Best aluminium alloy for the masts

Nobody is lying; don't be sarcastic. The precipitation hardening produced by MG and Zn is clearly inferior. Again, these precipitating phases without copper only produce a "medium strength" product. Copper is the only element that can make aluminum as strong as mild steel. Copper is hardly "the enemy of aluminum" as you stated. Anyway, the word 'enemy' is not a technical metallurgical description. So what does that mean, anyway? That it makes aluminum corrode? Well nickel does that even more so, and without providing any benefit as an alloying agent. So why isn't nickel the "enemy of aluminum"?

My understanding of the term 'exfoliation' is that it refers to the metal separating along grain boundaries. This makes it synonymous with intergranular corrosion. Is this wrong?

Jimbo

 

Jimbo
In regards to "the enemy"...there are many culprits that can be classed as enemies to aluminium. But copper is used frequently on boats as pipes. The aluminium, as noted before, is in the finished product state, not metallurgical for enhancing properties, that is an entirely separate issue. Copper, when in solution, via sea water/spray, just eats aluminium faster than a kid in a chocolate shop! The presence or otherwise of copper in chemical composition form is not the issue.

as for your last point, hhmmmm..interesting question which is not so easy to answer simply. So rather than putting my own interpretation, ive gotten a few books to refresh my memory just to make sure i explain correctly and don't mislead. I alwasy refer from memory and the passage time doesn't always help the mind!. Exfoliation corrosion is a form of corrosion that spreads along planes parallel to the direction of rolling (strain hardening - ie tempers such as H321 etc, which is the reason for the ASSET test requirement when using these tempers, as well as using hot rolling process). Between these planes are very thin sheets of sound metal. The build-up of corrosion products causes the corroded zone to swell, peeling away leaves of metal like the layers of an onion, hence the name “exfoliation corrosion”, as I have briefly explained before. When im teaching students i use a reem of paper to explain this, it is good visually.

However, intergranular corrosion is caused by the difference in electrochemical potential that can exist between the actual grain and the grain boundary zone where intermetallic compounds, such as the beta-phase Al3Mg2 phase for magnesium alloys, can precipitate. The dissolution potential of this intermetallic is very electronegative: -1150 mV SCE compared with the grain of -750 mV. Intergranular corrosion can occur when three conditions come together:
1.presence of a corrosive aqueous medium,
2.difference in potential of at least 100 mV between the intermetallics and the solid solution,
3. continuous precipitation of intermetallics in the grain boundaries.

Given the 400 mV difference in potential between the beta-phase Al3Mg2 phase and the grain, aluminium magnesium alloys are sensitive to this form of corrosion under well defined and well known conditions. (Also in slow cooling artes of heat treable alloys). They depend on the conditions of working and the conditions of service.

As the this form of corrosion advances, it reveals itself by lifting up the surface grain (hence intergranular) of the metal by the force of the expanding corrosion products occurring at the grain boundaries just below the surface. This advanced attack is commonly referred to as exfoliation. But the initial mechanism is intergranular corrosion on the grain boundaries which results in the exfoliation. Not sure if this helps?

 

Jimbo

this website is nicely explained on the subject:

http://www.asnt.org/publications/materialseval/basics/oct03basics/oct03basics.htm

 

Thanks for the very informative posts!

Jimbo

 

So you strongly disagree with aluminium associations and books on metallurgy used in university level and at the same time say they are not lying, that's quite an interesting perspective. At least you now finally acknowledge that other precipitates exist, a major advancement compared to your earlier comments on the subject.

Who has stated that and in which post ?!?

Please provide a quote and make sure if it is the claim made by the person who posted it or if it's instead quoted from another site like aluminium association web page and is therefore not just the opinion of the poster. Please make sure you understand the difference of these 2 possibilities.

 

Different poster, sorry

 

So, just to wake up this thread. I want to make/have made a 6.2m mast for my inland waterways vessel. It is to replace a very heavy wood mast that is too difficult to raise and lower plus the ongoing maintenance. As in my profile pic, it needs a cross trees for flags and a nav light fitting. Commercial vessels have typically a 3 step mast - lower section 80mm 2-3mm wall, middle 65mm 2mm wall and top of +/- 1m 2mm wall of 50mm. Cross tree at top of 65mm.
The boom in pic is immaterial.
So, would 6082 T6 do the job??

 

From what I gather from the thread so far, 6082-T6 is the way to go.

 

Indeed.
However, looking at your boat.... I would try to get 5083-O grade and temper, as extruded 5083 is more common these days - since this thread goes back 11 years!!
Since you may be able to find section sizes of drawn tube in 5083, given that 5083 is more corrosion resistant than 6082, and the unwelded and as-welded strength in the 5083 remains the same.

But, if strength is not an issue, 6082 is generally cheaper in extrusion form than 5083.

 

I read the thread here and a few curiosities.

My powercat uses Sparcraft s830 mast sections as cross members. I thought I had memorized the alloy, but could not find a callout on Sparcrafts website. From memory it was 6082-T6, but a cert would have been good. I can go check the computer later.

Anyhow, the beams are subject to splash and AH discussing the ill effects of copper have me questioning my bottom paint.

Could it be trouble to use copper af paint?

 

I was under the impression that copper is being phased out due to the toxic effects in the water.

Is Copper Bottom Paint Sinking? - BoatUS Magazine https://www.boatus.com/magazine/2012/february/copper.asp

We switched to Interlux ablative paints about 20 years ago with good success. Mainly the paint sloughs off over time and the repaint then does not require sanding etc. A quick pressure wash and then repaint.
Normally you use multiple coats with the first coat, after the applicable primer, a different color than the finish color. Ie primer, example light gray, then 3 or more coats of example blue. The light gray will begin to show through
when it is time to repaint. We used Micron CSC, but I see that Interlux has a new one out.

 

I went with Vivid White which is a copper tinged white. Probably ugly as hell with some algae.

It isn't on yet, thus the asking.

http://www.pettitpaint.com/media/4461/vivid-product-data-sheet.pdf

Says cuprous thiocyanate; so some copper present. I just don't like the idea of it sloughing off and corroding my beams.

My apologies to the OP for the unplanned hijacking.

 

Correct.
The International Maritime Organisation (IMO) placed a global ban on the use of tributyltin (TBT) in antifouling systems. Ostensibly because of the leaching out from antifouling paints into the marine environment, as it turned out that TBT is highly toxic in the marine enviorment.

Yup... i would avoid it.

Why not just leave the aluminium bare?..it forms its own oxide layer and becomes highly resistant.

 

I was going to leave the ally bare until the boat moves to salt.

The paint was going on the boat bottom.

But there is no protection from splash for the two aft(er) beams.

 

So what??... the ally oxide is the protection!

Plenty of ally boats about that are not painted.