M_D,
Since you don't run the racks with only a few parts, you are probably minimizing the problems mentioned above.
What I didn't consider was that the conductivity of Ti is about 16X less than Al. This is good because it will reduce the current draw of its large surface area. This is not so good because this increases the voltage drop in the anode connections, meaning you need more voltage for a given current density, thus you dissipate more power in the electrolyte. If the racking was aluminum, and it wasn't masked off to minimize its exposed surface area, this would be a serious problem.
What you are seeing is consistent with what would be expected:
1. Small loads show a lower than normal resistance, because the much larger SA of the rack (compared to the load) is dominating the total resistance. CC is almost useless in this case.
2. Large loads show a higher than normal resistance, because of two reasons;
A. More load SA, so the rack SA may not dominate. i.e: The effect of the parallel resistance of the rack itself is reduced.
B. The lower conductivity (higher resistance) of the titanium is dropping more voltage through the rack and via the contact points to the load. The higher the current, the more voltage dropped (V = I x R).
The lower conductivity means that the actual contact resistance with the work can never be as low as methods not using titanium. This is why Ti racking makes thermal problems (electrolyte temp. rise and contact burning) when used for Type III anodizing. Its not reasonable to expect numbers like mine when using racking, too many variables.
As always, there's never a free lunch.
I may take you up later on the loan of some Ti racking for experimentation.
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