rec

Topic: Pitting of 6061?

Hi.  Just joined the forum today.  My company manufactures 6061 components for the outdoor sports insutry.  100 %  need to be anodized.  We have recently run into a pitting issue on numerous components.  All are machined and then wet tumbled (mass finnished) with a slightly alkaline burnishing compound and plastic media. 

From there we use an ultrasonic cleaner and another slightly alkaline cleaning solution to remove any residue resulting from the mass finnishing.  The parts are then put into a drying oven (200 degree F max). We then send out these parts to be anodized. Type two decorative in a variety of colors. 

Our problem is that on any given batch of parts coming back from our anodizers( we have multiple) there is a white crystaline growth and pitting. 

We use both import and domestic stock.Either solid, drawn, or extruded. All is 6061-t6 and approximatley one inch diameter.  This has been a never ending nightmare of ours for a few years now. 

Any one have a suggestion on were to look or start? This issue comes and goes.  It is not confined to one specific part or color anodize.  We think it may be a stock issue.  Can there be such a difference in alloy compostion from batch to batch or supplier to supplier? 

Thank you 

Last edited by rec (04/03/2009 - 06:47 PM)

lightmetals

Re: Pitting of 6061?

Regarding pitting, one place to look is the metal or alloy itself.  Track whether the problem comes from one supplier.  The reason I suggest this is that hydrogen is soluble in aluminum.  This means that the gasses dissolve in the alloy in the cooling process during heat treat.  This can be caused by excess moisture in the casting furnace or excess humidity during heat treat.  Later during extrusion or some other processing, these pores of gas form an interface or void in the metal and when the oxide grows to the interface, a blister can form.  It is possible that this is the cause of your problem.

DustinGebhardt

Re: Pitting of 6061?

With a problem like this, you have to start checking everything.  Cut samples from each lot of stock that comes in and have them tested metallurgically.  Tag each lot as it goes through the process.  There has to be some way that you can quarantine each lot (or each vendor at least).  If you can nail it down to one vendor, then you can go back to them.

We use steel wire in various sizes from 1/4" to 3/4" dia.  All of the stock comes from the same supplier.  Yet we, too, have problems with the alloy from lot to lot.


Your problem is not all that uncommon.  Just do the legwork and track down the issue.

-Dustin Gebhardt, CEF

Advanced Manufacturing/Finishing Engineer

Moen

Sanford, NC

rec

Re: Pitting of 6061?

Thanks for the quick replies.  We are currently attempting to identify the culprit stock.  With multiple suppliers who in turn use multiple mills/suppliers themselves it is a chalenge. We have noticed our high end stock (Alcoa & Kaiser) are free of these issues.  A bit costly but it is a start.  If it was too much fun they would not call  it work correct?  I will keep you informed on our findings.Thanks again.

LeoH

Re: Pitting of 6061?

Based on your observation of white crystals this sounds very much like galvanic pitting. This is an anodizing defect that occurs in the anodize process. It is characterized by tiny piles of white crystals over tiny microscopic holes. It may occur in patterns forming straight lines or on sharp edges like threads of the parts. It happens during the anodize process but to be fair it is aggravated by any existing corrosion on the parts. Also sometimes there are segregations in the base metal that are a factor in the pitting corrosion as well as the galvanic pitting that happens in the anodizing process.

The defect is based on tiny voids or weaknesses in the anodic film that concentrate galvanic currents in post anodzing process, typically dye or seal. These anodic voids may be caused by existing pits in the metal. Although there are ways to deal with this within the anodize process it can be difficult if the metal has existing pits. These pits can be hard to observe pre anodize. The situation calls for a cooperative effort from both the metal processor and the anodizer.

For your part observe the location of the pits. Sometimes the basic metal as supplied may have metallurgical defects that occur in particular areas. In addition, it is not uncommon that the parts sat wet at some point and this may cause some microscopic pitting type corrosion. A careful microscopic metalographic examination of the parts is suggested.

The anodizer needs to examine his process for stray currents in post anodizing processes and racking techniques. It is common that some parts some of the time may be defect free. There are also pit stop electrical devices that can prevent galvanic pitting even on imperfect metal.

This is a fairly common but often misidentified defect that can be very problematic to resolve. More and rather detailed information is needed to provide more specific and useful advice.

rec

Re: Pitting of 6061?

Thanks Leo.  That is exactly the problem.  We have researched this in the past and have brought it to the attention of numerous anodize vendors of ours over the years.  Most of the time it has been recieved with flat out denial and refusal on their end.  So we are always looking to qualify new vendors because of this. It is refreshing to hear from a non involved party that it not 100% something we are doing in house.

Thanks,

Shawn