Coffee Machine Corrosion (hot water) 1

[some1else]

Here is a thought problem for your weekend brainstorming pleasure.

A local coffee company who leases industrial sized coffee brewers is having a corrosion issue with a coupling in one of their models. This coupling/nut connects the hot water storage tank (which operates at 190-205F) to the distribution tube that dispenses the hot water over the coffee grounds.

Teflon tape is used on the threads, but obviously doesn't last, as when the coupling is removed, it has a bluish glassy scale on 1/4 of it with corrosion that typically penetrates in a 1/8" diameter pit. The coupling is always submerged at least 3 inches below the hot water level. It is (supposed to be) made out of 300-series SS. Picture is attached.

The company rep mentioned that they have serviced 7 of these machines in the past year and all of them have had corrosion on this coupling. 2 of those machines had a filter, the others did not. None of their other machines have the issue. He was thinking that it could be some sort of stray current issue, due to how much the pit resembles an arc strike spot.

The only thing I can think is that these couplings were made of defective steel, which means that all of these machines need the coupling replaced.

Are there any creative ideas from the vanguard?

 

[Metaljon]

I think a full metallurgical assessment is required as there are so many possible causes. You should get them to engage a local metallurgist. I suspect wrong material grade for the application. Does this vessel get cleaned using the hypochlorite tablet dosing?What grade of stainless are they using?

 

[metengr]

Have the failed coupling subjected to a proper failure analysis as mentioned above by a metallurgical lab. If this problem happened on a small population of machines it could be a material issue, this has to be confirmed.

 

[some1else]

There won't be a full blown failure analysis- this is a relatively minor issue. It is interesting because of the way it occurs, not because it is a risk to life, limb, or the coffee company.
I offered to look at it pro bono and I plan to do some microscopy on it at a friends lab (and maybe EDX). I was just wondering if anyone had any ideas in the mean time. Thanks for the recommendations for an assessment, and I hope my relaxed approach to this particular problem is not percieved as unprofessional conduct.

 

[bimr]

You should approach this two ways.

1. Determine the properties of the water;

http://www.virtualcoffee.com/sep_water.html

2. Check the source and material construction of the fitting.

People usually propose "stray current" when they do not understand what is happening.

 

[EdStainless]

What does the whole assembly look like?
Galvanic corrosion is a possibility, but defective material is as well.
You are going to need some analysis work done, check composition of the parts.

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Plymouth Tube

 

[blacksmith37]

I think that looks pretty unusual (after doing failure analysis for about 35yr). Ranks with lightening strike traveling down an aluminum cable from a transmission tower and burning a 1" hole in line pipe laying in an open ditch.

 

[EdStainless]

The fittings aren't coated or plated are they? If they are then small defects would lead to rapid localized corrosion.
Have them skip the teflon tape, it could be making things worse by forming tight crevices with only a few local 'holes'.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[racookpe1978]

OK. You've covered the "bad" fittings pretty well.

Now, what about the "good" ones? How many are corroding, vs how many are good? (1-1 ratio? 10-1? 2-1? 100-1?)

What's in common in the good ones? Maybe there is a water supply issue (lime or acidic for example) in one region that isn't in the second where no problems occur. Or the reverse - the lime in one region from hard water is "sealing" the corroded fittings that leak in the other region.

Have you microscopically disected three good fittings and three corroded ones? What is in common with the bad ones' appearance that differs in the good ones' metallurgy?

 

[redpicker]

Is it leaking at the fitting? It kind of looks like corrosion/errosion to me. THis might just be that the models that develop this problem have a design issue that prevents this coupling from getting a good seal and develops a leak.

I agree with bimr, "stray current" is often code for "I can't think of anything else that could be wrong".

rp

 

[some1else]

@racookpe1978 - practically speaking, we won't be able to grab additional "good" samples from clients at this point. And the interesting thing about the water is that its only corroding this coupling. It all makes me think that the coupling is metallurgically defective.

@redpicker - not sure how it could be corrosion/erosion as the pit is focused in one location and isn't elongated. The water is hot, but it moves through the coupling at about the same speed as your Mr. Coffee brews a pot. Its a trickle.

@EdStainless - I'll repost if we get some microscopy and let you know if the sample was stainless plated. It would make sense for it to corrode at the thread roots where the plating is most likely to be thinned. The is only in contact with stainless, so if there is any galvanic coupling, it would have to be inherent to the component itself.
As far as crevice corrosion, I'll ask the repair technician if the teflon is torn up when he disassembles the machines.

Thanks for your insights.

 

[redpicker]

some1else
What is only a trickle through a 0.375" diameter tube can become much more agressive when forced through a 0.003" wide crevice. For errosion/corrosion to be a mechanism only requires the fluid to be moving fast enough to remove the corrosion products faster than they form. I'm not saying that this is what is happening here, only that from what little information we have, I find errosion/corrosion to be a more likely explanation than "stray current".

rp