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


A discussion started in 2005 and continuing through 2017 . . .


Q. We used to plate nickel phosphorus between 10-12% P. What is the matter with acetic acid spray saline when the phosphorus is higher than 12%? (about 13-15%)

Roberto Capria
plating ENickel - Pilar, Argentina


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).

James Watts
- Navarre, Florida


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.

Ron Duncan

- LaVergne, Tennessee

Ed. note:

It is our sad duty to alert the readers to the passing of Ron Duncan on Dec. 15, 2006. For those who would like to know more about him, a brief obituary opened Update No. 13 of our Metal Finishing Industry Update Podcast.


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.

Roberto Capria [returning]
- Pilar, Argentina

Electroless Nickel Plating
Wolfgang Riedel


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 B733-04 [link by ed. to spec at TechStreet]. 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]?

Ken Vlach
- Goleta, California
contributor of the year honored Ken for his countless carefully
researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work
which the finishing world continues to benefit from.

To minimize searching and offer multiple viewpoints, we combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition.

Massive corrosion of electroless nickel plated aluminum parts


Q. Aluminum parts of a radar system that were coated with 17 microns of electroless nickel 10% phosphor showed massive corrosion after about eight months service outside.

Using a SEM microscope with a EDX microanalyzer I found some very small pores in the nickel. They were were filled with chlorine type material.

There are other areas that have a gold flash over the nickel which were corroded severely even worse than those with the nickel without the gold.

What could be the mechanism?

Yosef Zaklas
Materials Engineer - Ashdod, Israel

Electroless Plating
Mallory & Hajdu


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 B733-04 [link by ed. to spec at TechStreet] and on page 186 of Electroless Plating, Mallory and Hajdu =>

Ken Vlach
- Goleta, California

Porosity test for EN plating on aluminum


Q. I'm trying to find a method for indicating porosity of electroless nickel plating on aluminium. I've heard about hydrochloric spot test, but can't find the standard. Are there any other methods (that are simple)?
Anyone please help me. thanks!

Shazatul Akmaliah
university student (researcher) - Malaysia

February 9, 2008

A. Try the Alizarin test according to ASTM B733-04 [link by ed. to spec at TechStreet] -- 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."

Gabriel Schonwald
Bnei Berak, Israel

Porosity in base metal causing plating problems

February 3, 2017


Q. I am having plating problems such as visible peeling and blistering and voiding in areas. Typically we are plating small precision machined housings from 6061-T6 aluminum alloy. The plating is typically electroless nickel and then gold. We did some cross-sectioning and SEM analysis and are finding micro-pores (micro-voids) in the base metal. These voids get plated over and entrap contamination, plating fluids, rinse fluids, etc. and then bleed out or pop out during further processing of the part. Has anyone encountered this porosity problem in the base material and what can be done to mitigate it?

Bob Diamond
Mechanical Engineering - Hauppauge, New York, USA

February 6, 2017

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.

blake kneedler
Blake Kneedler
Feather Hollow Eng.
Stockton, California

February 7, 2017

Q. Blake,

Thanks for your response to my aluminum 6061-T6 porosity question. The porosity appears in random areas of the machined parts. These parts are typically machined from rectangular bar stock and consist of machined troughs, holes, etc. It appears that the deeper the machined cut the less porosity indicating that the porosity is closer to the exterior surfaces of the raw stock.

Bob Diamond [returning]
Mechanical Engineering - Hauppauge, New York, USA

February 7, 2017

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.

Mark Baker
Process Engineering - Phoenix, Arizona USA

simultaneous February 8, 2017

Q. Thank you for all of your responses... I can answer the following:

Yes, we aluminum-oxide media for blasting and/or glass bead blast the parts. That process leads me into another topic. We have found that aluminum-oxide as the blast media can get embedded into the base material and not fully cleaned off by the machine shop or the plating shop. This embedded aluminum-oxide leads to additional plating adhesion problems. We are trying to change to having our machine shops use glass beads as the blasting media.

The issue of contaminated cutting fluid etching into the base material has never come up in our analysis. That is interesting and I will bring that up to the team here working on this problem. Thanks!

Bob Diamond [returning]
- Hauppauge, New York, USA

February 8, 2017

A. A while ago I encountered a similar problem with a specific batch of cnc turned and then milled parts sent to me to be anodized, after dying the parts porosity was observed near some outer edges of a dish-plate sort of shaped part where sulfuric would leach out and eat up the dye, material was 6082, the pores are barely noticeable at 20x magnification before anything was done to the parts.

This wasn't the first time I anodized those parts for the same customer, same material, same everything, and after some back and forth with the customer and the people that turned/milled the parts, the problem "magically" went away never to be seen again.

I suspect that it was due to contaminated cutting fluid they used (oil emulsion) that was left too long on the part during some operation and etched into the material, my solution was just to etch them longer in a bit more concentrated caustic and at higher temperature to enlarge the opening of the pore, and then spend more time in rinsing baths to get all the stuff out of them before consequent steps, was able to salvage that batch of parts, at the cost of a cosmetic (d)effect - longer etching times tend to make 6082 very matte surface texture - which wouldn't be so bad with 6061

Janis Ziemelis
- Riga, Latvia

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