I hope some of our resident "metal-allergists" will respond, when I put up a thread on polishing brass, it was mentioned that ammonia can cause cracks to develop in brass items. That information has opened more discussion on ammonia and brass among some friends, I think it would be interesting to know what experts can tell us.
Well, all I know is, it's actually the copper in brass that ammonia has a chemical reaction with.
I'm not a metallurgist. Never even played one on TV. But my old Aircraft Materials And Processes textbook (Titterton, 1956) contains reference to "Season Cracking" as a problem with some brasses and bronzes.
When I look on Google for season cracking, here is one of the entries that I found:
Cracking in brass caused by ammonia attack
Brass is susceptible to stress corrosion cracking, especially from ammonia or substances containing or releasing ammonia. The problem is sometimes known as season cracking after it was first discovered in brass cartridges used for rifle ammunition during the 1920s in the British Indian Army. The problem was caused by high residual stresses from cold forming of the cases during manufacture, together with chemical attack from traces of ammonia in the atmosphere. The cartridges were stored in stables and the ammonia concentration rose during the hot summer months, thus initiating brittle cracks. The problem was resolved by annealing the cases, and storing the cartridges elsewhere."
My guess is you'll polish a million times before you see cracks forming from exposure to the ammonia in your brasso. Just be careful storing your brass lamps in a horse stable though.
I find this interesting because the failure of brass items due to cracking is something I've been aware of for many decades, not just car parts, but any number of brass items with some age on them.
Since most if not all of the sheet brass items which have succumbed to cracks were formed by spinning, drawing or stamping, it was easy to conclude that the stresses and/or work-hardening inherent in them was to blame. One could presume that vibration in automotive applications also played a part.
if corrosion were a prime cause, then why do we often find brass items that were nickel plated also cracking ?
I rather think that the example given of ammunition stored in proximity to a horse stable doesn't have a lot to do with age cracks in brass auto lamps, any more than polishing with Brasso will destroy the brass on your Model T.
I posted the information below before in other threads regarding ammonia (some of it repeats what Dick Fischer posted above):
Many people think that Brasso is very bad for brass because it contains ammonia. However, I found out that many folks in the musical instrument business - hobbyist and professionals alike - have no problem with it. (Folks who clean brass clock works also seem to have no problem with using ammonia.) The concentration of ammonia is small and it is used for only a short period of time.
I found out much of the information regarding ammonia and brass fracturing goes back to when the British made the mistake of storing ammunition in horse stables. Long term exposure to the ammonia in the horse urine caused the brass cartridge cases to crack, especially where they were crimped on the bullet. Long term exposure to high concentrations of ammonia can cause season cracking, especially if the brass has already been stressed.
I've come to the conclusion that this is not a concern when using low concentrations of ammonia for short periods of time, especially when I read the arguments for and against on the internet. Also, many of the folks who are adamantly against using Brasso will be surprised if they ever research the ingredients of their favorite brass polish which actually also may include ammonia.
I've used Semichrome polish for 25 years and it contains traces of ammonia- you can smell it. I don't recall any cracking on my brass. Hopefully someone more knowledgeable will chime in here at some point.
Regarding Brasso. I had an old can from several years ago and it worked pretty good. The new can I bought is pretty much useless in comparison.
I only use the cracking as an excuse when I get lazy and my brass needs some polishing, like now.
I have made some covers to keep the horse pee off.
Gene's comment echoes what I've posted before: The current Brasso is not the same formula as the Brasso we used in the army fifty years ago. I dislike the current version intensely.
I think some are missing the concept that there are three essential components of brass cracking ...... stress, corrosive agent, and time.
Yes, ammonia exposure seems to be a factor in brass cracking. But ammonia alone is seldom a problem.
When some materials are placed under a high and constant load, they become far more vulnerable to corrosion at the points of highest stress. This is particularly true of aluminum and brass. Rather than seeing red rust or white powder on the entire surface of a part, all you see is a crack.
It's helpful to think of a piece of metal as made up like a poorly knitted sweater. Everybody has seen a knitted garment with some tight puckers and some loose loops. Now, imagine that we pull hard on the garment in an effort to stretch it. Where will it break first ? At the tight puckers, of course.
A cold formed metal part has tight spots and loose spots, just like the sweater. We just can't see them with the naked eye. That's why some metal parts have to be stress relieved before use.
Note that the google quotation said that cracking of the cartridges could be resolved by annealing. Annealing after forming is a form of stress relief.
So, why do we not see cracking in spite of using a polish that contains ammonia ? Maybe the part was stress relieved. Or maybe it simply wasn't formed to a degree severe enough to leave high stresses. Or maybe the short time exposure to ammonia while polishing wasn't enough to let the corrosion fully act.
But my point is that you have to consider the three elements of stress corrosion: Stress, Corrosive agent, and Time. Absent one of those three components, the cracking may never occur in our lifetimes.
Too many words. I know. Engineers are like that.
Seems like vibration would also be a factor,.....or would that be considered "stress"?
It's OK Dick, some of my best friends are engineers ( my son, too ! . . .) I think I'm understanding you to say that brass can be corroded or corroding without the corrosion being apparent ? and that corrosion would result in cracks at a stress point ?
That would seem to indicate keeping brass well polished would be advisable preventive maintenance ?
Any road, it sounds like the British Indian Army in the 20's had even more problems than failed ammunition . . . if ammonia concentrations in horse stalls were that extreme, their horses would certainly have been unsound - lung problems and bad feet from epidemics of thrush !
In my initial post I filed a disclaimer about not being a metallurgist. Sure wish a real metallurgy person would speak up.
But my understanding is that stress corrosion isn't the same stuff as surface corrosion. Stress corrosion implies that the material becomes more susceptible to corrosion in the presence of tension along the grain lines. The stress corrosion doesn't spread on the surface, but rather grows down into the material, following the line of greatest internal tension.
I think this would be an entirely different process from surface corrosion.
An instance of stress corrosion that I personally observed was in a forged aluminum wing beam. The beam had a machined pocket that cut through the forging parting line. Apparently the forging was not entirely stress relieved after forming. A crack grew right down the parting line while the rest of the beam showed no sign of surface corrosion at all. In that case, the crack line was ground out (wasn't too deep) and the area shot peened. Shot peening puts the surface of the material under residual compression rather than tension. Thus no new crack was likely to develop.
Dick, thank you for your reply, that answers my question, it would be nice if someone with the urge for metal would give us a bit more detail.
I guess it's all pretty much academic, unless you have control over fabrication, there's not much to be done to prevent it, or if cracks begin to appear in your T's brass parts.
What detail would you like a bit more of? I tend to get carried away and the cheap seats chirp when I try to explain things in general...ha ha
Have a specific question ? Throw it out and I'll try to answer.
What has been said above is a decent explanation and just to add to it...the culprit IS corrosion by definition in the sense that the base metal gets compromised with something else, you just don't see it because it is microscopic.
There has to be 3 things for those cracks to 'happen'
1- An environment that contains the active ingredient that wants to fill microscopic voids in the base metal (In this case Nitrogen; for Steel it is Hydrogen; for Austenitic Stainless it is Chlorides; for Aluminum it is Chlorides also)
2- A relatively high stress level (Cold working without stress-relief)
3- A material that when 'worked' makes a receptive host. (Tougher to explain but the grain structure originally formed up tight tend to become opened microscopic Swiss-cheese as the material is being worked)
2 outta 3 doesn't count, you have to have ALL three and even then the cracking is still a maybe.
Keep in mind that 100 + years ago, they probably did not think to anneal out stresses as a general course, because stress release can cause warp, and the temp needed to stress-relieve would also draw out the hardness.
Thanks for jumping in, George.
I was starting to have that queasy feeling that comes with telling somebody more than I know.
While on the topic of brass, are there any books that have been written on the topic of how to restore antique in areas such as annealing , soldering and restoration in general ? Whenever I go into any library or bookstore I have had practically zero luck in this regard.
George, thank you for you post. I guess the question most of us would have would be, is corrosion likely to have already infiltrated stress points in antique items, or is it possible to reduce, or forestall the development of corrosion in our brass car parts ? I'm also curious as to what kinds of environmental exposure are likely to develop cracks, which kinda brings us back to the ammonia in Brasso . . . ?
I think you did a fine job of explaining it.
I'm not aware of a book as such. There may be for musical instrument restoration. The general issue with brass and why people say an amateur can not do shade tree annealing is that there is really only about a 10 degree temperature range where annealing actually happens. Under anneal and you never know it as it comes out the same as it started. Over anneal and it goes soft but just how soft is always a question. Thus the whole thing is a learned art and not a plug n play science.
Ditto on the soldering...there are those that swear by silver solder...others who swear by a 63/37 solder...and still others who swear AT 60/40 shelf solder.
First of all, think of the odds. I mentioned 3 things that have to happen at the same time. I didn't say the culprit was Ammonia, rather, said it was Nitrogen. So how does the Nitrogen come out of the Ammonia? Not so easy as something has to scavenge the Hydrogen at the same time. Second, the potential fissures are there already, they were the result of manufacture. Third, once actual stress corrosion cracks start, they tend to propagate quickly. Fourth, it may not be the Nitrogen from Ammonia that is/was the trigger...as long as the other chemical element 'fits' in the fissure in the metal, cracks happen, it is just that Nitrogen size and typical fissure space are most favorable.
Take all that, and realize also that stress corrosion cracking still haunts the brass industry at every turn in the road even today, and put on your 'don't worry about it' hat. I imagine that keeping tarnish away in the first place would be a good start as tarnish itself under the right circumstances can also become a trigger.
Put a piece of formed brass in a bath of Brasso in a high humidity area when you pull it out and you will probably have corrosion cracks within a week! They may be minor, but they qualify. Keep it polished or use the passive chemical covers and your odds of stress cracking happening are about the same as winning the lottery.
George, thank you VERY much for the informed and detailed response much appreciated !
Put a piece of formed brass in a bath of Brasso FOR A FEW DAYS in a high humidity area...