Metalurgy of brass in rosin - Page 1
Forum section: electronics / general

author: ConKbot
author_rank: Super Contributor
timestamp: November 17, 2015, 01:50:00 pm
content: I had a new to me occurrence happen the other day. I had some yellow brass parts which needed to be soldered after being machined. To ensure the surface was nice and clean, oxide free etc, I boiled them in RMA flux. After the rosin and parts had got to temperature (Rosin was smoking, a good bit, and solidified fully afterwards, definitely > 200C ) I pulled them out, cleaned off the rosin, and the parts were pure copper colored on the surface (not just more red, completely copper). Obviously the zinc was removed from the surface, leaving just copper. But I'm wondering if any metallurgy type folks have a better clue as to what happens. Like if the zinc is dissolved into the rosin? If it ends up going below the surface making a zinc poor layer over top of a zinc rich layer? Ive tried searching but can only find stuff about people soldering, or using zinc chloride flux.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 17, 2015, 02:10:48 pm
content: The zinc leaches out. It depends on the grade of brass but, under the right conditions, water will remove the zinc leaving a copper sponge-like material. Try looking-up: "brass de-zincification", particularly with regard to boiler making/fittings and model engineering.

author: Ian.M
author_rank: Super Contributor
timestamp: November 17, 2015, 02:25:48 pm
content: Hot rosin has free active Abietic acid, and is quite agressive. (seehttp://hackaday.com/2010/07/16/decapping-integrated-circuits-with-sap/)Normally its hot (and thus active) for a few seconds. You boiled the parts in ACTIVATED rosin, so there were more acids for far longer.I'd bet that it has preferentially etched the Zinc out of the surface leaving spongy copper. Hopefully its not too deep as spongy copper is about as weak as peanut brittle! As long as the tinning fills all the voids down to the base brass, it will be OK, but if the de-zinced layer is too thick, I would expect trapped oganozinc salts, and porosity.A similar effect can happen due to electrolytic action if dissimilar metals are in contact with brass in a wet environment. Vessels have sunk before now due to the wrong grade of brass being used for underwater fittings. (Arguably there is no right grade of brass for salt water use, but so-called DZR brass is better than most)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 17, 2015, 06:19:30 pm
content: Yes, dezincification is a known phenomenon.Presumably, it affects the top 100nm or so of the material, enough to become visible. (Light isn't perfectly reflected off the surface of metals, but interacts for some depth. The physics of this is because the electrons in the metal can't move instantaneously, and the light wave isn't a perfect wave itself, but is made of photons, which penetrate into the material as they pass, analogous to skin effect at low frequencies. The colors of copper, brass, gold and cesium are due to their electrons responding somewhat slowly, leading to absorbed rather than reflected light in the blue to UV range. All metals absorb somewhere in the UV, by the way -- they're just not "pink enough" to see in visible light! The penetration depth corresponding to this absorption or reflection is on the order of 100nm.)I don't know offhand if it leaves nanoporosity or anything -- brass is generally a solid solution of zinc in copper, so it wouldn't be dissolving whole crystals, leaving a microscopic sponge, but rather affecting the surface (and at least some of the bulk, given the optical result).In any case, dezincification is well known around acids, especially related to soldering, and to corrosion in corrosive or marine environments.Tim

author: ConKbot
author_rank: Super Contributor
timestamp: November 18, 2015, 01:54:46 pm
content: Cool, thanks for the information. The layer isn't too heavy, a light scratch reveals brass underneath. The rosin was only at its smoke point for about 30 seconds, and they went from somewhat grungy (old stock, darkened surface) to bright copper. I'll do some pulling twisting and prying testing to make sure they are soldering fine.Spongy brass sounds interesting, maybe Ill boil a piece of brass for longer, and then soak it in sn100 solder to see if I can convert the surface to bronze.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 18, 2015, 06:47:34 pm
content: Soldering produces a thin interface layer, so you will succeed -- just in a matter of micrometers rather than much visible, and going from Sn100 on the outside, to a series of ceramic-like intermetallics, down to about 20% where you finally get a workable "bronze", then ordinary brass/copper underneath the rest.The whole transition layer might be microns thick, unless it's driven deep with excess heating. Which is why tight solder joints can end up wholly seized if they are overheated (sometimes seen on burned components in PTH PCBs, or when adjusting/dismantling copper water pipes) -- the intermetallics grow into the solder area and cement the joint together!An electroplated layer of tin might be thin enough to see turning 'bronzey'. Though tin oxide tends to have that color initially as well, so you might not know.There's an old trick: rezincification! Take salt water, drop a penny in, add a mild acid (vinegar?), and drop some zinc granules on the penny. It turns silver, being electroplated by zinc! It gets better. Heat it to nearly the melting point of zinc (you'll know when that is, if it's a U.S. Cent dated >= 1982..) and notice the color change. Yup, the layer is thin enough that the zinc diffuses into the surface, forming a (little more than decorative -- probably 100s nm) layer of brass!On a related subject: colloquially, "bronze" is copper with tin, but technically, it's any copper alloy: a superset. The type of bronze is identified by its major alloying component, or assumed to be a combination of the most common elements paired with copper (namely, zinc, tin and lead: 85-5-5-5 is a very common red bronze with reasonable strength, excellent castability and excellent machinability). Brass is simply "zinc bronze", if you will. There are a number of more esoteric bronzes, like manganese bronze (strong; light color), aluminum bronze (quite strong, corrosion resistant, difficult to cast and machine), tellurium bronze (or 'copper' if lightly alloyed so that it is still pink; adds strength and machinability, preferred over lead for use in water fixtures, for obvious reasons) and so on.Copper is pretty versatile stuff, it's kind of too bad it's moderately expensive; though steel is cheaper, stronger, and even more versatile, so it's not a big deal.Tim