Mold growth on 2024 Aluminum and 303 SS

[quizzical1]

Water changes from ~60F to 150 F every 5 minutes. After a month of cycling between hot and cold, the rig was disassembled and this growth was found. Manifold is Aluminum (2024, not anodized) and the device is 303 SS. Manifold images show before and after cleaning - a wire brush and CLR Mold/Mildew foam was applied multiple times, but its been difficult to remove.


can someone identify the growth, and share insights for cause and prevention?

the discoloration on the cup (brass like appearance, is easily removed when polished) may be evidence of corrosion in addition to the mold issue.



thank you,

 

[TugboatEng]

Looks like aluminum oxide. Is this a closed loop cooling water system? You need to add corrosion inhibitors and perform some basic water chemistry stewardship.

 

[quizzical1]

Thank you for the prompt response. With a little elbow grease I was able to remove most of the surface contaminants and revealed some serious pitting.

Does the pitting and red tint correlate with Aluminum Oxide? could that be depositing onto the 303SS and causing the brown discoloration?


FYI the device will be used in fuel/oil, but testing is conducted in water, dissimilar metals are not present in the actual application.



thank you,

 

[TugboatEng]

Aluminum oxide is white. The brown may be iron oxide from the 303 stainless steel. Brass will be fairly to very tolerant of these conditions, 303 stainless and aluminum alloys will not.

 

[EdStainless]

The Al Oxide will take on whatever color there is of contaminates in the system.
I would expect the Al and 303 to both corrode in this environment.
I would expect the red tint to be Fe from the 303.
Is your 303 modified with S or Se?

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P.E. Metallurgy, consulting work welcomed

 

[quizzical1]

Gentlemen, thank you for the insights. I would like to share some follow up thoughts and gauge your responses. Attached is a simple mock up of the rig - do the setup and results suggest galvanic corrosion? I think we can just change the Aluminum block to 303SS and alleviate most of the issue if so.

the metallurgy report is attached- this shows the before and after for the 303SS. Before corrosion, ID is clearly 303SS, after corrosion its reporting more Copper.

would this be an expected result given the info shared so far?

thank you,

 

[Compositepro]

The red tint is probably copper deposits from the aluminum.

 

[EdStainless]

The Cu on the surface is from the brass. Cu is somewhat soluble in hot water.
It ends up on the SS because of the relative galvanic potentials.
303 is very susceptible to biofouling and MIC (microbially influenced corrosion) because of the large S inclusions.
Then the pitting initiates along the inclusions.
There are other choices for free machining SS. Outokumpu makes grades call Prodec which are very good. A couple of other mills offer similar products that less sensitive to corrosion.

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P.E. Metallurgy, consulting work welcomed

 

[quizzical1]

Thanks everyone. I think the conclusion is the 2024 has to be changed to a more compatible material. maybe the brass as well but its basically sitting on the al block with a gasket and stagnant oil, so im not too worried about it.

Ed, I think the copper is coming from the 2024, that's where we see the pitting and there is no direct flow path between the brass and the plated 303.

 

[TugboatEng]

If you're using non-conductive fluids such as oil, you should have minimal corrosion issues. Fuels can be more complicated but the modern ultra low sulphur content of fuels really does minimize these problems as well.

 

[Compositepro]

2024 alloy contains 4.4% copper.