Purpose of Petrolatum compound in Cleaning Procedure?

[ThereAre4Lights]

I looking at bonding and grounding of aircraft components. This process includes surface cleaning to ensure good conductivity between contacting parts. My main sources are AC 21-99 and SAE ARP1870A. The two have directions on cleaning of aluminum surfaces and I am trying to reconcile them and make sure they are communicating the same processes.

My question is that in the AC it says:
"Apply a coating of petrolatum compound to bonding or grounding surface of aluminium structure and clean surface thoroughly, using stainless steel wire brush with pilot as shown in Figure 13–4. Wipe off the petrolatum compound with a clean dry cloth."

The SAE document does not mention the use of a lubrication and I cannot think of a reason for it. Does the petrolatum help remove material? Soften the surface? Allow the brush to move more smoothly across the surface? I am not sure. Does anyone know? I wonder how important it is and whether we are OK sanding without it.

Any info would be great.

 

[aussiejohn2]

Hi ThereAre4Lights,
In the electrical world (e.g. busbar joints) it is common to brush to remove the surface layer of aluminium oxide. This is done through grease with the idea being that it prevents the exposed aluminium from immediately re-oxidising. Aluminium forms an oxide film in seconds-to-minutes (

https://hal.archives-ouvertes.fr/jpa-00223499/document).

Sometimes the joint compound also contains chemicals intended to remove the oxide. In these applications, normal recommendation is that the compound is *not* removed before forming the joint.
John.

 

[WKTaylor]

TA4L...

Unable to identify USA FAA AC 21-99 related to electrical bonding and grounding...

ARE YOU REFERING to Australian CASA AC21-99 Aircraft Wiring and Bonding ????????

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[ThereAre4Lights]

Thank you John, I'll read more on that.

Wil, yes that is correct, my mistake.

 

[WKTaylor]

Are You working to USA or Australian requirements?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[WKTaylor]

TA4L...

I was baffled by the grounding point surface preparation per CASA AC 21-99.

The verbiage for petrolatum use was inconsistent and is alien to me...

Petrolatum
Petrolatum compound
Petrolatum-zinc compound

Also the use of a rotary SStl wire brush to clean oxides at grounding-points on aluminum is alien to me. SStl wire brushes are forbidden for use on aluminum structural parts in USAF application.

Normally I would recommend using abrasive woven nylon pads [hand or jitter-bug or rotary] or abrasive-rubber bristle brushes to prep the surfaces; then clean with solvent and fresh process wipes; then apply clear conversion coatings (conductive) MIL-DTL-5541 Class III to bare aluminum; then the assemble the ['water-break' cleaned] hardware; then the ground point is checked for resistivity across the joint; IF OK, then the joint is entirely over-coated [buried] with 2-part sealant to prevent environment moisture, etc from attacking the dissimilar material stack-up.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[ThereAre4Lights]

Hello Will,

I am working toward USA requirements but the CASA reqs provided additional detailed information - I figured I'd gather the AC 43.13-1B, SAE ARP1870A, and the CASA documents to make sure I covered my basis and didn't miss anything. The SAE document also specifies a wire-brush for aluminum cleaning:

Your description of the process aligns with my understanding and process-write up. I think the area that I have swapped is the abrasive cleaning and the solvent cleaning. I thought we chemically strip paint first and then use abrasive action after to get what did not come off. ?

We have some paint stripper on hand but I'm on the fence about using that on aluminum. I've ordered alumniprep which I think will do the trick after abrasive cleaning. My coworkers seem to agree that the petrolatum is not needed. I may have to specify to our guys not to use the regular paint stripper though.

 

[enginesrus]

Petrolatum, Vaseline, Petroleum Jelly, its a thin grease that protects against corrosion.
Wipe it off with a clean dry cloth and it leaves a very thin film of corrosion protection.

 

[WKTaylor]

TA4L...

Obviously paint/sealant stripping is always step #1... by chemical stripper or very low abrasive means to prevent metal smearing [a very real problem with SStl wire brush].

CRES, Brass, steel bristle [alloys] are highly dissimilar to all aluminum alloys... IF fine particles from the brush become embedded in aluminum surfaces they can become a corrosion nightmare. WORST CASE: brushes [also] tend to be used indiscriminately between metals... so abraded particles from one job [EX: cleaning steel of rust/etc] can be transferred to another surface such as aluminum making situation far-worse. Gahhhhh@^%&***^%$$^^%$^%!!!

NOTES.
One form of CPC [MIL-C-11796] is petrolatum-based heated for ease of misting/spraying [petrolatum specs I'm aware of... VV-P-236, A-A-54492, MIL-P-37649, etc].
There is a petrolatum-zinc-dust anti-seize compound... A-A-59313... but I have never heard of it being used to aid in electrical bonding/grounding.
There are lubricants for electrical contacts... and a few corrosion preventative compounds [CPCs]... that are 'electrical conductivity' friendly.

IF You can find a copy of USAF T.O. 1-1A-14 [or NAVAIR 01-1A-505-1 or TM 1-1500-323-24-1] ORGANIZATIONAL, INTERMEDIATE, AND DEPOT LEVEL MAINTENANCE - INSTALLATION AND REPAIR PRACTICES - VOLUME 1 AIRCRAFT ELECTRIC AND ELECTRONIC WIRING... You might find that very useful... and more consistent with my experience.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]