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Galvanic Corrosion in 303, 304, 316 Steel 1

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Jack Benson

Jack Benson

Industrial
Jul 11, 2023
101
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.
20200120_141528_iykbxz.jpg


Is there any risk of galvanic corrosion if we connect this flexible steel tube directly to the heat exchanger: (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:
I have also read this thread:
Any help would be appreciated.

thank-you

Jack
 
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There is not a significant difference in galvanic potential between 304 and 316 stainless to be concerned with for fresh water applications.

The L is more of a concern for welding. L grade stainless has better corrosion resistance after welding.

303 stainless steel should not be used for water applications as it has very poor corrosion resistance.

Many appliance hoses are non-conductive as would work to galvanically isolate. However, be mindful of the hose end material, they are usually brass.

FYI, PEX fittings are available in stainless steel so you could build a PEX based isolator with a brass fitting on one end and SS on the other. Expansion type PEX would be best.
 
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thank-you for your reply.

i have seen that most flexibles have brass in the end of the hose so i am avoiding them

at the moment we connect the PVC fittings onto the heat exchanger but they are bulky and make out appliance wider than it needs to be
I have seen these which are 316 steel
i would be able to connect 4 of these directly onto the heat exchanger and then i would use a PVC connection to avoid potential galvanic corrosion.

with the PVC parts, i cannot connect 4 elbows directly onto the heat exchanger as they are more bulky, and once i have connected 1, it stops the second one from screwing on.

your advice is attaching 316 elbows onto a 304 heat exchanger for fresh water applications is safe?

the water will be grey water so there will be soap, hair etc in the water.

i appreciate your help.
 
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Yes, the 316 to 304 connection is safe. Crevice corrosion will be your biggest problem. Gray water is very dirty and without pumped flow (velocity) for scouring a film can form inside of the pipes. This film can create the conditions that lead to crevice corrosion.
 
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the grey water is pumped so its not stagnant and the we backwash daily with clean water

i will look into Crevice corrosion.

thanks for the heads up
 
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The biggest problem with gray water is all of the Cl.
Soaps contain a lot of salt and this is what will drive the crevice corrosion.
The formation of a surface film will reduce heat transfer, so you will need to watch this.
I think the idea of coupling 316 fittings to the 304 HX and then making the plastic transition later is a good one.

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

thank-you.

I have joined a start-up and am taking over the product development as the person who was doing the role (although very qualified) could not solve a lot of problems i have been able to resolve.

But i am not an engineer and have always been concerned with both the fouling of and potential corrosion in the heat exchanger.

The backwash has so far prevented any fouling, but we do not have any devices that have been installed for longer periods of time.

the system has PVC connections to the 304 HX as i have explained, but the heat exchanger we are using is copper brazed plate heat exchanger.

in my mind, if there is potential for galvanic corrosion outside the HX, surely there is potential for galvanic corrosion inside the HX.

but i do not know the science, and I have struggled to find someone that would be able to advise me.

any insight would be appreciated.

thank-you

 
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Ed,

thank-you.

I have joined a start-up and am taking over the product development as the person who was doing the role (although very qualified) could not solve a lot of problems i have been able to resolve.

But i am not an engineer and have always been concerned with both the fouling of and potential corrosion in the heat exchanger.

The backwash has so far prevented any fouling, but we do not have any devices that have been installed for longer periods of time.

the system has PVC connections to the 304 HX as i have explained, but the heat exchanger we are using is copper brazed plate heat exchanger.

in my mind, if there is potential for galvanic corrosion outside the HX, surely there is potential for galvanic corrosion inside the HX.

but i do not know the science, and I have struggled to find someone that would be able to advise me.

any insight would be appreciated.

thank-you

 
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The Cu and hopefully the braze will not be bothered much by the Cl.
But any S (or any S compounds) can cause issues.
The freshwater flush will really help this.
Cu is very flow rate sensitive, too high and erosion becomes a big problem.
Yes, the stainless threads are on the outside of your HX and that is where crevice corrosion is a risk.
At least this can be watched.
Where does the SS connect to the Cu HX?
There is a risk of accelerated corrosion of the Cu in this location.


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

again thank-you for the reply.

can you confirm which these short hands are:

Cu = Copper
CI = ?
S = Salt?

which parts can the 'S' cause issue to?

water flows though the HX at approx. 6 litres per minute.

You say "Cu is very flow rate sensitive, too high and erosion becomes a big problem." -> are you talking about the copper brazing inside the Hx?

currently no copper parts connect to the HX and I do not plan to connect copper to the HX again - it was only in the early versions like in the picture above.

We glue PVC parts to the inlets/outlets at the moment
Hx_qkvfrt.png


as we discussed about, i will glue 304 elbows to the HX then attached the PVC parts to those.

I appreciate your advice
 
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Cu = copper
Cl = chloride usually from salts, present in all soaps but very high in liquid soaps (it is how they control the hardness or viscosity of the soap)
S = Sulphur

So the HX is SS with Cu braze alloy?
The braze joints may be your weak link.
Be very careful in what you use as a thread sealant on the SS.
Make sure that is designed for use on SS.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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This is the HX
We currently use Loctite 480 to attach the PVC to the SS threads in the heat exchanger. No issues so far.

I plan to use FiletFix III to connect the 304 elbows to the HX. No issues when used to connect threaded brass parts.

The Sulphur & Chlorine can damage the copper brazing inside the HX? Is there a way to manage this.
 
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Keeping the velocities high will minimize the risk of fouling and freshwater flush will help dilute corrodents.
Do you know what the dimensions of the internal passages are in the HX?
Do you have any way to estimate the internal velocities?

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
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I can email them to ask for the internal velocities - does this help at all?

Hx_Spec_p4gmew.png


at the moment we backwash a few hours after the last use.

should we be backwashing more quickly after grey water has passed through the HX
 
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Do know that you have picked the absolute worst type of heat exchanger for this application. The small, convoluted flow paths are prone to clogging. The brazed construction makes cleaning impossible. With shell and tube configuration you can run the dirty fluid through the tubes which are easily cleaned. An anular tube configuration is the most flushable.
 
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We believe we can manage the fouling. We have plenty of hours use for this and the backwash is enough given the grey water we are recovering heat from.

What I am concerned about is the potential for corrosion inside the HX.

 
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are you able to comment on the likelihood of the copper in the HX being corroded?
 
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Provided sulfides are not present and depending on the brazing alloy used you'll possibly see dezincification of the braze material before the copper corrodes. There is a very high ratio of brazed area to copper area in brazed plate heat exchangers which means the braze alloy will be anodic to the copper until the zinc is depleted.
 
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The HX manufacturer has confirmed that there are only the 2 following materials in the HX

304 Steel
Pure copper (brazing material)

How does this change affect your previous post?

Thank-you
 
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So the plates are 304 and copper is the brazing alloy? That's not good. You have a very high ratio of surface area of stainless steel to copper which means the brazed joints are going to corrode away very quickly if there is any conductivity to the fluid. The type of heat exchanger you have is for clean, non-corrosive fluids.
 
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