, I suppose.
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Presumably it is buildup of crud (ie rust and possibly some Fe3O4) on the surface being etched. Neither are great conductors. The more alkaline the liquid, the more favourable the formation of hydroxides.
>Obviously there's chlorine being offgassed
There shouldnt be. The idea of the etching is that the chlorine being formed at the anode attacks the steel (oxidizing iron metal, Fe(0) to Ferric ion, Fe3+) and gets reduced back to chloride ions in solution. If you are actually getting significant quantities of chlorine gassed off, that is chlorine wasted and everything near it getting corroded. I'd suggest that this would be due to having the current too high. The anode (workpiece) shouldn't really gas at all. Overall, the cell is turning iron into hydrated iron oxide (rust) and hydrogen. Chloride is not consumed. You can regard it as a catalyst.
>And at what point do the reaction products include sodium chlorate & hypochlorite?
I'm not sure, its a complicated reaction and I'm not an electrochemist. If chlorine is being gassed off, some of it will dissolve (the more alkaline, the more so) and you start geting the oxy-ions of chlorine. You'd start getting these immediately if you started with inert (eg carbon) electrodes in salt water. With a steel anode, the steel gets oxidized rather than the chloride.
Note that under mildly acid conditions, where the iron stays in solution rather than dropping out as rusty stuff, you still get crud. Iron deposits (very badly!) on the cathode, hence there is a good reason for bagging it If the solution is sufficiently acid, it iron won't deposit and you'd only get hydrogen.
>molasses as a brightener
>corn syrup to improve the surface finish
Both contain glucose, classic brightener especially in nickel plating.
>Mylands Black Polish...shellac-based
Definitely likely to chip if you scratch it.
>I've never used cap & ball
Look closer, that's a Snider! Thoroughly modern breechloading rifle ever since it was converted sometime in the 1860s!
>so the use of a micrometer rear sight seems slightly incongruous to me
everyone at my shooting club would agree with you....but they thought the peep sight on the Brown Bess was silly too.
>is the accuracy enough to benefit from it?
It means I can hit the target which I couldn't with the open sights. Once you have bullets that fit and an appropriate load (a lot of effort and an excuse for contrapting various special tools!) the Snider is a not-inaccurate rifle and great fun to fire.
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Hmm, didn't know that was a known brightener - Googling tends to overload with proprietary compounds for electroforming. Fine if I'm doing anything commercial, but for just having a play I don't really see the need to go fully tech on it.
Oops. Mea culpa! The oldest I've used were the No4's; I'd fixated on the hammer mechanism and overlooked the breech. If the sight improves accuracy, then that is all that matters really! Although I have to admit to a fondness for more modern optics and overpriced image intensifiers.
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>I have to admit to a fondness for more modern optics and overpriced image intensifiers
Anyone remotely fond of anything optical doesnt put it near a black-powder gun!
These things belch out smoke, fouling and dirty lube!
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