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]

Tap water has salts so it will be conductive to a degree.

I have added Dielectric unions to the design.

 

[Jack Benson]

The dielectric unions i have found are carbon steel to brass, not SS304 to brass.

the steel on my system is SS304

is that going to be an issue?

 

[EdStainless]

The CS fitting will be anodic to both the SS and brass and as such is likely to suffer noticeable galvanic corrosion issues.
The other thing is that you listed 303 in the title of the thread.
People love making fittings from 303 because it is easier to machine.
But the pitting resistance of 303 is nearly non-existent.
The same stringers that act to assist machinability also serve as pitting initiation sites.
Think of it as built in crevices for corrosion to start in.
The irony is that the modern lean duplex grades (2101 and such) machine better than 303 and have better pitting resistance than generic 316.
They also cost about the same as 316, but market adoption has been slow, the chicken and egg syndrome.

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P.E. Metallurgy, consulting work welcomed

 

[Jack Benson]

All steel is now 304. When I started this post I did not have full information.

I am not finding Steel 304 dielectric unions. Another supplier is offering iron to brass dielectric union.

I didn’t expect it to be difficult to find SS304 to brass dielectric unions

 

[Jack Benson]

i have spoken to 3 manufacturers in China to source a dielectric fitting between ss304 & brass.

no-one makes it that i have found.

i have been offered:

IRON - BRASS
STEEL C45 - BRASS
STEEL Q325 - BRASS

after changing the layout of our product to accommodate the dielectric fittings, which took 3 days, i am quite deflated.

I wonder if this is why so many people complain that the dielectric fittings do not stop corrosion on water tanks, as they might be connecting to the wrong type of metal to the IRON/STEEL thread on the dielectric union

 

[Jack Benson]

https://www.nuflowmidwest.com/will-...stainless,minimal risk of galvanic corrosion.

https://blog.brennaninc.com/mixing-...tainless,little chance of galvanic corrosion.

Both these website suggest that SS304 to brass has a low risk of corrosion

 

[Jack Benson]

Is anyone able to suggest how I move forward if SS304 to Brass dielectric unions are not available

 

[TugboatEng]

Use compression fittings to incorporate a small section of non-metallic tubing to isolate the two piping sections.

 

[Jack Benson]

using compression fittings and plastic pipes will re-introduce a process in production where failure is likely.

We preciously used PEX pipe with PEX fittings and we had no many issues with cutting pipe lengths accurately, bad joins, pipes bursting etc.

I only want to use parts that can be bought complete like the flexible hoses that we are currently using.

Are there any other options.

 

[MintJulep]

https://www.hartindustries.com/dielectric-insulated-unions/

 

[Jack Benson]

Thank you MintJule. I got a quote from them but they are a bit out of my price range

I googled to try to find a consultant that could advise me and I found someone that gave me some time on the phone.

They suggested putting the ss304 parts in water with the brass parts and use a multimeter to detect if there is a voltage.

In tap water there was no voltage. I added salt to the water to test if i could get a reading and in salt water I got 0.1v [when I put one probe on the ss304 part and the other probe on the brass part].

This is very unscientific test i know.

How sensitive should my multimeter be to be able to identify that there is enough voltage between the two metals that would cause galvanic corrosion?

Is 0.1v enough? I will invest in a better multimeter by Fluke. Any recommendations?

 

[EdStainless]

The thing that I worry about with the brass is de-zincification.
In general 0.1V is not enough to drive serious galvanic effects.
I find it hard to believe that you don't see a voltage in the tap water.
Do you have a conductivity meter?
That might be more telling.

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P.E. Metallurgy, consulting work welcomed

 

[Jack Benson]

I can buy a conductivity meter. Are the cheaper ones going to be OK?

https://uk.rs-online.com/web/c/?searchTerm=conductivity+meter

This is the multimeter I used Kewtech KT200. Again I'm happy to invest in a Fluke one.

The multimeter was in DC setting for the tests. Definitely nothing in tap water when I tested.

 

[EdStainless]

Yes, they show a few under L100 that look fine for this work.
They are very handy; I use them for quick checks in the field and in the plant (how clean is your rinse water?)

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

 

[Jack Benson]

I ordered the Milwaukee MW806 MAX 4-in-1 pH/EC/TDS/Temp. Combo Meter with ATC earlier today

I have been looking at multimeters and I will probably order the Fluke 179. It will record from 0.005 mV according to Fluke technical support.

They did some testing today in tapwater with a brass threaded connector, an SS304, threaded connector and a piece of copper pipe. There was a 0.1 V reading between the copper pipe and the brass connector. No other combinations resulted in voltage.

I will run the test again when I have the Fluke multimeter, which is much more sensitive.

I then introduced some aluminium foil just to test what happened, and there was a 0.5 V reading between the aluminium and all the other metals. There is no aluminium in my system, but I was just interested.

 

[Jack Benson]

We have a brass threaded G1/2" nipple that is glued into a stainless steel small water tank that has a heating element.

This tank was made before i joined the company so i cannot confirm if it was SS304.

(on the top is what is left of the fibre washer, its not corrosion).

I removed the brass threaded G1/2" nipple.

The white residue is the glue.

There looks to be green marks on the brass.

From what i read this is oxidisation - not galvanic corrosion - is that correct?

This is the end that would have been submerged in water inside the stainless steel water tank.

This water heater has not been used in a long time so it was not full of water so the thread would likely have been wet but also exposed to air.

these marks are not green, more like grey. Do you know what

This is a picture of inside the brass connector (taken from the end that was inside the stainless steel water tank

thank-you

 

[TugboatEng]

Brass is subject to dezincification. That will produce a grey colored oxidation. If you clean it off you'll see red copper left behind.

 

[Jack Benson]

thank-you for confirming the dezincification.

the water in this section of our system is clean tap water.

is the cause of the dezincification related to dissimilar metals (i am starting to google to understand this).

from the pictures, did you see any sign of galvanic corrosion?

Does nickel plated brass suffer from dezincification?

thank-you

 

[EdStainless]

If you scratch at the surface you can compare different areas.
Areas with dezincification will have some a somewhat porous appearance and a more pronounced Cu color.

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

 

[Jack Benson]

This is a Steel brained EPDM home with brass (and nickel plated brass connectors).

It was used on the grey water side of the system (the water was contaminated with shower soap / shampoo).

This is a new part:

This is a part that has been used. It was connected to a threaded PVC connector (not a metal connector):

What type of corrosion is this?