bicyclop(at)pacbell.net
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 Posted: Mon Apr 01, 2013 9:34 am Post subject: Corrosion prevention was Blind riveting ground to airframe |
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Howdy Bob,
I think you're referring to alodine which is a conversion coating. The proper (by the book) way to prep aluminum for paint is to wet scrub it with alumaprep and scotchbrite, rinse thoroughly, and soak it in alodine until it starts to get that golden color and rinse again. Then it should be primed as soon as it is completely dry. The alumaprep cleans the surface. The alodine corrodes the surface layer (a few atoms deep) so that a barrier to further corrosion is created and provides a bit of "tooth" for the primer to follow. A problem with this process on completed structures is the difficulty of getting the phosphoric acid based alumaprep and alodine rinsed out from between riveted structures and every little nook and cranny. It especially tends to rear its ugly head on repaints and will later show up as filoform corrosion under the paint around rivet heads and lap joints.
I did it this way for much of my early build and used the solvent base Akzo epoxy primer that you can't get anymore. It would be an interesting experiment to compare the areas that I did that way to the stuff that I've just wiped with acetone and sprayed with self etching primer in twenty or thirty years.
Alodined aluminum without primer will probably perform about like alclad since both processes work by creating a very thin layer of aluminum oxide on the surface as a barrier to more damaging corrosive processes. As a prep process for paint, it used to be the only real way to get primers to grab onto aluminum. This was before epoxy primers. The instructions for these are now mostly about cleaning and sanding the surface before application. Completely cured epoxy primer is extremely tough and very difficult to remove which makes it a good long term undercoating for paint. By the way, the wash primer that Van's sprays quickbuild kits with is a form of self etching primer and is a lousy moisture barrier. It is intended to be painted over within hours, not left to stand on it's own.
Corrosion only occurs in the presence of an electrolyte, usually moisture contaminated with dirt, dust, and whatever airborne acids are around. The whole purpose of any of the available treatments to prevent or stop the spread of corrosion is to exclude air and the moisture it contains from the vulnerable metal. The barrier can be a few molecules thick or a nice coat of paint over well adhered primer. The latter is obviously more robust and less susceptible to tiny breaches. Another approach is to spray the interior with CorrosionX or equivalent every few years (after painting the aircraft please - it weeps from between joints for a long time).
Between joints is the hardest area to protect and that's why I recommend that if you're going to prime any aluminum, faying surfaces and extrusions are the best places to spend your energy. All steel, except for stainless, must be primed and painted in any case. Powder coating works well on steel, too.
Pax,
Ed Holyoke
On 3/29/2013 6:18 AM, BobsV35B(at)aol.com (BobsV35B(at)aol.com) wrote: [quote] Good Morning Ed,
Care to comment as to how well Anodizing works?
I Anodize all small parts before painting.
Happy Skies,
Old Bob
In a message dated 3/28/2013 10:00:08 P.M. Central Daylight Time, bicyclop(at)pacbell.net (bicyclop(at)pacbell.net) writes:
[quote]With primer barriers, particularly on the aluminum. Most sheet stock in use on kitplanes is alclad and doesn't generally need primer. All extruded angle and bar stock is not clad and needs to be primed to prevent corrosion. Many folks prime all faying surfaces (where two pieces fit together) and it should certainly be done at dissimilar metal joints.
Ed Holyoke
On 3/27/2013 11:16 PM, B Tomm wrote: [quote] How does the the stainless firewall riveted with aluminum rivets to aluminum sheet and angle stock play nice together?
Bevan
From: owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com) [mailto:owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com)] On Behalf Of Ed Holyoke
Sent: Tuesday, March 26, 2013 9:35 PM
To: aeroelectric-list(at)matronics.com (aeroelectric-list(at)matronics.com)
Subject: Re: Blind riveting ground to airframe?
Stainless and aluminum don't play nice together. The aluminum will donate ions and corrode. Cad plated steel is the AN way. The cadmium is more anodic than the aluminum and acts as a sacrificial barrier. Eventually, it has no more ions to donate and loses it's value as a corrosion preventative. Time then for new hardware. Another approach would be to prime your hardware and install it wet.
Ed Holyoke
On 3/26/2013 7:25 AM, Jay Hyde wrote: [quote] (at)font-face { font-family: Wingdings; } (at)font-face { font-family: Cambria Math; } (at)font-face { font-family: Calibri; } (at)font-face { font-family: Tahoma; } (at)font-face { font-family: Consolas; } (at)font-face { font-family: 3DȬourier; } (at)page WordSection1 {size: 612.0pt 792.0pt; margin: 72.0pt 72.0pt 72.0pt 72.0pt; } P.MsoNormal { MARGIN: 0cm 0cm 0pt; FONT-FAMILY: "Times New Roman","serif"; FONT-SIZE: 12pt } LI.MsoNormal { MARGIN: 0cm 0cm 0pt; FONT-FAMILY: "Times New Roman","serif"; FONT-SIZE: 12pt } DIV.MsoNormal { MARGIN: 0cm 0cm 0pt; FONT-FAMILY: "Times New Roman","serif"; FONT-SIZE: 12pt } A:link { COLOR: blue; TEXT-DECORATION: underline; mso-style-priority: 99 } SPAN.MsoHyperlink { COLOR: blue; TEXT-DECORATION: underline; mso-style-priority: 99 } A:visited { COLOR: purple; TEXT-DECORATION: underline; mso-style-priority: 99 } SPAN.MsoHyperlinkFollowed { COLOR: purple; TEXT-DECORATION: underline; mso-style-priority: 99 } PRE { MARGIN: 0cm 0cm 0pt; FONT-FAMILY: "Courier New"; FONT-SIZE: 10pt; mso-style-priority: 99; mso-style-link: "HTML Preformatted Char" } SPAN.HTMLPreformattedChar { FONT-FAMILY: Consolas; mso-style-priority: 99; mso-style-link: "HTML Preformatted"; mso-style-name: "HTML Preformatted Char" } SPAN.EmailStyle19 { FONT-FAMILY: "Calibri","sans-serif"; COLOR: #1f497d; mso-style-type: personal-reply } .MsoChpDefault { FONT-SIZE: 10pt; mso-style-type: export-only } DIV.WordSection1 { page: WordSection1 } OL { MARGIN-BOTTOM: 0cm } UL { MARGIN-BOTTOM: 0cm }
Hello Sacha,
I have the same problem in South Africa. For non structural parts I simply use Metric sized bolts and nuts; usually stainless steel ones to protect against corrosion. Stainless steel and aluminium have relatively close galvanic potentials as far as I can see. I have given up trying to source, or even understand (!) the imperial nut and bolt thread system; its far easier to walk into a bolt store and ask for M6 x 25mm bolts and M6 nuts. J
For structural parts you must however use AN fasteners and they are not too difficult to understand (unless you need to order a tap or die…) ; I order them directly from Aircraft Spruce in the US.
Johannesburg Jay
From: owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com) [mailto:owner-aeroelectric-list-server(at)matronics.com (owner-aeroelectric-list-server(at)matronics.com)] On Behalf Of Sacha
Sent: 26 March 2013 03:58 PM
To: aeroelectric-list(at)matronics.com (aeroelectric-list(at)matronics.com)
Subject: Re: Blind riveting ground to airframe?
Does anyone know what the METRIC equivalent of an AN fastener is and where I can buy some? I'm in Italy, so preferably somewhere in Europe, though I'm willing to order from the US if necessary. I've been going nuts (no pun intended) trying to look for some but can only find a few manufacturers, nothing for the retail buyer.
On Mar 26, 2013, at 13:42, rayj <raymondj(at)frontiernet.net (raymondj(at)frontiernet.net)> wrote:
[quote]
Looks like a great source for fasteners. I wanted to be sure the ones I chose were to an specs. I ended up ordering a big assortment of small AN screws. If I could have gotten an stuff with 6 lobe recess, I would have prefered it. Definitely preferable to Phillips.
| Quote: | | Raymond JulianKettle River, MN. "And you know that I could have me a million more friends,and all I'd have to lose is my point of view." - John Prine |
On 03/26/2013 07:20 AM, William Schertz wrote:
[quote]
--> AeroElectric-List message posted by: "William Schertz" <wschertz(at)comcast.net> (wschertz(at)comcast.net)
Also note that they carry 100 degree flat head with Torx drive, much nicer for avoiding stripouts like phillips
Bill Schertz
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