What kind of corrosion is this?

[KPRick]

Looks like pit corrosion. Extrusions are typically different alloy than cast or sheet aluminum products.


Some resources:


https://haluminium.com/Product/5-cor...nium-profiles/


http://www.almet-marine.com/images/c...vironments.pdf

[betwys1]

Originally Posted by KPRick

Looks like pit corrosion. Extrusions are typically different alloy than cast or sheet aluminum products.


Some resources:


https://haluminium.com/Product/5-cor...nium-profiles/

Here's an extract:
"Intergranular corrosion is pure aluminium is not prosperous intergranular corrosion, intergranular corrosion reason has something to do with the improper heat treatment, alloying elements or intermetallic compound along the grain boundary precipitates out, relative to the grain as a corrosion cell anode, intergranular corrosion caused by acceleration."

The second pointer is much more readable.

[betwys1]

Originally Posted by team karst

One thing not mentioned is the aluminum alloy that was used. If it was 6000 series like 6061, then bad, with its content of copper in the alloy mix. 5000 has nil copper. But, you will need a portable XRF gun to analyze it. Fast and non-destructive, but you need to get one. Try a high end scrap metal yard, they have them. An employee will tag along, since they are around $20k to buy.

5000 series may contain up to 0.8% copper ~ can be lower.
6061 contains 0.3% copper
2024 contains 4.0% copper

MatWeb - The Online Materials Information Resource

[team karst]

5052 is 0.1% max Cu. But is usually a rolled, weldable product. There are other marine Al alloys. I have no idea if this mast is bent and welded, extruded, etc

[Exile]

I have what appears to be this sort of corrosion going on with an aluminum, electric headsail furler which obviously takes a lot of seawater up on the bow. It was originally painted by the factory but a widespread problem with paint coming off ensued. I tried having it re-painted with Awlgrip (after etching) but after awhile the same problem occurred, after which I stripped all the paint off and left it bare aluminum. That was several years ago, but now I've noticed quite a bit of pitting. I thought I'd try polishing it with an electric drill and a nylon brush attachment, if nothing else to try and remove any remaining bits of Awlgrip which may be causing the problem. Could Awlgrip even be a cause??

Someone recommended Naval Jelly to pacify the aluminum. Anyone second that, or have another recommendation? Or should I just go with the 2-part etching systems that are widely available (albeit $$)? I plan on leaving it bare aluminum.

[s/v Jedi]

Originally Posted by Exile

I have what appears to be this sort of corrosion going on with an aluminum, electric headsail furler which obviously takes a lot of seawater up on the bow. It was originally painted by the factory but a widespread problem with paint coming off ensued. I tried having it re-painted with Awlgrip (after etching) but after awhile the same problem occurred, after which I stripped all the paint off and left it bare aluminum. That was several years ago, but now I've noticed quite a bit of pitting. I thought I'd try polishing it with an electric drill and a nylon brush attachment, if nothing else to try and remove any remaining bits of Awlgrip which may be causing the problem. Could Awlgrip even be a cause??

Someone recommended Naval Jelly to pacify the aluminum. Anyone second that, or have another recommendation? Or should I just go with the 2-part etching systems that are widely available (albeit $$)? I plan on leaving it bare aluminum.

I recommend sanding with 220 grit, followed by the new Awlgrip CF Wash Primer, followed by original Awlgrip topcoat.

If there is any galvanic action going on, nothing will help but if it’s just the weather then this will be okay. The new CF technology may be a step up from acid etching for DIY (no direct experience yet… just bought it, now find the time for first test)

[Exile]

Originally Posted by s/v Jedi

I recommend sanding with 220 grit, followed by the new Awlgrip CF Wash Primer, followed by original Awlgrip topcoat.

If there is any galvanic action going on, nothing will help but if it’s just the weather then this will be okay. The new CF technology may be a step up from acid etching for DIY (no direct experience yet… just bought it, now find the time for first test)

Thanks Jedi. I didn't know about this new product. What's interesting is many of the small pockets have a tiny speck of what I assume is old Awlgrip or primer inside. Could this be causing a reaction? As for painting again, I'm a bit gun-shy of trying this again, and don't mind the look of the bare aluminum. Unless the new CF plus Awlgrip is the only way to protect the casing. Replacement furlers are big boat bucks!!

[thinwater]

Originally Posted by s/v Jedi

I recommend sanding with 220 grit, followed by the new Awlgrip CF Wash Primer, followed by original Awlgrip topcoat.

If there is any galvanic action going on, nothing will help but if it’s just the weather then this will be okay. The new CF technology may be a step up from acid etching for DIY (no direct experience yet… just bought it, now find the time for first test)

Something like this. No water, no corrosion.


As for the structure, isolated pits have little effect on strength, but the only way to know is thickness testing. Ultrasound is simplest (as an API tank inspector I do this a lot). Pro units are expensive, but you can get something very serviceable for about $100 on Amazon. You will find other uses I am sure. Read up on the use.

• Calibrate on something of known thickness.
• Variable or poor signal, like depth sounder, means a bad reflection. The usual cause is a rough surface caused by corrosion. A good surface gives a good echo.
• Loose paint is a problem.
• It will not see faults much smaller than the transducer.
• You need a couplant. Shampoo and conditioner work just fine, and are easier to take past TSA on airplanes than some custom weird gel!

[s/v Jedi]

Originally Posted by Exile

Thanks Jedi. I didn't know about this new product. What's interesting is many of the small pockets have a tiny speck of what I assume is old Awlgrip or primer inside. Could this be causing a reaction? As for painting again, I'm a bit gun-shy of trying this again, and don't mind the look of the bare aluminum. Unless the new CF plus Awlgrip is the only way to protect the casing. Replacement furlers are big boat bucks!!

No, I can’t imagine what is going on. What you call “pockets” are they pitting? Awlgrip does not cause any reaction, but when it was pitted before painting, the pint inside there is more protected

[Exile]

Originally Posted by s/v Jedi

No, I can’t imagine what is going on. What you call “pockets” are they pitting? Awlgrip does not cause any reaction, but when it was pitted before painting, the pint inside there is more protected

I'll post some photos in a bit. I don't want to mischaracterize what may or may not be going on.

[Cadence]

I had suggested Ospho but had to look it up. Probably still have the black spots but I could not recall what it turned rust into? OSPHO is a rust inhibiting coating that provides a sound base for subsequent primers and paints. When applied to rusted surfaces, OSPHO causes rust (iron oxide) to chemically change to iron phosphate - an inert, hard, black substance. If it is from some grindings getting on the boom it may stop more damage. Apparently highly acidic (.08) so maybe wetting with a toothpick on the spots would be prudent.

[KPRick]

Not all aluminum alloys work well in an extruding press. The aluminum alloys most often used for mast and boom extrusions are 5000 series (5083 and 5086) and 6000 series (6082, 6061 and 6063). The 5000 series generally have slightly better corrosion resistance then the 6000 series, but they are both suitable for marine applications. Typically both alloys are solution and age treated ( think heat treating for aluminum noted as a temper at the end of the alloy designation such as -T5X, -T6X, etc.). The 6000 series can be "heat treated" to a slightly higher strength than the 5000 series. The 6000 series are more common and cost less. You will more typically find 6000 series if mechanically fastened joints are used. If any welding is involved, both alloys go back to -T0 temper or annealed condition adjacent to the weld joint. The 5000 series is stronger in the annealed condition in heat effected zone of the weld, so it is most often selected if there are any welded joints at structural points.


https://www.wellste.com/marine-aluminium-extrusions/
https://www.azom.com/article.aspx?ArticleID=4193


Short article on avoiding corrosion of aluminum in marine environments


https://www.shapesbyhydro.com/en/mat...-environments/

[Exile]

Here are some photos of my ProFurl electric headsail furler. If you're able to zoom in you can see at least one larger recess with what looks to me like Awlgrip or primer that wasn't sufficiently removed when the unit was stripped. Also around a couple of fasteners. The bulk of it looks more like alum oxide to me but not sure. The unit was stripped years ago and I didn't think alum oxide could form on aluminum just from being exposed to salt water alone.

In any event, I'm considering using a nylon brush attachment or scotchbrite pad on a drill (or maybe dye grinder) to try and remove it all. After that I'll look into something to pacify it (thanks for the tip on Ospho Cadence), if necessary. I prefer to leave it bare aluminum again rather than attempt another paint job.

[s/v Jedi]

Originally Posted by Exile

Here are some photos of my ProFurl electric headsail furler. If you're able to zoom in you can see at least one larger recess with what looks to me like Awlgrip or primer that wasn't sufficiently removed when the unit was stripped. Also around a couple of fasteners. The bulk of it looks more like alum oxide to me but not sure. The unit was stripped years ago and I didn't think alum oxide could form on aluminum just from being exposed to salt water alone.

In any event, I'm considering using a nylon brush attachment or scotchbrite pad on a drill (or maybe dye grinder) to try and remove it all. After that I'll look into something to pacify it (thanks for the tip on Ospho Cadence), if necessary. I prefer to leave it bare aluminum again rather than attempt another paint job.

What you think is Awlgrip, I think is aluminum oxide. I would take this path: Ultimate Aluminum Cleaner / Restorer

Also, when it’s clean, protect it. RejeX or the new ceramic coatings.

[KPRick]

Ospho is for steel and not aluminum.


Ospho Rust Treatment - Since 1947