A readers question about white streaks

A reader of AnodizingWorld has some specific problems with streaks. He received a personal answer by email from me, but i would like to share my answer to him with the rest of you. He asks:

  • Are the white streaks purely “material” related issue and linked to the extrusion process?

  • Why can one anodizer remove it and another can´t?

  • What would you recommend in terms of etching time and current density to eradicate or to minimize these white streaks from appearing?

  • Will the streaks appear deeper inside the material or is it mostly appearing near the surface?

  • Will buying a bigger diameter bar and removing more material from the surface helps?
As I mentioned in my post about streaking, Streaking defects on extruded, anodized aluminum, there are a lot of different streak defects. Most of the streak defects are material related, some of them you will find way down in the material and others are in the top layer.

In a later post I mentioned the importance of knowing your microstructure, Microstructure of aluminum alloys. So try to make a light microscope image of the surface or even better a Scanning Electron Microscope image. In this way you will know what you are trying to get rid of.

The reason that one anodizer is able to remove it and another one isn´t is probably due to the different ways Anodizers anodize parts.

You should always ask for a process description with all the steps involved. They don´t have to give you the chemicals or concentrations but a step by step process description.

An acid etch can sometimes be better for extrusion related defects than caustic etch. Regarding time of etch you have to test, it is dependent on what´s in the microstructure.

For the current density, higher current density and low bath concentration would make your layer more opaque and depending on your microstructure (again) perhaps hide the white streaks.

Buying a bigger diameter will only help you if the streaks (microstructure which gives the streaks) are found in the top surface of the aluminum bar.

If you find this article useful and you would like to know more please contact me blog@aluconsult.com