PAR and 7075 alloy???

[elton10]

What exactly is meant by the term PAR? Also can LCD anodizing be used on 7000 series alloys?

[NeoMoses]

PAR = Peak Anodizing Resistance. It's the point you see while anodizing that has the highest resistance. And, since V = I*R, and I is fixed (we're all Constant Current anodizing, right?) it's also the point where voltage peaks.

Why do we care about PAR? In theory, when peak resistance is reached, the anodizing layer is at it's thickest.

This next part might not be 100% correct, but this is how I visualize it:

Resistance in a wire is defined as: R = (1/A)*(Resistivity*Length) where resistivity is an intensive property of the material used for the wire and A = cross sectional area.

In a similar manner, I believe we can adapt this to anodizing. While anodizing at a constant current, we see the resistance rise up to a certain point, then start falling again. What changes to cause this increase in resistance? The thickness of the anodizing layer. The cross sectional area can be assumed constant, and the resistivity of the anodized layer should be constant, so in theory, the voltage (during constant current anodizing) is also an indicator of layer thickness.

I'm working on some testing right now to verify this hypothesis. Hopefully we'll have some quantification of it soon.

[elton10]

Neo

As always thanks for the helpful info.. Any notion as to whether LCD anodizing will work on 7000 series Al alloys?

[Fibergeek]

Figure 1 on page 20 of the LCD Instructions shows anodization curves for T-6061, T-2024 and T-7075. As shown, T-7075 anodizes without incident using the LCD process. Other 7000 series alloys should anodize just as well.

[elton10]

Neo

Thanks.. 7075 is exactly what I will be anodizing.. along with some 2024 and possibly 6061. Im going to cal Caswell and see if I can order jsut the instruction book for now... Ive been asked by an associate to set-up a line to experiment in anodizing and dyeing component parts of a bicycle brake assembly

[Fibergeek]

You can download the LCD instructions here:
http://www.caswellplating.com/kits/lcd_ano.pdf

[NeoMoses]

Fibergeek,
Does the wire resistance analogy above sound correct to you? I'm still working on some anodizing test pieces to send to you for measurement, but unfortunately time has been at a premium lately. Again, thanks for all your help.

[Fibergeek]

You can look at it that way.

It works the same as thick film silk screened on resistors, as was used in old fashioned (70's) hybrid circuits. The measured resistance in inversely proportional to surface area.

I have explained it before like this:

Imagine that the entire anodize layer is composed of a myriad of microscopic resistors, all of the same resistance. When the surface area is large there are more of these resistors; all in parallel, so the measured resistance of the entire area is smaller.
Now imagine the same microscopic resistors, but the surface area is smaller, now there are fewer resistors; again all in parallel, so the resistance measured is larger.
Fewer resistors in parallel yields a higher overall resistance.