EN plated parts failing salt spray. Is phosphorous level too high?

A. Hi Jim. If you can give us a chart or report about that galvanic compatibility of mid phosphorous EN with aluminum I'd appreciate it, because I didn't know that and it surprises me.

Meanwhile, as we await other responses, we appended your inquiry to a thread where several very sharp guys, including Ron Duncan (RIP) seem to be implying that trying to tweak phosphorous content for corrosion resistance or galvanic compatibility is unlikely to achieve much.

Luck & Regards,

A. Why? ... is it failing a QC test? Higher Phosphorous EN solutions cost more, are harder to control and do not get as many tank turnovers as lower P. Some shops have no problem, but more do have problems with it (opinion).

A. Salt fog type tests (whether neutral or acetic acid ) are mostly porosity tests. The phosphorus content of a high phosphorus coating does not have a significant effect of its results. If you are seeing reduced performance, I would suggest that you look at other factors like bath age or substrate quality, and not whether the coating is 11% or 14% phosphorus.

Most commercial high phosphorus EN Bath will not produce coatings with more than 12% Phosphorous, unless they are plating at a very slow rate.

Q. Always we worked about 10-12% Phos. But two months ago we discovered serious corrosion with acetic acid salt spray testing. At different laboratories, they informed us phosphorous content was 7%, 11% and 15%!
I would guess initially that 7% is a problem, but then when it appears 15%, I don't know now. So, I ask you if the 15% is a probably the cause of the corrosion.

Not enough information, so some general suggestions for determining the problem. As noted above, the P content isn't the root cause.

Poor corrosion results can be caused by inadequate surface preparation, insoluble impurities (inadequate filtration) or soluble impurities (buildup with metal turnovers and impure chemicals).

First, does the EN pass adhesion (bend) testing? If so, surface preparation may be OK.

Second, do a porosity test of as-plated specimens. E.g., alizarin test on aluminum, per ASTM B733. Failure would suggest better filtration is required.

Since your bath is quite old, the buildup of soluble impurities seems likely. How many metal turnovers [has it run]?

A. The mechanism is galvanic corrosion. Occurs wherever aluminum contacts a more noble metal in the presence of electrolyte (moisture + chloride). The voluminous Al(OH)₃ corrosion product lifts off the EN plating in large pieces.

There are many details (alloy, pretreatment, zincating, strike plating, dissolved & particulate impurities in the EN bath, rinsing and post-treatments) which may be involved. Chloride is present in many EN solutions, but EN plating should be non-porous and final rinsed in hot DI water. Was the EN activated for gold plating using HCl solution? Possibly, surrounding areas can be masked off and the gold applied by brush plating.

Use adhesion and corrosion testing on EN-plated test coupons to qualify the processing. Use at least 25 microns EN for outdoors applications. Passivation in 1% chromic acid at 50 °C for 15-20 minutes will improve corrosion resistance (especially on aluminum). Restrict any post-plating adhesion baking to 190 °C max (or omit entirely). [Heating high-phos. EN above ~220 °C yields a cracked, crystalline structure cf. the as-plated metallic glass]. For critical applications, require the plater to use ~new EN solution, 2 MO's (metal turnovers) max. A porosity test using sodium alizarin sulfonate is described in ASTM B733 and on page 186 of Electroless Plating, Mallory and Hajdu .

A. Try the Alizarin test according to ASTM B733 -- 04 paragraph 9.6.4
"Alizarin Test for Aluminum Alloys: Wipe the plated part or specimen with 10 mass % sodium hydroxide solution. After 3 min contact, rinse, and apply a solution of alizarin sulfonate prepared by dissolving 1.5 g of methyl cellulose in 90 mL of boiling water to which, after cooling, 0.1 g sodium alizarin sulfonate, dissolved in 5 mL of ethanol is added. After 4 min contact, apply glacial acetic acid until the violet color disappears. Any red spots remaining indicate pores."

A. Bob,
Yes, this is a common problem, unfortunately. There are many possibilities but I will just mention one. First, a few questions. You mention problem "areas". Part to part, are the pits located in random areas on the part or are the pits always in the same places on each pitted part? Are the pits located on a common face or diameter? The reason I ask is that if in fact, for example, the pits are only on an OD and an outer dimension, there is a chance that specifying a machined surface in a given area will require the machinist to order an oversized blank or oversized stock then mill away the pores to achieve the requirements. Or one can specify blank size or stock size to be used. In my experience, it's best to inspect for pores prior to plating.

Hello, Blake has some very worthwhile info here. I was curious if you do any bead or sand blasting on the parts prior to pre clean? Nice cross section by the way.

A. Hi Eddy. You may be highly experienced in plating on aluminum or in electroless nickel plating but not the combination, but we're don't know until you tell us :-)

One potential step-by-step is:
Non-etch alkaline cleaner
Alkaline etch
De-smut
Zincate
Nitric acid stripping
Zincate
Alkaline copper plating
Electroless nickel plating
Tarnish inhibitor
(Rinsing between each of those steps)

We have about 2 dozen threads specifically about the combination of electroless nickel plating on aluminum.

Regards,