Selective EN Plating on Aluminum

[WayneCothran]

Hello All,

I have a project that's been eating my lunch for few weeks now. I'm trying to selective plate an area of aluminum (7075 alloy) using the EN process. I'm having to sand out a sandblasted surface to a slick finish. I can make the process work about 20% of the time. Biggest problem is poor adhesion of the nickel. I'm using the zincate of course, and Caswell suggested I use the De-Ox/De-smut as well. I get 0% results when using the de-ox before zincate. The zincate directions as I understand them are contradictory. I've found that if I let the zincate drip dry, the EN won't adhere. Tape pulls it right off. So, after the second fresh bath I zincate for 15-30 seconds, rinse with distilled water then straight to the EN bath. Then cross my fingers. Two parts out of ten are perfect. The directions also state that as the zincate depletes more outgassing happens. I've read on this forum that I should see violent outgassing immediately. I see only a few bubbles, but I get a mostly uniform, dark, dull grey look after zincate-ing for the aforementioned time. What should I be looking for? I'd greatly appreciate specifics. I goin' broke buyin' chems and I'm about to be bald from pulling my hair out. BTW-when the plating fails, I get to do it all again. I'm removing the failed plating with duct tape, and the stubborn patches with a Roloc Scotchbrite pad, then sanding 220, 600, 1200 then cleaning, zincate, EN. I can see two distinct layers while removing the failed EN. First the nikel, then a darker layer. Once I get it looking white I begin sanding to remove scratches.

[craige]

Once you have the part polished out you have to make sure that it is clean. This is where the water break test is done. If the part passes, then you would immerse in the Deox for 7 to 8 minutes, rinse and go into the zincate. When removed from the zincate, just shake off the excess and go immediately into the electroless bath. Adhesion issues arise from the part not being thoroughly clean.

[WayneCothran]

I failed to note every part passes the water break test prior to de-ox and/or zincate.

[WayneCothran]

"Once you have the part polished out you have to make sure that it is clean. This is where the water break test is done. If the part passes, then you would immerse in the Deox for 7 to 8 minutes, ..."

The instructions on the bottle of de-ox clearly state an immersion time of 1-3 minutes. Would the real immersion time please stand up?

The only conclusion I can draw after 23 more attempts is that the Caswell EN system will not reliably plate aluminum.

[woodjames]

The de-ox will take as long as you need it to. Any smut (basically particles that aren't aluminum depending on the alloy) that arises from cleaning operations needs to be removed. It's always dark and rubs right off. A clean white paper towel rub will show it's existence. So basically when you see your aluminum as a very white metal you're good to continue. Some alloys take longer than others and cast is the worst.

Your successive plate actually resides on the zincate layer, and if that gets attacked you'll get nowhere. The real problem exists in that nickel is more noble than the zinc, so it'll attack it rather than plate on it. The attack is the dark area that you find under the nickel.

I can assure you it can work, it's just a very grey area without all the information and correct techniques.

Ph has a lot to do with how quickly the zincate can survive before plating commences. The problem you're having is more than likely resolved by using a different metal to hang your piece due to electrochemical potentials.

Zincate has it's own set of problems, but really needs a good neutral rinse before plating. Look for a dove grey color. If it's too dark, it's smut, and if it's too light you didn't do it long enough. Dove grey is the only way.

[WayneCothran]

James,

Thanks for the information. I was hoping you were still participating in this forum. I read many of your posts and you really seem to know the business of plating. I originally wrote an epistle describing what I'm doing and lost it due to time out on the forum. The material I'm attempting to plate is 7075 aluminum. At least that's what I've been told. I have not had it tested.

I'm unclear on when to check with a paper towel for the existence of smut. After immersing five parts in the de-ox for the 3 minute max stated on the container I see a uniform white color on the surface to be plated. It's almost as bright as a sheet of copy paper. I assume one wouldn't rub this surface with anything after de-ox treatment. (?)

After failing with a relatively fresh bath I made up another. The first part failed, the next two were perfect, the next almost perfect. Then hit or miss afterwords. With a fresh bath I noticed much more outgassing than with the previous bath with the first few parts. Is this an indicator of things working correctly? Although, I have noticed greatly reduced outgassing with parts that turned out good, just not perfect.

I'm using a clear lacquer as a mask hence the 'selective' plating in the title. I'm pre-heating the parts to about 100 degrees F prior to spraying. Then I heat them again to 100-110 degrees F after they're dry to the touch for thirty minutes to ensure curing. Could this be contaminating the bath?

I'm hanging the parts with copper clad steel filler rod, BUT this is NOT in contact with the bath. I'm only immersing the area to be plated plus ~1 1/2" into the bath. So, the only metal in direct contact with the bath is the aluminum. There's not much heat loss and 300 watts is enough heat input to maintain 190 degrees F. BTW - how critical is temperature control when plating aluminum?

I've since started a third bath and noticed nickel deposits on the side of the 6 gallon bucket (tank) supplied with the large EN kit. There's 'sheets' of nickel on the bottom. Nickel deposits on the mist control balls, as well as on the heaters. Seems I can plate everything except the parts!

I'm assuming your reference to pH is that of the bath. Can you elaborate on the pH and how to control this? Will the zincate step need to be lengthened after longer immersion time in the de-ox?

Many thanks.

[woodjames]

Before you go too far, try getting the steel down in to the tank while still maintaining contact with the part, this is especially critical when the part first contacts the solution. You'll have to account for the plating out of the nickel sheets somehow.

Far as the de-ox goes, if you've got that white a piece you're fine. If you used a strong alkaline cleaner, you'd notice the aluminum turns a darker shade.

Keep in mind too that more often than not, and with the exception of cast it's best to double zincate. De-ox, zincate, rinse, de-ox again till bubbles are gone, rinse, zincate, rinse, plate. The second app of zincate is much more forgiving, there's a much larger window to achieve the correct color. Ohh, and it makes a huge difference using zincate if you can hang your part with an aluminum wire. I can prove why, just take my word for it. But then the aluminum wire needs to be changed before the nickel plate.

I've had the same zincate bath for a long time now, no additions.

-Jimmy.

[WayneCothran]

Thanks, Jimmy, for the guidance. The project has fallen through, so I don't know when or if I'll be able to try your suggestions. I've got a bitter after taste with this so I may not take in another plating project that involves aluminum.

In regards to hanging the part during the zincate process: Since only a small portion of these parts were to be nickel plated, I simply held the part with a gloved hand and submerged enough of the part to zincate the area to be plated. Lifted the part and using a spray bottle filled with distilled water rinsed the excess zincate before going into the EN bath. I was thinking I had read one of your posts where you said it was a good practice to do that to help preserve the EN bath.

Since most of this part was masked off with lacquer and about a three to one ratio of masked vs. exposed aluminum was in the bath. Could that be a reason so much nickel precipitated out of solution and fell to the bottom of the tank? I saw no traces of nickel attached to the masking, however. I have to believe this was the case because after about three parts the bath was spent. The exposed surface area was less than 30 square inches and I was using a 40 pint bath in order to get the surface submerged. I was making adds according to the 30 square inches and not the entire part. Parts were in the bath for 30 minutes. I haven't attempted an area calculation of the entire part as the geometry is rather complex. But, I'd hazard to guess what was submerged was approx 180 square inches with less than 30 in2 of that exposed.

I would really like to understand this failure.