hard anodize, mil-std-8625, type III 4

[dho]

I remember many years ago I read one article about hard anodize. it said the top layer of the HARD anodize would be not dense. so, the proper tech was to build up more and grind off the top layer, say .001". now, I search for that article and do not have any luck. anyone can support that statement, or dispute?
thanks.

 

[CoryPad]

Hard anodizing is porous throughout, which is why it is standard practice to seal the porosity (e.g. with boiling water, nickel acetate, etc.). Grinding usually is for adjusting the thickness of the layer, or to remove it altogether.

 

[tbuelna]

CoryPad-

What you noted about the porosity of hard anodize is correct. But the common sealing processes you noted can reduce the abrasion resistance of the anodize coating surface. If you take a look at the relevant sections of MIL-A-8625F shown in the attachment, it states that sealing of type III anodize should not be used unless corrosion protection is a primary concern, since it reduces the abrasion resistance (hardness?) of the coating.

I did not see anything in the spec to indicate the properties of unsealed type III anodize are not uniform through the coating thickness. However, the way I was told that the hot water sealing process works is by causing the oxide molecules at the coating surface to swell and close off the adjacent pores. So finish grinding a type III anodize coating that was sealed using boiling water would appear to effectively remove an outer layer that has reduced hardness. But doing so would not make any sense since the finish grinding would also remove the sealing that was just applied.

 

[dho]

grinding serves two purposes. 1) bring it to dimension. 2) bring it up to finish.
will the grinding remove the SEALED top layer is subject to discussion. 1) does the sealing go through the full anodize thickness. 2) if sealing does not go through the full thickness, expose the un-sealed anodize should INCREASE the wear resistance (yes, it defeats the purpose of sealing).
but i did read one paper saying the top layer is not dense as the bottom layer.
thanks to all.

 

[CoryPad]

Terry,

I agree with you. I only wrote about sealing because that is the only thing I have heard related to porosity. The sealing process creates hydrated hydroxide gels that fill the pores in the oxide layer.

dho, it is true that there is a non-porous layer adjacent to the aluminium surface, but it is very thin and you could not grind to it and expect to have the performance of the hard anodizing layer. For a nice schematic illustration of this anodizing structure, take a look at Figure 10 here:

http://psec.uchicago.edu/Papers/Chemical_Reviews_Anodic_oxide_films_on_aluminum.pdf

 

[WKTaylor]

A few added thoughts RE Type III [hard] anodize finishes.

Hard anodize is an exceptionally brittle coating that can initiate premature fatigue cracking of the base metal. IF the base metal is anodized on end-grains or re-entrant-grains, then this can REALLY aggravate fatigue crack initiation and potentially initiate stress corrosion cracking. Fatigue life reductions of 90% [IE: 10% expected life, actual] are possible without careful attention to details!

IF at all possible, the surface to be Type III anodized should be shot-peened to maximum authorized intensity and at least double coverage. The anodic layer will build on the thicker shot-peened [compressed] layer and will retain the value of peened-compressive surface for retarding thru coating cracks into the base metal.

NOTE.
It is extremely important to ensure all sharp corners/edges are generously radiused before/after anodizing [spec has recommended minimums for each type coating Type [=thickness]. This include holes drilled/reamed after the anodize has been applied. My rule, based on long experience, is that small open holes, fastener holes and/or bushed holes should NOT be anodized. These holes should be finish-machined after anodize is applied [may pilot the holes, but must ensure final ream completely removes and anodic coating]. These reamed holes must have their edges carefully radiused [high quality], and all bared-metal should be coated with chromate conversion coating and sealed with wet primer or sealant ['wet assy']. Reasoning is simple: anodized coatings are brittle! One... or worse... two anodic coatings merging at a sharp corner create not just a higher potential for cracks, but also a great potential for coating to chip/flake off. The ragged/brittle edges remaining as chips are liberated expose base metal to corrosion and are stress risers for crack initiation [cracking generally accelerates in the presence of corrosion].

NOTE.
Paint [primer] will never sick to a sharp edge. Radiused edges allow thin paint coatings to form a continuous surface film that ‘wraps-around’ the edge. Sharp edges result in two-paint films trying to merge, but the sharp-edge discontinuity prevents the films from joining. This results in a micro-thin coating on the sharp edge that will likely chip off with/without assistance of chipping metal along the edge.

NOTE.
IF other areas of the part need to be anodized, but need corrosion protection only, then mask-off these areas and anodize them separately to Typ I, IC and IIB [thin films, preferably sealed].

NOTE.
I know of one application where a semi-polished [8--16-microinch Ra] type III anodized surface is coated with a baked-on solid film lubricant [300F long-bake]. This is followed by a burnishing [dry buffing] of the SFL coat to semi-mirror finish 16--32 microinch Ra] that tolerates sliding hard synthetic rubber seals and is highly resistant to jet-fuel, hydraulic oil, moisture and... ultimately... corrosion/erosion.


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[WKTaylor]

dho... a minor point... the spec You cited is actually MIL-A-8625 not MIL-STD-8625.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[dho]

sorry.
yes. should be MIL-A-8625.

 

[dho]

http://psec.uchicago.edu/Papers/Chemical_Reviews_A...

"It has been known from as early as 1932 that the anodic oxide film on aluminum consists of two regions: an outer region of thick porous-type oxide and a thin, compact inner region lying adjacent to the metal."
unfortunately, not quantified.

 

[CoryPad]

Figure 10 quantifies it at the Angstrom level.

 

[tbuelna]

CoryPad- That 1968 paper from Diggle, Downie & Goulding was great. Much of the chemistry described was over my head, but I was able to grasp the basic concepts. Section D.1 and figure 15 give a nice description of how the hot water sealing process closes off the pore mouths.

wktaylor- Thanks for the comment about shot peening prior to hard anodize to inhibit propagation of coating cracks into the aluminum substrate. I don't use hard anodize much and I had not heard of this approach, but it seems to make sense. Any suggestions where I can read more about it?

 

[CoryPad]

Agreed, Wil's suggestion is solid gold.

 

[WKTaylor]

Been a rough last week... wife's major surgery was very hard on her/me... but she's on the up-swing and her friends are taking-over with her recovery next few weeks; so I am able to get back to work [different city] with the old boy dog stuck with me for care/feeding.

A few additional points before, I have to dig-in deep at work and disappear from E-T scene for awhile...

DHO…
My initial answer ‘presumed’ a hydraulic cylinder made from a thick wall drawn aluminum tube or a forged block where grain is fairly uniform.

IF on the other hand Your cylinder is made from a die forged cylinder body, there are other considerations that apply [and would also apply to a non-hydraulic forged fitting or component]. I have seen many forged-cylinder-bodies crack on exterior or secondary features … that followed the recommended practices I’ve mentioned above for the ID [SP-IDs/Type III HA/ground to bore-dimensions/etc]… but failed to deal with the forging-parting plane grain disruptions. In these cases fracture occurred on external or internal features machined into, or close-to, the DF parting plane [such as bored-holes, threaded ports, lugs, etc]. These failures occurred along grain boundaries that abruptly turn-outward at the DF parting plane. Simple features such as machined DF flash machined back to cylinder contour, or lugs/lug-holes should also be shot-peened [or hole-cold work processes] after machining. But internal features on/adjacent to the DF parting plane, such as threaded ports, holes, grooves etc will still present a major stress/fatigue problem not easily dealt with… short of a DF body re-design that moves the parting plane away from these features.

The issue of metallic materials, material grain, finishes, crack initiation/stable-crack growth/unstable-catastrophic crack growth [rupture], W/WO significant cyclic loading and W/WO corrosion elements present is highly complex… hurts my brain.
In an attempt to ‘look really smart about this subject’ [yeah, right], I have attached a zipped file with several ‘light reading’ tech reports extracted primarily from DTIC website <<

http://www.dtic.mil/dtic/

>> spanning the breadth of this issue in an attempt to shed light on very important co-related concepts.

NOTE. There are a LOT of other documents/papers ‘out-there’ that I’ve read/absorbed over the years that contributed to my base of understanding… begged-borrowed-stolen… that I CANNOT allow to be viewed in this forum due to corporate proprietary, or DoD FOUO, reasons.

I suggest starting with the following… then poking around the others as time permits.
Shop-Awareness Briefing
NAVWEPS 00-25-559 Tips on Fatigue.
USAMERDC Report 2339 The Effect of Surface Coatings on the Fatigue Strength of Aluminum

Listed [below] are some important documents/resources on this topic that are available thru the organization listed.
SAE HS-84 Manual on Shot-Peening
ESDU 92015 Guide to the effect of shot peening on fatigue strength.
AD0735409 Shot Peening for Improved Fatigue Properties and Stress-Corrosion Resistance [Battelle]
Forging Design Handbook, ASM 1972

MIC [Metal Improvement Corp] has a significant technical library on all aspects of peening and life enhancement [Fatigue, SCC, EXCO, Fretting, etc] by various peening methods.

http://www.metalimprovement.com/

NOTE. Peening now comes in many flavors... it is mind boggling.
Manual or automated blast-peening
Metallic media shot blast-peening.
Glass bead blast-peening
Ceramic bead blast-peening
Rotary flap-peening
Vibratory peening, Ultrasonic peening
Laser shock-peening
Peen-forming

Personal NOTE. For a host of reasons I prefer Ceramic Beads for Blast-Peening [and peen forming] processes for aluminum alloys.

Hmmm... as usual I have once-again gone 'out-in-left-field' on this response...so-be-it.

Rut-Ro... will have to break the zip file into (3) pieces, separate postings.

http://files.engineering.com/getfil...ue_Corrosion_SCC_Shot_Peening_&_Coatings1.zip

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[WKTaylor]

RE previous posting... Zip file part 2 attached.

I Think I'll stop at this one... too many documents would be way-too confusing.

http://files.engineering.com/getfil...ue_Corrosion_SCC_Shot_Peening_&_Coatings2.zip

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[dho]

thanks a lot, wktaylor. wish you the best.
actually, my application is a thin wall cylinder. the piston (reinforced teflon jacket seal) 0.1" DA oscillating inside at helo blade Hz. the pressure is low in dozens psi. wear is my main concern. we currently build up .002plus and grind off .001minus for size and dense anodize layer.

 

[tbuelna]

Just to be clear, this application involves a low pressure hydraulic piston actuator with a short stroke and fairly high frequency. But what surface do you intend to hard anodized- the piston OD, the cylinder ID, or both?

If you require a cylinder ID coating that is extremely durable and works very well with marginal oil lubrication conditions, I would suggest taking a look at Nikasil type coatings designed specifically for use on aluminum cylinder bores. It is an electroless nickel matrix with silicon carbide particles embedded in it. After plating, some of the outer surface of the nickle is etched away leaving tips of the SiC particles protruding slightly. The SiC tips are then given a diamond finish hone to create uniform plateaus surrounded by valleys just a few microinches deep, which retain a super thin film of oil in operation. The coating is designed to work with a mating piston skirt surface of plain aluminum. The electroless nickle matrix easily tolerates the extreme thermal conditions and surface contact pressures of an engine cylinder without cracking, spalling or flaking. Probably a much better choice than hard anodize for an aluminum cylinder bore. It won't cost much more than hard anodize if you can find a vendor that has existing tooling for your bore size. But if you have to make tooling it can get a bit pricey for small quantities.

 

[WKTaylor]

dho... sorry for my last rant.

NOTE: tbuelna's suggestion RE nikasil coating the cylinder inner bore may be a winner.

T... any hard data source [authoritative] on the Nikasil process? I have been looking for process and performance data on-off for a few years...

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true.
o For those who believe, no proof is required; for those who cannot believe, no proof is possible.
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can break them like an artist. [Picasso]

 

[tbuelna]

wktaylor-

The original Nikasil coating was developed by Mahle. There are several vendors in the performance auto/MC aftermarket that rework Nikasil coated cylinders. I'm sure they would provide you any technical data they have or explain the process in more detail.

I erred in my previous post describing Nikasil. It uses electrodeposited nickel and not electroless nickel.

One issue with using Nikasil is that the available equipment is designed for engine cylinder bores that have certain L/D characteristics. So the equipment may not work with hydraulic cylinders that have much greater L/D ratio.