Lye as an aluminum cleaner?

You can say that again. I printed that text out for my keeper file.
Excellent information. Thank you.

Don...

And the problem with KOH is???

Jim


(NOT Drano, as it
contains potassium hydroxide, in addition to the sodium hydroxide)

In article ,
"RST Engineering" wrote:

And the problem with KOH is???

Jim


(NOT Drano, as it
contains potassium hydroxide, in addition to the sodium hydroxide)

Too reactive!

You are telling me that KOH is more reactive than NaOH? That's not what
they taught me.

Besides, this whole thread is remarkably void of solution concentration data
.... just that "lye" is really bad for aluminum. Hm. How about a gram of
solid NaOH in a hundred liters of water? Ten liters? One liter? A hundred
milliliters? What is the optimum cleaning-not-disintegrating concentration
and what time span do we use for soak?

Jim




"Orval Fairbairn" wrote in message
news
In article ,
"RST Engineering" wrote:

And the problem with KOH is???

Jim


(NOT Drano, as it
contains potassium hydroxide, in addition to the sodium hydroxide)

Too reactive!

Michael Horowitz wrote:
I've taken my aileron apart and notice corrosion on the thin aluminum
parts, so I'm reading around about cleaning prior to alodining. I've
taken a stainless steel brush to the surface and removed most of the
corrosion, but it's really a PITA.

Someone suggested using lye as a cleaner. Anyone have any experience
using supermarket lye in that role? - Mike

------------------------------------------------------------------------------------------------------------

Dear Mike,

The suggestion by 'Someone' is a nice example of Conventional Wisdom,
which is always WRONG even though based on a kernal of truth.

The kernal of truth is that sodium hydroxide (ie, common lye without
any additives [meaning you can't use drain cleaner] get the stuff used
for making soap) has long been a standard in the preparation of
aluminum surfaces. Then comes a host of 'unimportant details' always
ignored by the Conventional Wisdomites, such as the exactly strength
and temperature of the solution, how many minutes (or even seconds!)
the part(s) is to be dipped, and the number & nature of the following
neutralizing rinses.

But the main fallacy here is that your primary interest is NOT
surface-prep but elimination of corrosion.

Airplane don't use much PURE aluminum. What we use are aluminum
ALLOYS. For aviation use, the most common alloying elements are
copper, magnesium and zinc, with manganese, silicon and tin being less
common.

'Corrosion' in airframes falls into two broad categories with WATER
common to both. In the first case, water reacts with the alloying
element, producing by-products with then react with the aluminum. In
the second case IMPURE water reacts with both the aluminum and its
alloying elements.

About the only time you see the first case is when the water comes from
snow. In all other caes, including rain water, due to contaminants in
the atmosphere (mostly sulphur but LOTS of otheres) the water is
sufficient impure to react directly with both the base metal and the
alloying element. (Ever heard of 'acid rain'? It's even more damaging
to airplanes than it is to forests, people, stone buildings and so
forth.)

The type of 'corrosion' we're dealing with is usually an oxide (but it
can also be a sulfide, halide and so on). To get rid of the corrosion
you need something that attacks it more vigorously than it attacks
either the aluminum OR its alloying elements. Which is why lye is a
bad, bad idea for corrosion removal. (Lye LOVES aluminum! It eats it
all up and rubs its tummy for more.)

Phosphoric Acid (as found in Coca-Cola and other soft drinks) is a
good, good, idea when it comes to cracking the code for aluminum
corrosion, since it loves the oxides more than the aluminum itself.
(Notice the can your soda-pop came in?)

Phosphoric Acid that has 'Aviation Certified' on the label costs a lot
more than Phosphoric Acid which does not.

For light corrosion, lay the brush aside and try a wad of aluminum foil
as your scrubber. Next step up is a Fine-grade non-metallic abrasive
pad. In each case, allow the acid to treat the scrubbed, abraided
surface.

For thin gauge sheetmetal, that's about as far as you can go since any
pit produced by the corrosion (or scratch resulting from your efforts
to remove it) whose depth exceeds 10% of the metal's thickness, is
grounds for rejection. (Sections of the wings of military aircraft
which must be walked upon during normal servicing are often forty to
sixty thou in thickness, not because the airplane needs that amount of
strength but because those surfaces must also serve as walk-ways.)

When the depth of the corrosion is 10% of the material's thickness,
you then estimate the AREA of the corrosion. If it covers more than
20% of the surface area, the part is usually tagged for replacement.

---------------------------------------------------------------------

Wherever two aluminum-alloy surfaces are in close contact, if water is
present it will be drawn into the gap (and there is ALWAYS a gap) by
capillary action. To prevent having our airplanes dissolve like
aspirin tablets -- even with alloys deemed 'non-corrosive' (which is
bull****) -- it is standard procedure to ALWAYS give such surfaces a
wipe of zinc chromate before assembly. Better still is to give each
component an overall spritz of zinc chromate. Yeah, it adds a tiny bit
to the cost, weight and assembly time. But you are seeing what happens
when those standard practices are ignored.

So don't use lye. Unless you know how to, when to and why. Do use
phosphoric acid, then neutralize it good with copious amounts of
BOILING HOT water (and use an oven or a heat gun to make sure all the
cracks are water-free after). THEN give it a spritz of zinc
chromate... because you CAN'T apply alodine, et al, to an assembled
structure. (Not only is it a waste of time & money, you're liable to
PROMOTE a new form of corrosion (chromate-ion growth) in the cracks
between the assembled parts.

-----------------------------------------------------------------------------

Don't shy away from making yourself a set of new ailerons from scratch.
With a corroded but otherewise undamaged aileron as your model the
task is not as difficult as it might seem.

-R.S.Hoover

PS -- There's no mystery to the alloying elements in aviation-grade
aluminum, they form the basis of their SAE nomenclature such as 2024,
6061 and 7075. The leading number indicates the PRINCIPLE alloying
element; 2 = copper, 6 = magnesium, 7 = zinc and so on... if that's
news to you, grab a copy of the ASS Co. catalog and give it a read.

With regard to corrosion, if you never took chemistry you might find it
worth your time to obtain a Periodic Table of the Elements and a good
basic text, such as Linus Paulings 'General Chemistry,' (probably out
of print). Rivet together a few coupons, do a few kitchen-table
experiments, add a bit of time and you will end up with a better
PRACTICAL knowledge of corrosion than most A&P's. You may then use
your corroded coupons to test various methods of removing that
corrosion, which is an education in itself, especially when you see
that the 'aviation-grade' chemicals (costing up to five times as much)
do no better -- and occasionally worse -- than chemicals from your
local paint store.

Naval Aviation exists -- literally! -- upon a good knowledge of
corrosion control. There are some outstanding Navy manuals written for
the average guy (or at least, the average sailor :-) taken from the
technical manuals, methods & procedures used by Grumman and Douglas
(ie, traditional builders of Naval aircraft) who can still teach Boeing
and Lockheed a trick or two when it comes to corrosion prevention and
aircraft maintenance in a corrosive environment (such as bobbing around
the ocean on one of them big grey things... I forget what you call
them... ) -- rsh (USN, retired [and Master of the Run-On Sentence])

On 14 Oct 2006 12:38:50 -0700, "
wrote:

So don't use lye. Unless you know how to, when to and why. Do use
phosphoric acid, then neutralize it good with copious amounts of
BOILING HOT water (and use an oven or a heat gun to make sure all the
cracks are water-free after).

But be careful here; 20xx and 60xx can handle 350F for a pretty long
time with few problems, but 7075 is less tolerant. A heat gun can
overdo what you want if you aren't careful. I don't know a good way
to know when you overheat; those optical thermometers are probably
pretty good, though.

Limit those temperatures to 250F for safe measure.

I wrote:


When the depth of the corrosion is 10% of the material's thickness,
you then estimate the AREA of the corrosion. If it covers more than
20% of the surface area, the part is usually tagged for replacement.

-----------------------------------------------------------------------------------------

Which caused someone to ask,

"Can you expand on that?"

-----------------------------------------------------------------------------------------

Mebbe. It's really the inspector's call, not a rigid rule.
Where the corrosion occurs is the clue. With Alclad you can expect to
see it on edges, that feathery looking discoloration that may not be
significant. But could be.

You also tend to see it around rivets, where it may brush away, giving
the impression it isn't a problem.

If it's fairly extensive, such as on every edge or around every rivet,
even if it looks to be only minor surface corrosion the inspector will
ask (or, SHOULD ask...) to see what's UNDER those edges, requiring you
to drill out a few rivets, separate the parts and take a peek at what's
underneath.

Sometimes there's no problem. Sometimes. But the edge-corrosion may
have etched a line in the lower part creating an unacceptable
stress-riser. Or the edge-corrosion my have bled back farther UNDER
the piece than on the exposed surface.

Drilling out a rivet, you sometimes find the edges of the hole has
turned into a powdery circle (lots of riveters store the rivets in
their MOUTH, treating each rivet and the hole it goes into to a free
dose of spit). A corroded rivet hole may clean-up by going up HALF a
size, in which case you may be required to dismantle the thing,
re-drill and re-assemble with new, plus-size rivets.

Lotsa stuff like that.

Corrosion begs the question: "How bad is it?"

If you treat only the VISIBLE corrosion you leave the question
unanswered. The inspector's job is to come up with a definitive answer,
which then dictates whether the part can be repaired or if it needs to
be replaced.

Okay, so it's a royal pain in the ass and the part is probably good for
another twenty years. Try looking at it this way: You're safe, feet
firmly on the ground, probably warm & dry over in the corner of some
hangar. So you take the time to find the answer; to leave nothing to
Chance. Do the job right, it's a life-time sort of thing. And yes,
there's several ways to interpret that :-)

-R.S.Hoover