Brass fitting corrosion
Brass fitting corrosion
(OP)
I have an corrosion mechanism in a machineable brass Swagelok fitting (copper,30%zinc,2.5% lead). The fitting has was in use 20+ years in a PH 10 steam environment with <10ppm of hydrazine (which decomposes to ammonia). The part failed at the first thread of the fitting. The fitting appears to be yellow brass colored on the outside with a pink, copper color on the inside. I initially thought it was dezincification however EDX analysis showed almost identical composition in both areas. The only difference was a small increase in oxygen in the copper colored area. The yellow brass area shows a normal microstructure after etching however the pink copper area shows a finely porous surface with no grain structure. If this is not dezincification, what else can it be?





RE: Brass fitting corrosion
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Plymouth Tube
RE: Brass fitting corrosion
RE: Brass fitting corrosion
No one got hurt and all fittings are being replaced with stainless.
RE: Brass fitting corrosion
RE: Brass fitting corrosion
I would tend to agree with the SCC as suggested by swall.
Some people might have heard that SCC was first observed in brass cartridges. During the civil war, some army units had stored brass cartridge ammunition in chicken coops. The ammonia from the chicken manure caused SCC failure of the brass. Modern cartridges now require annealed brass.
The pink copper color inside the fitting could be the remainder of thread sealant compound. Difficult to say what thread compound might have been used 20 years ago. If Teflon tape was used for thread sealant, then the steam temperatures might have been hot enough to decompose Teflon. Chlorine and fluorine from decomposed Teflon would be very aggressive on brass.
RE: Brass fitting corrosion
RE: Brass fitting corrosion
The way I understand it,SCC is a cracking mechanism. This copper colored material on the ID of the fitting is not SCC. It appears to be oxygen related corrosion. This copper colored area is a woody, slightly porous material with no grain structure. EDX dot maps show copper, zinc, and lead separated out in what appears to be flakes (seen at 8k magnification in the SEM). There is slightly less copper in the porous area but the major difference is that this area has more oxygen. This is unusual for a condenser where oxygen is kept low however this may be an area that has not been purged. This fitting has been in use 20+ years and currently all brass fittings are being replaced by stainless.
Teflon tape is not used as a sealant. EDX analysis did not show any fluorine or other unexpected contaminants. In any case, the tape would be on the outside of the threads. This copper colored corrosion is on the inside where it is exposed to the environment. The best I can come up with is that it's an oxygen corrosion mechanism.
RE: Brass fitting corrosion
Cu + O2 -> Copper oxides
then some time later
Cu oxide + N2H4 -> Cu(metal) + H2O + N2 (gas)
I'm neglecting the role of ammonia, but it pretty much is a side player here. Ammonia (well ammonia+water) will dissolve the oxides, and tend to wash the surface, and you would see something more like pitting corrosion; probably both reactions occur (otherwise you'd see growth of the surface towards the i.d. as the base metal becomes more porous or less dense).
Clear as mud? I probably am missing something too.
RE: Brass fitting corrosion
...figure one exposure every year, 20 years, perhaps .005 inch of corrosion progress per cycle...sound plausible?
RE: Brass fitting corrosion
Normally, I could not agree more with your posts. However, in this case not sure that I would agree. The zinc is still present as mentioned above. If you had copper remaining with no zinc, this would be dezincification. In this case, the OP has a layering effect with copper and zinc.
RE: Brass fitting corrosion
Dealloying
Dealloying is a corrosion process in which the more active constituent metal is selectively removed from an alloy, leaving behind a weak deposit of the more noble metal. Copper-zinc alloys containing more than 15% Zn are susceptible to a dealloying process called dezincification. Selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide in brass. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass by weak, porous copper that would eventually penetrate the brass, weakening it structurally and allowing liquids or gases to leak through the porous mass.
Plug-type dealloying refers to the dealloying that occurs in local areas; surrounding areas are usually unaffected or only slightly corroded. In uniform-layer dealloying, the active component of the alloy is leached out over a broad area of the surface. Dezincification is the usual form of corrosion for uninhibited brasses in prolonged contact with waters high in oxygen and carbon dioxide (CO2). It is frequently encountered with quiescent or slowly moving solutions. Slightly acidic water, low in salt content and at room temperature, is likely to produce uniform attack, but neutral or alkaline water, high in salt content and above room temperature, often produces plug-type attack.
Brasses with copper contents of 85% or more resist dezincification. Dezincification of brasses with two-phase structures is generally more severe, particularly if the second phase is continuous; it usually occurs in two stages: the high-zinc ? phase, followed by the lower-zinc ? phase.
RE: Brass fitting corrosion
Sorry, I knew I wasn't being clear. De-zincification implies the zinc goes away entirely (due to its oxide's high solubility in water). Here, the zinc (and other alloying metals) is getting left behind in the porous deposit, although we don't know how much relative to the original alloy content. I'm just trying to point out how the zinc and other metals could get reduced and re-deposited by the hydrazine. I used copper in the illustrating reaction by mistake. The zinc (and lead, tin, etc.) can get oxidized and reduced in the same manner as the copper, only at different rates depending on pH, nobility of the base metals, etc.
It is possible, the copper is not oxidizing at all, just the base metals, but then the re-flooding of the area with hydrazine solution is reducing and redeposting some of the zinc/lead/tin ions.
Dunno, not a materials expert, just know a little bit about hydrazine, and wanted to throw out the possibility of a cyclic oxygen exposure to account for the corrosion being seen here.
RE: Brass fitting corrosion