Ni plated Al corrosion or?

[Yosemite_Sam]

Hello-
some much needed corrosion advice needed. These parts are from a water cooling assembly of a tool. Top and bottom platen, process cooling water of about 280kOhm-cm resistivity (RT to about 70C at most) that runs through these parts. Not pictured is the SS-made diverter that sits in between. The parts are said to have been Ni plated Aluminum. We were told that this problem would not have occurred had we employed a dedicated chiller with a an additive for the water. For whatever reason it wasn't done and now we have this growth. I haven't attempted sanding or chemical etch yet. Not sure if there is pitting.
If anyone can help with
a) how to identify what is corroding chemically
b) how to remove the growths with minimal damage to the surface itself
Much appreciated

 

[EdStainless]

What is the Al alloy?
How long has this been in service?
What Ni plating was used? (electrolytic or electroless, straight Ni or NiP)

So you have a lot of Ni and SS in the circuit and Al. And no corrosion inhibitors being used?
My first hunch is the natural porosity of the platting resulted in corrosion of the exposed Al.
You need a closed loop system with the correct corrosion inhibitors in it. If you don't need it colder then it does not need to be a chiller, just a tank, pump, and heat exchanger to be cooled by the outside water.

If you looking to save these pieces then clean them gently (perhaps blast with dry ice or baking soda or corn cob). Then you can examine the surface below and decide on the next step..

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P.E. Metallurgy, consulting work welcomed

 

[Yosemite_Sam]

@EdStainless

Thanks for the reply. I don't have anymore info on the Al or type of Ni plating. But definitely no corrosion inhibitors are being used. Part has been in service at least 3 years and maybe 5+.

 

[EdStainless]

The this is likely end of life for this one.
Why are they Ni plated?
There are Al alloys with better corrosion resistance (52XX).
But a closed loop with inhibitors is the right way to handle this.
You can buy the inhibitor package that is used in auto engines, without having to take the glycol.

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P.E. Metallurgy, consulting work welcomed

 

Scrape some off and send it off for X-Ray Diffraction (XRD) analysis. I have frequently used this method for identification of corrosion products. The report will give not just elements but compounds in approximate percentages, which is very useful in understanding corrosion processes.

Look for a petrology lab; they use this test for analyzing core samples. Around $200 cost.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Yosemite_Sam]

good idea. I have access to XRD. will check. Thanks

 

[TugboatEng]

Why would you run untreated water in a closed loop system? Is this exposed to a product such as food where contamination is a problem? Treat that water as you would an automotive cooling system. I'm not suggesting using automotive coolants (you don't need the glycol for anti-freeze) but you need to make the decision between a sodium nitrite or carboxylic acid based inhibitor. I would contact your local water treatment specialist. Your problem is that inhibited coolants can't protect corroded surfaces so you'll need to figure out a way to clean them (usually sulfamic acid). Oh yeah, fire the guy that didn't treat the cooling water.