Where the
world gathers for metal finishing
Q&As since 1989


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Magnesium anodes prevent pitting in aluminum anodizing

Q. I would like to have information about how to set-up a Galvanic Corrosion Protector capable of preventing galvanic corrosion/pitting of aluminum parts processed on titanium racks in anodize dye and seal tanks. I know of at least one product on the market, but I would prefer to manufacture my own equipment to avoid high equipment costs.

Any help would be appreciated.

Bill Crider
- Tulsa, Oklahoma
1998

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Magnesium Bars to eliminate Current in Post Anodize Treatments

Q. Hi,
We have been experiencing major galvanic pitting in anodizing 7075 T6 aluminum parts. Out of our nickel acetate seal, the parts looks as though someone sprinkled salt on them, and as you wipe away the salt, black pits are very apparent. We are racking on a Ti Hi-Lode racking system, and has come to our attention that the difference in conductivity between the Aluminum Part and Ti rack, along with Ti ability to be a capacitor out of the anodize bath is creating a potential causing this pitting. We have added Magnesium bars to the bath, and when connecting the bars to the flight bar via a copper cable, the problem was eliminated in small test lot Qtys. Now, we have begun to increase the square footage, and the pitting is reappearing. Is there a ratio of magnesium to AL,Ti,Tank volume that needs to be maintained? Why is the Ti racks discharging in the Nickel Seal as opposed to the dye? Any information on the use of Magnesium in post anodize treatments would be greatly appreciated as this information seems hard to find and our chemical supplier seems to not be too familiar with the practice.

Jordan Somerville
Plating/Anodize Employee - York, Pennsylvania
October 4, 2010

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Q. Thanks for your response Willie. The tank is polypro, and was made up with DI water, however during a period of DI regeneration, the tank was topped off with City water. Our city water is of decent quality, and in fact, our dye tanks were made up completely with city water. In any case, the thought of contamination in the seal bath has crossed my mind before. I will research chloride levels in post anodize treatments and run a chloride analysis on the bath. Some other interesting details about this issue: After connecting the magnesium bars directly to the part load, the pitting was removed completely from the outside of the part, the only occurrence of pitting appears on the inside of the part where solution has the chance to pool (these parts are conical and racked in a horizontal fashion).

Jordan Somerville [returning]
- York, Pennsylvania
October 6, 2010

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Q. Thank you for your response Robert (Thanks are extended for your numerous other posts throughout this website which have helped me troubleshoot numerous problems). I have checked the V potential between my SS coil and Rack and have come up with ~1.08 Volts as you predicted. Was this value determined though the difference in Oxidation/Reduction Potentials of SS and Ti (I am having trouble finding good information on the voltage potentials of these metals)? Or what lead you to suggest this number? These same voltages can be seen throughout the circuit we have created from the rack to the mag bars mounted on SS angle iron on the side of the tank.

 

To me, this would suggest that we are successfully removing this galvanic current. We have recently installed Di-electric unions on our steam coils to eliminate outside sources of current. Thanks you for your continued input.

Jordan Somerville [returning]
- York, Pennsylvania
October 11, 2010

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A. Marc, someone in the past was sold the tanks and if they are taken care of, they last an extremely long time.

James Watts
- Navarre, Florida
October 14, 2010

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Q. I have an automated line with SS saddles and pick-ups for my flight bars. If I put something in between my saddles and pickup to insulate them from my flight bar, would it stop the pitting or would I need the magnesium rods?

Robert Rea
- Keene New Hampshire usa
June 8, 2015

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Pitting on green dyed anodized parts, but not black dyed parts

June 5, 2017

Q. Hello,

We are having some issues with anodized parts from an outside supplier. We get the same parts anodized with both green dye and black dye. We are seeing some pitting on parts that have been dyed green. When the parts come they look fine, but after oiling and being put on a machine that puts the parts through several repeated movements, we are seeing (what we think) is pitting. Both the green parts and black parts go through the same oiling and movement process, but the pitting is only occurring on the green parts. While the parts are moving, nothing rubs against the area that is pitting. We recently changed the type of oil we use, but again, the black parts get the same oil and we are not seeing any issues on the black parts. I was told there were similar issues with the old oil, so I don't think it has anything to do with the oil. I attach a photo to try and better convey the pitting, if it is pitting.

Could it also possibly be some kind of build up? I am relatively new to surface finishing chemistry and we have never seen this issue from the baths in our facility, just this outside supplier. We are going to try to work with the supplier, but would like to have some idea what is going on before we go back them. This is happening on nearly 50% of parts.

Thank you for any insight.
Danielle

Danielle Derrington
metallurgy lab employee - Gallatin, Tennessee, USA