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Pitting Defect on Bright Finished Aluminium Extrusion

Q. We are getting a similar pitting problem in electroless nickel on machined aluminium. The pits are different to those caused by gassing during plating. I believe our problem is caused by contamination of the cleaning solution , Tri-acid in this case.

Andrew Madden
Employee - Exmouth, U.K.
March 15, 2022

⇩ Related postings, oldest first ⇩


Q. Pitting Defect on Bright Finished Aluminium Extrusion

Pitting occurs only with Chemically Bright Finished anodised Aluminium Extrusion
Pitting occurs primarily with one extrusion Manufacturer
Pitting occurs predominantly with specific aluminium profiles, not all of the profiles

Other Manufacturers Extrusion & other extrusion profiles on the same load do not pit
Pitting occurs predominantly on one surface of the extrusion, not all over the extrusion.
The same surface of a profile pits, irrespective of its orientation in the tank.

There is nothing apparent of the surface of the "mill finish" extrusion prior to finishing.

The Extrusion is finished immediately after being taken out the extrusion case

Thoroughly cleaning the "mill finish" extrusion with "Methylated Spirits" prior to finishing does not reduce the incidence of pitting.

De-greasing the extrusion prior to the bright finishing process does not reduce the incidence of pitting.

A 1 min caustic etch prior to the normal Bright processing will stop 99% of the pitting

Pitting is evident coming out of de-smut tank, though, its is believed to occur in the Chemical Brightening Tank "Non-Pitted" (Low-or-No-Pitting) bands (4-10cm wide) occur 4-6 times along each length

When laid side-by-side, these "Non-Pitted" bands tend to line up with one another across the extrusion length.

It is believed that some type of contaminant has landed onto the upper facing surface during the extrusion production process after the extrusions have been "cut-to-length"

It is believed that this contaminant penetrates into the Aluminium surface, and then reacts in the Chemical Bright Finishing tank with the tanks reagents.


At this stage, the main suspects are;

Lubricant used in the cut-to-length saw

Carbon/Graphite used in the extrusion process

Any other agents that may be introduced in the "Aging Process"

Any ideas on what the cause of the problem might be are greatly appreciated

Fabio Chiavaroli
- NSW - Australia

Q. We have the same defect. Altering the % nitric acid in the bath seems to have little effect. Do you use a long life etch? and is the material of acceptable quality if you etch prior to brightening?

Rodney Gutsell
- QLD, Australia

A. Have you compared the distances between the non-pitted region with the roller/belt spacings at the extruder?

Alan Castle
- Gloucester, United Kingdom

A. I would check quality of the Al billet before being extruded. Sometimes a bad AlTiB grain refiner is used and some TiB2 agglomerations and oxides are present in the billet producing final defects on the profiles.

Leopoldo Galan
- Aviles, Spain
October 31, 2009

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