Galvanic Corrosion in 303, 304, 316 Steel 1

[Jack Benson]

Hello,

We are developing a heat recovery device for domestic hot water (grey water not black water).

The heat exchanger is made of 304 steel.

At the moment, we use PVC connections to the 2 inlets / 2 outlets to ensure there is no contact with copper or brass fittings.

I would like to change some of these to steel parts. They can be 303, 304 or 316 steel.

The reason for the PVC connectors connected to the heat exchanger is to prevent galvanic corrosion.

Is there any risk of galvanic corrosion if we connect this flexible steel tube directly to the heat exchanger:

https://www.coditherm.com/produits/flexible/flexible-extensible/flexible-extensible

(I work for a French start up - your browser should translate).

Other fittings I have seen contain 316 or 316L steel. Would these be suitable?

Does the L (Low Carbon) change the risk of experiencing galvanic corrosion?

I have seen tables such as this:

http://www.npfasteners.com/technical/galvanic-corrosion.htm

I have also read this thread:

https://www.eng-tips.com/viewthread.cfm?qid=88097

Any help would be appreciated.

thank-you

Jack

 

[Jack Benson]

They are able to use nickel or steel to braze (although it will be more expensive).

is nickel sufficient, or do we need a stainless steel brazed heat exchanger?

 

[EdStainless]

Ask what braze alloys they are using.
Most Ni braze will more corrosion resistant and more galvanically similar to the SS.

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P.E. Metallurgy, consulting work welcomed

 

[Jack Benson]

thank-you.

i have asked the question - i will hear back tomorrow

 

[Jack Benson]

Nickel or Steel brazed heat exchangers are 5 times the price.

There is a company in the shower waste water heat recovery space that has been going 14 years.

They used a copper brazed steel (316) plate heat exchanger.

It is a passive system so the waste water flows though the heat exchanger by gravity (as opposed to ours which is pumped).

Their system self cleans weekly, not daily.

In 14 years, they have not had any heat exchangers fail due to corrosion.

I have discussed the issue with them, and they acknowledge on paper there is a risk, but in practice the risk has not materialised.

I discussed our application with the heat exchanger manufacturer, and they think the copper brazed option is sufficient. They say the Nickel brazed one does has a very similar risk to the copper one.

I like to have the certainty, I am still very uncertain on the risks this issue poses for me.

 

[EdStainless]

It makes me wonder what braze alloy they are actually using.
If this sounds good to you then go with it.
This isn't one of those 'doomed to rapid failure' applications, it is just that there are risks.
The risks sound fairly small in your case.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[btrueblood]

Seems to me a shower effluent stream is a lot less concentrated in salinity/chloride than say a clothes washer (bleach), or dishwasher/sink drain. Not sure how much different, but because clothes- and dish-washers are mostly automated, you could fairly easily test several brands and capture effluent for testing. Or better, run corrosion tests on the effluent with sample braze joints and evaluate corrosion rates.

 

[Jack Benson]

Thank-you for all your input.

I have also discussed our use case with the heat exchanger manufacturer and given that similar products have used the same type of heat exchanger in the same application for a long period, i am comfortable with the level of risk

 

[Jack Benson]

How much risk is there of Galvanic corrosion from Nickel plated brass fittings being connected to steel fittings?

https://www.monarchmetal.com/blog/galvanic-corrosion-common-questions-answered/

This shows that steel and nickel are ok together - i know plating is very thin so this might not apply

thank-you

 

[dik]

Can you find out?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Jack Benson]

the brazing material in the heat exchanger is pure copper.

The heat exchanger itself is SS304

(i think earlier i incorrectly wrote that the hx was SS316)



i should give details of how the system has evolved as this will affect the answer

connected the heat exchanger are 2 tanks - both made of SS304 steel like the heat exchanger. connecting the tanks to the heat exchanger are two SS304 female G1/2" elbows with free nuts

I want to know the risks of connecting the nickel plated brass connectors to the ports of both the SS304 tanks

thank-you

 

[TugboatEng]

The heat exchanger you have selected will not tolerate your intended service. It is meant for non-corrosive service with clean and treated water. Address this issue before worrying about your fittings.

Otherwise, a brass and stainless pairing is subject to dezincification. Here is a brass valve installed in some stainless steel saltwater piping on one of my boats.

 

[Jack Benson]

The water flowing through the side of the heat exchanger/metal tanks shown above will be clean water (no impurities). Will that make a difference?

I note what you are saying about the hx being meant for clean non-corrosive water.

 

[dik]

Thanks...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TugboatEng]

What about the other side? Is that going to be clean without impurities? If the heat exchanger corroded through it doesn't matter which side the corrosion occured on.

Also, the plate heat exchanger is the most sensitive to clogging. You have made the worst choice possible for gray water.

You have presented yourself with a very challenging problem. Your goal of heat recovery from a low quality source has little value. Therefore, you have little money for investment so you have to keep all costs to a minimum. This excludes all materials that WILL work in your situation. Now you have to experiment with materials that MAY work. A welded 316 SS heat exchanger MAY work provided you have sufficient flow to keep it clean. A 90/10 CuNi heat exchanger MAY work provided there are no biological processes that produce sulfides. When weighing cost vs risk, the 90/10 CuNi is likely your best option.

 

[Jack Benson]

TugboatEng - i appreciate your concern. We are building on a system that has been on the marketplace for 10 years and has not had a failed heat exchanger in the same application. We have a backwash system for unblocking. We have the system working on customer sites.

I am trying to develop the next build of the system, and I need help on some of the design choices.

on the side of the system where there is clean water, i need to transition from nickel plated brass to steel at the 2 points that i have mentioned (not directly on the heat exchanger)

as the temp of the system is low (max 40C) we have been successfully gluing PVC G1/2" threaded parts between the steel & brass.

I am trying to ascertain on the clean water side if its necessary to have this dielectric break

 

[dik]

If for commercial purposes, do you intend to passivate the stainless to remove the 'rust' stains?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EdStainless]

Jack, a lot of this has to do with the quality of the water.
What is the conductivity of the water? And all of the other characteristics.
If the conductivity is low then the risks of galvanic corrosion are low.
In high conductivity water or water that is aggressive in other ways you will have issues.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Jack Benson]

this will be tap water, direct from the mains.

At the moment it is targeting the French market, but we will expand in the EU.

 

[weldstan]

Tap water quality can vary significantly (at least so in the USA). Similarly in France or not?

 

[TugboatEng]

The mineral content can vary quite a bit. Tap water isn't supposed to be corrosive to the pipes that deliver it so there should be limits to how bad it can be.