Gold plating problem to ALuminum 6065-T6, help is needed. 1

[lling]

Hi everyone,

I am very glad to find this site. I hope some experts here can help me to solve our problems.

We are a millimeter wave components and subsystem company. Most of our housings are made of Aluminum 6061-T6, type III soft gold plating, per MIL-G-45204. The detail plating specs are:

PER MIL-G-45204, REV C
100 micro-inches(min) of type III gold over 100-150 micro-inches of electroless nickel per MIL-C-26074, Rev E,Class4. 200 degree C Bake Test for 5 minutes.

Occasionally, we got some RMA(returned parts), because gold is lifted from aluminum floor, blisters are also found, which was believed was caused by the moisture trapped under nickel and gold before plating, which is the conclusion from a Material Analysis Lab. In order to get rid of moisture, after discussed with plating company, our plating specs was changed to:

PER MIL-G-45204, REV C
100 micro-inches(min) of type III gold over 100-150 micro-inches of electroless nickel per MIL-C-26074, Rev E,Class4. 350 degree C adhesion Bake Test for 15 minumtes, before plating, after nickle plating and after gold plating.

After the 350 degree C high temperature baking, about 50% parts failed. Some of our engineers question: is 350 degree C too high for Aluminum 6061-T6?

My questions: do you have the same experience ? How did you solove the problems? What is the standard bake temperature for Aluminum 6061-T6 parts? Please contact me if you have any clue. Thanks.

 

[kenvlach]

1) The adhesion bake temperature is way too high (unless satisfied with Al 6061-T0).
MIL-C-26074E:
"3.4.2.3 Class 4 coatings. Coated heat-treatable aluminum alloys shall be heated
between 240 ^oF to 260 ^oF for 1 to 1.5 hours to improve adhesion of nickel deposit."

2) The adhesion bake is irrelevant to the cause of the problem and is actually just helping blow off the EN plating by during trapped moisture underneath to steam. In this case, it serves as QC to reveal poor plating.

3) The probable cause is poor zincating. Double zincating is preferable for wrought alloys, and the thinnner the final zincate the better (as long as 100% coverage). Many platers believe "if a little is good, more is better." But as zincate is porous and holds moisture, the opposite is true. Typically, 15-30 seconds is OK, 45-60 seconds is guaranteed failure.
Follow the 2nd zincate with rinsing, DI rinsing and then alkaline EN strike.

 

[lling]

Thanks for your valuable reply. I will check with our plating company about zincate, and cleaning procedure.

Some of us suspect this plating problem might caused by poor material. Is that possible? We have been asking for metal certificate from the machine shop, but how can we know if the material cannot meet its' standard? Is there any military or aerospace qualified material supplier?

Thank you very much!

 

[kenvlach]

There's no problem buying aerospace quality Al in California. Lots of AMS & ASTM & QQ specs. Depending on material form, maybe AMS 4150 & ASTM B211 apply:

http://www.sae.org/technical/standards/AMS4150L

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B211.htm?L+mystore+ambn8757

But, it's probably impossible to buy any non-ASTM 6061 within the US. While thick sections (forgings, etc.) may be a bit off-spec (soft in the center) on hardness/strength, defects in 6061 that affect anodizing or plating are exceedingly rare (unlike 2024 & 7075). Unmachined extrusions very occasionally show an 'orange peel effect.' Unless plating a raw extrusion or doing something unusual (like machining with dull tooling & using sulfurized cuttting oil & prohibiting etching in the cleaning process), I'd guess the chance of material defect due to the 6061 as 1 in 10,000. Whereas, 'white rust' or substrate corrosion of EN-plated Al, Mg or Zn is common. E.g.,

 

[lling]

Thanks Kenvlach, so you think the problem was caused by plating procedure rather than material? Do you know if there is a complete plating procedure for the parts in our industry? Could you help? Thanks.

 

[kenvlach]

It's frequently said that 90% of plating defects are due to human error. That number is low, as most materials errors are the result of human error.

I have very limited gold plating experience but much on aluminum. A systematic troubleshooting approach works well for most plating systems. Where does the defect appear in the Al/Ni/Au? Are all process and QA steps being followed? Are you doing the hot water rinsing (Para. 3.7.1) and adhesion testing (Para. 4.5.2 et seq.)? The hot water rinse also has QA value, as defective plating often blisters. What process sequence does the plater's shop traveler show? Are there signatures from the processors & QA people?

Note that MIL-G-45204C was canceled in 1988; possible replacements given are AMS 2422, 'Plating, Gold' and/or ASTM B488, 'Electrodeposited Coatings of Gold for Engineering Uses.'

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B488.htm?L+mystore+ovxu3529

Also, ASTM B253, 'Standard Guide for Preparation of Aluminum Alloys for Electroplating,' is useful.

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B253.htm?L+mystore+ovxu3529

Among the pretreatments for aluminum, the zinc immersion (zincate) with alkaline electroless nickel strike is very commonly used.

Further info sources.
Check the finishing.com Letters (& search archived Letters) for gold plating problems & responses, and the Products Finishing site for articles & a plating clinic:

http://www.finishing.com/Letters/index.html

http://www.pfonline.com/Scripts/Search/SearchMainPF.asp?qt=Gold Plating

http://www.pfonline.com/dp/clinics/index.cfm?cat=PC12&zone=PLATING

 

[mighoser]

See MIL-HDBK-5H page 3-264 for reduction in RT mechanical properties from elevated temp/time exposure for 6061-T6. Baking at 400F for one hour causes about a 15% reduction in ultimate and yield strength. I just looked, even 350F for 30 minutes wouldn't have any effect. In fact at this temperature the impact begins between 10-100hrs. I would check oven charts for time and temp for the failed parts. Alloy 2024 T3/4 and 7075-T6 are more sensitive. Bottom line: you are right on the edge of causing a problem and dropping the temp and increasing duration is highly recommended.