Pitting of electroless nickel over 2024-T351 1

[edrush]

Is electroless nickel plating on 2024-T351 prone to pitting? I have an assembly with multiple materials in contact (alum, nickel plated aluminum, SS, bronze, titanium, and sometimes carbon fiber). The whole system should be electrically isolated from ground (i.e. the ground)

The nickel plating is flaking off after 3 months service, primarily on o-ring sealing surfaces. I believe I see pitting in a single part off the shelf before assembly. The majority of pits are very small and appear to be very shiny in the pit (no apparent corrosion product). I have limited microscope capacity and the scale is small enough that I can't be sure it's not an artifact from machining. I also see larger pits, some of which appear to be a perfect spherical indentations and others which have odd shapes. The odd shape pits are more what I would expect from a corrosion process. The confusing thing to me is the very smooth surface in these "pits". I would expect to see some corrosion product.

The plater makes is sound like plating 2024 is like falling off a log (i.e. they do it all the time).

Any advice/experience will be greatly appreciated.

 

[kenvlach]

Not necessarily a simple problem. I have some info, but also some questions:
What was the plating specification for the EN?
Were certs included?
Was this low, mid or high phosphorus content EN?
Plating specifications allow for an adhesion bake at 250 F (125 C) for 1 hr (or longer) for heat treatable Al alloy: Grade 4 per ASTM B733, MIL-C-26074E and AMS 2404D. Was this specified?
Do you have any of the original material and certs?
What are the service conditions (liquid, T, flow rate, etc.)?

It is true that Al 2024 is EN-plated all the time, but it is also true that it can suffer adhesion and/or corrosion problems. Also, since the EN is a barrier layer only (no galvanic protection), once corrosion starts, it will spread at the Al/EN interface. Your description of the pits (smooth, shiny, w/o corrosion products), suggests pitting prior to the EN plating. This could indicate crappy initial material containing (non-solutionized) primary solidification phases, which can cause smears during machining and pitting during the plating pretreatments. This can be determined by metallography (primary solidification phases and refractory inclusions can be seen with an optical microscope, unlike precipitation phases from heat treatment). If original material is available, please examine.

Are the o-ring seals at the edge of threaded holes which were plugged during plating? A frequent problem is solutions undercutting and getting trapped (not rinsed) beneath edges of masking materials. Do some adhesion testing to see if this is the only problem area. Adhesion test: Where the plating is beginning to flake off, take a scalpel or other thin blade and try to separate more of the plating, beyond any initial corrosion. See the following for more info:
Adhesion characteristics of electroless nickel
thread332-64937

I do not recommend the adhesion bake w/o knowing the service environment, as the baking reduces the corrosion protection of the EN (microstructural changes occur in the initially amorphous EN). However, there are supplementary seals (extra cost option, not standard practice) for EN coatings.

Hope this helps. Please furnish the additional information.
Ken

 

[edrush]

Ken -

The EN is 4% P. It is baked after coating, but I don't know the time and temp (and I had better get them defined). I don't have certs.

There were no plugged holes or masked areas.

This is a manually operated vacuum pump which pulls vacuum on skin when walking. It can pull sweat through the system, but it is not typically connected in this fashion. Operates in ambient conditions (-40F to 110F) at approximately 1 hz. The majority of the problem occurs in the vacuum chamber, although there are problems elsewhere also.

Raw material problems seem unlikly, but you are correct, the symptoms seem to match this diagnosis.

I have done many subjective tests (generally beating, scraping, cutting and gouging) in an attempt to get the coating to peel off. It seems to have good adhesion everywhere except in the problem areas. The plater says adhesion problems typically exist over the entire part.

As I have looked more closely am beginning to conclude this is a lousy plating job. I see occasional inclusions in the nickel, debris under the coating, and small areas of what may be surface corrosion in the plating, or possibly a variation in composition of the coating.

I choose the 4% for linear bearing wear resistance. I may have to give it up and go to an amorphous nickel or a hard anodize. You are correct, once the coating is breached either the aluminum or the interface corrodes very quickly.

I just looked at one part which did not exhibit any wear (looked brand new). I peeled off a rubber bumper and underneath (an area where there is no contact or stress) I see flaking and a white corrosion product. I will find out what the composition of the corrosion product is. I question whether the air being pumped through the unit is an effective ion transport mechanism for galvanic corrosion, as after the first few steps there isn't much air left. But maybe the fact that the air is being pumped makes it a very effective ion transport system.

Thanks for your help!

 

[kenvlach]

Although sweat is actually quite corrosive, lack of adhesion is a plating problem which engenders corrosion.

“It seems to have good adhesion everywhere except in the problem areas.”
Is the o-ring in an ID or OD area? ID areas may not have been properly rinsed during pretreatment, and require good circulation of the EN plating solution. Problems also occur at racking contact points. Wiring is preferable, as EN plates pretty well under loose wiring as long as there is strong agitation of the solution.

“I see occasional inclusions in the nickel, debris under the coating”
Indicates a dirty shop and poor filtering of the EN solution.

“I just looked at one part which did not exhibit any wear (looked brand new). I peeled off a rubber bumper and underneath (an area where there is no contact or stress) I see flaking and a white corrosion product. I will find out what the composition of the corrosion product is.”
1) Adhesion problems are sometimes present in beautiful EN parts, as the EN plating will bridge over surface impurities.
2) The corrosion is aluminum hydroxide, possibly with some zinc from the zincate pretreatment. It may have begun forming as soon as the EN plated part was rinsed. EN plating is done at ~192 F, initial rinsing is at room T and a final rinse is in hot DI water (to avoid stains and aid drying), so the differential thermal expansions exacerbate any adhesion problem, allowing corrosion to begin immediately.

“this is a lousy plating job.”
Agree. Sounds like lowest bidder work.

Suggest finding a new vendor. Request to see their last 7 years of monthly qualification tests (especially, adhesion and salt spray), preferably signed reports from an outside lab.

If switching to anodizing, note that conventional (Type II per MIL-A-8625F) offers better corrosion resistance than hard anodize (Type III per MIL-A-8625F). Ask to see both the vendor’s salt spray (Type II) and abrasion (Type III) test results; again, preferably signed reports from an outside lab.

Good luck. One last suggestion: do some rigorous first article testing (salt spray, EN adhesion).
Ken

 

[edrush]

Thanks again Ken. I am not as focused on the pitting now, as I think you were correct - it is probably in the base material and may not compromise the coating.

One last statement. Since the nickel is more cathododic than the aluminum, from a theoretical standpoint this coating seems destined to fail unless it is completely coherent. In fact, from a corrosion standpoint nickel seems useless as the only substrates it could protect are stainless, gold, titanium, platinum, etc. I think it is unrealistic to expect the plating to be completely coherent on 99% of parts throughout the lifecycle.

 

[kenvlach]

Don’t give up so easily on EN plating. Its major uses are plating Al and steel for corrosion resistance and decorative purposes. EN is most of the plating thickness on many chrome plated Al wheels. Your descriptions indicate that the plater doesn’t do MIL spec work, did poor pretreatment, used lower quality chemicals (a high quality zincate is especially important), didn’t filter the EN solution, lacked quality controls, etc., etc.

See ”Properties and Applications of Electroless Nickel” for some good EN at

http://www.nidi.org/multimedia/tech...erature_pdfs/10000_technical_series/10081.PDF

The corrosion resistance of EN-plated Al can be improved by a post-plating dip in 1 wt% chromic acid/DI water at 120 F for 15-20 minutes. This chromates any exposed Al in pores or at edges of the coating, and passivates the EN. Especially beneficial on poor plating jobs. Can’t solve adhesion problems; that depends upon proper pretreatment.

If you do switch to anodizing, crappy Al 2024 (having primary solidification phases and/or refractory inclusions) will have pitting corrosion problems. Do salt spray testing.

Lots of luck,
Ken

 

[usjbh]

I am late to this thread but could not help but want to add my 2 cents. 2024 is a popular alloy for machining but is probably the worse alloy to use in an environment where there is corrosion potential. The 4% or 2 cents worth of copper added to improve 2024 machinability is its downfall. I would consider changing to 6061 or 6063 T6 or 5052 H32 (.1% Cu) to improve the galvanic coupling problems if you can take the tensile hit.
Another problem I've experienced in the past with 2024 is smutting or more spefically properly de-smutting when etching prior to plating. In my case it was for chromate coating but the problem exists regardless and can contribute to contaminating the surfaces.

 

[edrush]

Thank you for the input USJBH. 2024-T351 is very attractive because it has almost twice the elongation as 7075-T6 or 6061-T6. This translates to better fatigue life. Anodizing 7075 results in large fatigue strength scatter (varies with lot and anodizer), and hence risk. We already have a big chunk of our rear ends missing from this scatter.

But 2024 it is a pain in the a** to plate or anodize. My machine shop tells me it can't be anodized because it will burn up. Anodizers say it's difficult, but can be done. My european colleagues use anodized 2024 on many products and don't see a problem. Maybe it depends on the part, but I have tight tolerances and class 3 threads. So anodizing the 2024 seems problematic.

I am not sure the pitting is the problem with the EN. I believe the pits occur before the plating and hence the pits are plated. I am convinced I have good adhesion. The bottom line is that I have an o-ring reciprocating at 1 hz in a .0005-.0007 4% P nickel plated bore and it's exposed to salt water the bore has life of 90 days. I am not certain how the salt is getting to the substrate, but once it does the plating comes off quicker than Michael Jacksons pants at a slumber party.

I am considering a zinc sacrifical anode, sticking with the nickel and possibly using a duplex coating of high phos covered by low phos. But placement of the anode is a challenge. And the duplex coating may be risky (it is prohibited by the AMS spec).