pencils ? ... pencil lead reacts badly with Aluminium (or rather Al reacts with pencil lead).

Black ink ? ... what is added to the ink to make it "black" ? could it be carbon ?? Carbon reacts badly with Al.

In my history was with a c/- when we did a colour change for our cabin insulation. This was untracked (who cares about the colour of the insulation, completely interchangeable with original, no need to record substitution.)

Yep old colour ... silver, new colour ... black.

This created about the biggest "sh**-sh**" to unravel, once reports of corrosion started coming through.

another day in paradise, or is paradise one day closer ?

Carbon-black pigments in all paint, inks, dyes/stains, fillers, etc must be avoided at all cost... for exactly the reasons described above.

Exceptions will be for black paint markings over multi-layer/deep* coatings of [inorganic finishes for metals] + primer + top-coat [base/over-all-decorative coating] = isolation from harm.

OH yeah... and don't forget that built-up engine exhaust carbon-soot can become a defacto coating that is extremely corrosive. Paint coatings that are rough, porous, lusterless, etc tend to entrap/embed soot that 'bakes-in'. Insidiously, carbon soot can migrate into unsealed/poorly sealed cavities anywhere the engine exhaust can flow-to [including 'forward' by recirculation]. When combined with moisture... especially salty-oceanic-air... the drippy-wet-soot migrates by capillary action into microscopic openings/gaps and will damage most unprotected/thinly-protected* metal alloys... Mag, Alum, iron/steel, SStl, Cu, Titanium [especially with chlorides]... with some exceptions for the oxidation resistant super-alloys... fay surfaces, bearings/bushes, fasteners, etc.

* All organic coatings are permeable... thin/single-layer primer coatings will eventually be penetrated by moisture and carbon molecules.

** Combusted-fuel engine exhaust soot is not just 'pure carbon or carbon-oxides', it contains trace water and combusted-fuel-additives... such as anti-icing, anti-biologics, anti-coking, metal scavengers, lubricity enhancers, etc... adding to the mess.

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]

Not kidding, but the most benign markings you can use on metallic parts must be wax crayons.
You can get a "wax pencil" if you don't want to look like you're in kindergarten.
Orange shows up pretty well on everything.

Living the dream, problem is nightmares are also dreams

Simple issues/questions in our business RARELY-ARE SIMPLE... we often don't give much thought to the 'simple' and 'long-established tribal knowledge'. I can't tell You how many time questions/issues about 'simple stuff' and 'tribal knowledge' have driven me insane because the root/origin of the stock answer is... uhhhhhhhhh... lost/forgotten. I love to explore those rabbit holes.

As-in-everything there are exceptions... black colors can be 'imitated' by a mixture of colors [pigments] without carbon black... BUT without explicitly KNOWING that carbon black is NOT included in the ink or paint, then default to the known. NO, NOPE.

All, some philosophy... Sooorrryy.

We are in a challenging, exciting, evolving, technical profession that has both rewarding and unforgiving outcomes.

I have learned-to-learn from every one of MY experiences and from other's genuine experiences [spoken and read]... and to quickly discriminate facts from fiction [BS] by constant self-study and carefully connecting widely spaced dots using deeper knowledge + experience. It's amazing how much BS is 'out-there'.

When asked technical questions I always start from the beginning and work towards the answer(s), independently validating data and 'facts' and building the picture from scratch. Thankfully experience [real-life, analytical, knowledgeable contacts, etc] speeds things up tremendously.

As a service, liaison, materials/process and design engineer... and mishap investigator... the lessons-learned [LL] and knowledge absorbed from established and trusted tech data... and have been humbled too-many times to count by what 'I didn't know'. I am still on a interest-driven learning curve at 69.5-YO. OH, YEAH... Young engineers, old aircraft and the world of today are evolving challenges in learning AND TEACHING. Far-more-often-than-once, knowledge and experience from one 'source', has enlightened me on a seemingly unrelated problem, later-on.

As most of You know I collect quotes because I like to see the world from so many perspectives. I'll only present just few as Fud-4-Thot today... and the quotes in my signature block.

"Aviation in itself is not inherently dangerous. But to an even greater degree than the sea, it is terribly unforgiving of any carelessness, incapacity or neglect.” --Captain A. G. Lamplugh, 1930, pilot

"It is necessary for us to learn from others' mistakes. You will not live long enough to make them all yourself." -- Hyman Rickover, American admiral

"I'm not young enough to know everything.” --J.M. Barrie, writer

"I'm too old to know everything.” –Oscar Wilde, playwright

"If you done it, it ain't bragging.” –Walt Whitman [its often VERY humbling]

"It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so.” --Samuel Clemons, AKA Mark Twain

And for fun...

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]

Sharpies are forbidden on any high strength steel components. The acid in the ink will cause an initiation site for stress corrosion cracking (SCC). I believe Boeing issued a memo years ago.

It's not the acid, it's the chlorides. There are safe markers. Don't judge by the color.

https://industrialmarkingpens.com/collections/low-...

I have seen a document a while back that had pictures of cracks in the shape of letters from SCC due to the chloride content of the ink

Interesting thread. I knew about graphite from pencils causing problems on aluminum skins. I used to work at Mooney and this was a well-known no-no there. But I hadn't heard about the issues of chlorides or acids in Sharpie ink, and hadn't thought about the carbon dye in black Sharpie ink. I work at an general aviation OEM and we use red Sharpies to mark rivet rows on rib flanges. At this point the ribs are already primed so it the ink isn't contacting the bare aluminum so I would think this isn't a problem. I haven't seen black Sharpies being used in the shop, but then again I haven't looked that hard. I'd be interested in learning more on the subject so I can educate our shop folks on the possible issues.

Does anyone have and can port or share a link to the documents referenced above (Boeing memo ERAU82 mentioned and the document TugboatEng mentioned)? Or other documents/lessons learned on the subject?

Found it!

TugboatEng, thank you!

I couldn't help but think of this thread when I saw this fun article.

I say it's on-topic because it's about aviation, Sharpie markers, and even their color.

https://askthepilot.com/me-and-my-sharpie/

Sooooo... Tug... Your... 25 Oct 22 13:52.

RE the # 357 written in felt-tip pen that appears to be crazing where the numbers were written...

Please advise...
WHAT COLOR was the original ink on the 316L tube?
WHAT brand of marker was tested?

Hmmmmm... I cannot remember any smell of ascetic acid [or other acids] coming from my Sharpie permanent ink pens.

Found this article... What Chemicals Make a Sharpie Permanent?... There was NO mention of acids.
https://www.ehow.com/info_10055820_chemicals-make-...

... ...
Ingredients
While most washable inks are water based and designed to break down easily, Sharpies use water-insoluble ink that contains Permachrome, a pigment used in ink for ink-jet printers, and xylene and toluene as binding agents to give the ink longevity. Xylene is a clear solvent used in paints and varnishes, and toluene is typically found in crude oil and gasoline. The chemicals are relatively safe in the small amounts used in Sharpie markers but can be toxic if inhaled in larger amounts.
... ...

Regards, Wil Taylor
o Trust - But Verify!
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", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

Please see my comment on 25 Oct 22 01:55

I said it was NOT acids that caused the problem. Lots of inks have chlorides. Here is a list of a few colors that do:

https://www.siegwerk.com/fileadmin/DATEN/ProductSa...

My recommendation was to purchase markers that are listed as halogen free and not rely on judging them by their color.

Did you look up an SDS for "Permachrome"? It has a proprietary ingredient. We have no idea what it is.

Tug...

I concur that not all chloride containing chemicals have an acidic [or similar] noticeable smell... and may actually still exist in marker pen ink in tiny quantities.

Best plan is to consider using marker pens, ONLY on primed or painted surfaces... or clad aluminum only... and ensure the marking is 100% removed by solvent cleaning prior to protective finishing. AND avoid use on 3xx SStl: use paint pens [enamel, lacquer, etc] ONLY.

I have seen a NASA study of chloride pitting attack on 3xx SStl for ground equipment due to proximity of oceanic environments... and have witnessed odd [inconsistent] attack on 321/347 [welded] SStl bleed-air ducts.

Regards, Wil Taylor
o Trust - But Verify!
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", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov