Metal finishing Q&As since 1989
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Black Marks on Extruded Aluminum I-Beams
Q. Hello, my name is Ian Morris.
I was recently hired as a Quality Manager for a manufacturing facility building boat trailers. We use extruded aluminum for some of our trailers and I am trying to focus on the quality of the products provided by suppliers. I would like to address some aspects, but feel I would be better prepared if I could identify exactly what the defects are.
Can anyone tell me what the black marks are?
They run intermittently down the length of aluminum I-beams on all 4 sides, but are more prevalent on the insides of the "I". They are rough to the touch, seem to contain fine aluminum dust and do not wipe away. They will sand out with some effort.
I appreciate any support you are able to provide.
- Nashville, Georgia
February 14, 2024
A. Many years ago I worked at an aluminum processing plant that painted coil and extruded aluminum. This black streak is very common; not entirely sure what it is, but do know that aluminum turns black when in contact with caustic so most likely a corrosion product. Could be transfer from skin of billet. Before I left I moved on == I worked for a shot period as an inspector on the extrusion line. This was not treated as a defect.
Ron ZeemanRetired - Brampton, Ontario, Canada
February 21, 2024
Ron,
Thank you for your response. I imagine whether it is deemed a defect would depend on if it is preventable, which I would say it is due to its inconsistency as I receive some beams without these lines, and based on customer requirements.
Thank you for taking the time to respond, and I appreciate your input.
- Nashville, Georgia
February 22, 2024
A. I saw this 40 some years ago so I doubt it is preventable.
I believe they clean the dies with caustic prior to re-racking them in storage. If it was preventable plenty of time has passed for process improvements or the cost is prohibitive.
Retired - Brampton ON Canada
February 22, 2024
Again, thank you for your input.
I am not yet convinced this is unpreventable or cost prohibitive.
If as you say, this problem has spanned at least the past 40 years, I imagine there have been an innumerable amount of individuals who can attest to that.
The first step is to identify what these marks are and what is causing them.
- Nashville, Georgia
February 26, 2024
Q. We believe these marks are from the shipment. They are stacked on top of one another, which causes rub points where they are resting and vibrating during transportation. I assume these scrapes and rubs then collect dirt and corrode causing them to turn black in these sections.
If this sounds unreasonable, please let me know, but for now we have engaged with our supplier to ensure they are packaged properly and we will hopefully see this issue go away.
If not, I guess I will be back to trying to figure out what is causing it.
- Nashville, Georgia
March 13, 2024
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