White residue in heat exchanger

[Jimlad]

This post reflects something Stu Jackson reported in 2004 but I thought it worth reposting as it'd be good to hear of other member experiences.
I have a M35B with the 3" heatex.  It's about 10 yrs old and every year or so I need to clean out gunk from the sea water outlet elbow of the heatex. See the attached pics. It is a thick white paste that is easily scraped out with one of the Admirals' s best kitchen knives, but why is it there in the first place?
I'm wondering if it is a build up of deposit from the sacrifical anode in the heatex.  I tend to have to replace these every 4 months or so. The heatex itself is fine and has no residue inside it.  It's a pain to have to check this every year and I'm thinking of getting a fitting with a 90 degree curve rather than the right angle which seems to trap the deposit.
Views anyone?

Stu's 2004 post http://c34.org/bbs/index.php?topic=1487.0

[Stu Jackson]

Dave, my read is that it is from the salt precipitating out from the seawater where hot meets cold at that port of the HX.  I change my zinc quarterly, and usually have enough old zinc left that I do not attribute it to that.j  I could be wrong, wouldn't be the first time.  Another reason I continue to recommend that properly servicing the HX requires pulling it every couple of years.

[Ron Hill]

Dave : Not too sure what may cause a "Paste"?  

I'm wondering if someone didn't load the raw water pump with grease to get good a seal and that's the cause of the paste? The white is probably the old ZnO residue.  

A few thoughts

[Jimlad]

The good old taste test did not reveal any saltiness.  The paste feels coarse, not greasy, similar to a brazing paste flux.  My feel is that it is zinc residue but the colour is not quite right.  Zinc residue seems to be more of a dirty light grey. 

[lazybone]

What's it taste like?

Looks like nice runny Brie.

[2ndwish]

I'm no chemist, but I believe Zinc reacts in salt water to form ZnO- zinc oxide - "a white powder that is insoluble in water" according to Wikipedia. So rather than spreading the paste on a cracker, you might consider spreading it on your nose to protect against sunburn :-o. Not exactly sure why it would collect in the brass, but brass with its own zinc content is more reactive than copper,  so it might not be surprising that you would find ZnO collecting there (in addition to the sharp right turn).

After thinking about it for a few more minutes (I'm pretty slow), it occurred to me that the paste is likely to be ZnO from the corrosion of the brass itself- sticking there like rust. You might inspect the elbow carefully and for the couple of bucks it costs- replace it if it is suspect.

[2ndwish]

So sitting around waiting for my turkey to brine- thought I'd do a little web surfing on the subject-always a dangerous thing. A few searches on brass corrosion turned up a term "zinc meringue" (sticking with this thread's unfortunate food theme..) which is a term well known to plumbers, construction defect attorneys and someone named Yaofu Zhang, who's 2009 master's thesis on the subject of brass dezincification
(http://scholar.lib.vt.edu/theses/available/etd-12182009-203827/unrestricted/Zhang_Y_T_2009.pdf)

contains some (not all) photos that look shockingly like those seen in the original post. Bottom line: brass composition varies widely with some fittings and geometries being much more susceptible to leaching. When the meringue is removed, the surface may or may not exhibit pitting, but the structure will have been weakened anyway. The meringue made of any number of zinc compounds and may not be simple ZnO- so leave it off your nose.

[Jimlad]

Ok, thanks everyone for your contributions.  Am pleased to report I am  writing this from this side of the pearly gates so the taste test did not finish me off.  Neither did it taste like Brie tho :-(
I think it is ZnO, probably from the decaying anode, that is accumulating in the right angle of the fitting.  I'm going to check it on the same cycle as my heatex anode ie 3 monthly