Cast aluminium surface

[dogbural]

Hi,

Noticed some of cast aluminium part that was machined into the size had kept in warehouse and found colour change.
Attached images show before and after change.
Is it being oxidised?
If so, as this part is used as heat sink where the LED boards will be mounted on, I am wondering if the oxidised surface would affect on thermal and electrical conductivity ?
Any comments/advice will be much appreciated.

Regards,
MK

 

[dogbural]

"Before" image attached

 

[kingnero]

Yes - this is oxidation.
Yes, this will affect both thermal and electrical conductivity.

Good news is - it can easily be removed. Depending on how "clean" you want it to be, a manual or power driven wire brush will remove most of it (and smear a bit of it open on/in the surface), or chemical cleaning (NaOH will remove the oxides, but too concentrated and it will remove base metal as well).

 

[EdStainless]

If you apply a paste or compound to assist heat transfer, and you don't rely on this for an electrical connection then you may not need to do anything.
I would suggest that you look at using a flap wheel made with a flexible abrasive like ScotchBrite just to clean them some.

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P.E. Metallurgy, consulting work welcomed

 

[dogbural]

Thanks for the advise Kingnero and EdStainless.

Is there any way to prevent them to be oxidized? After machining them, we keep them in the box indoor.

Regards,
MK

 

[TugboatEng]

Wipe some oil on them. Without knowing your specific heat transfer compound, I would recommend a silicone based oils as it will likely be compatible with the compound so you won't need to remove it before assembly. DOT 5 brake fluid (not 5.1) is a really inexpensive source of silicone oil...

 

[EdStainless]

There are dips intended for use with Al, sort of oils buy really a corrosion inhibitor in a liquid carrier.
Contact one the people that supplies RP oil or VPI products.

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P.E. Metallurgy, consulting work welcomed

 

[Compositepro]

First, we are not looking at a cast aluminum surface. It is a machined surface on a cast part. The corrosion displays the pattern of the machining marks. It appears to me that the high points of the marks are corroding because they touched something that is causing the corrosion. There are even what might be fingerprints showing. The solution is to control the contamination source or protect the surface with a suitable oil or coating. Dirty rags might be one cause.

 

[EdStainless]

Comp, good point, we might even go back a step and ask what coolant they using in machining and are they keeping it clean and refreshed properly?
How are the parts cleaned after machining?
All of this matters.

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P.E. Metallurgy, consulting work welcomed