Best aluminium alloy for the masts

[deucalion]

I'm working for an extrusion company and I have lots of inquiry from our parthners for masts. I checked on the net and as I see, there are 4 or 5 kind of alloys to produce these masts.

Masts for the optimists, lasers or windsurf booms, it is generally used 6060/63-T5 or T6. This is a light alloy with low mechanical properties.

For the biggers 6061-T6 and 6082-T6 alloys can be used. I looked up for these alloys mechanical properties.

6061-T6 / 6082-T6
Ultimate Tensile strength 260-310 MPa / Min. 270 MPa
Yield strength Min. 240 MPa / Min. 200 MPa
Elongation %0,05 9-13 / 12
Hardness (Brinell) 95-97 / 80

I also see rarely 7075-T6/T9 alloys which has high strength but they are not so prevalent.

Are these informations right? What's the best aluminium alloy for the masts? Which one is the most common and why? What's the priviliged mechanical property and what are the minimum values? Is there any international standard of these mechanical properties for the mast production? Can anybody help me?

[Ad Hoc]

Depends if the mast is to be welded or unwelded when used.
6061 is popular in the US because the aircraft industry requires shed loads of the stuff, so it is always in stock. It does however have 3 times the copper content of 6082, which is the reason why most Class societies no longer allow 6061 to be used. 6061 has poor corrosion properties.

[whoosh]

Depends if the mast is to be welded or unwelded when used.
6061 is popular in the US because the aircraft industry requires shed loads of the stuff, so it is always in stock. It does however have 3 times the copper content of 6082, which is the reason why most Class societies no longer allow 6061 to be used. 6061 has poor corrosion properties.

well that is the most misleading post you could post,
after 30 yrears building alloy yachts I can tell you 6061 t6 stands up in all situations , especially masts
I used it on trailers too, its propertys are amazing,and I use it with Lloyds, ABS, approval

I used it(along with a zillion others) in all framing
where did you get your facts, !!!!!! it is the number one alloy of preference
It is against my principals now to give neg feed , but boy you tempt me

[deucalion]

As I see their mechanical properties are so close to each other, the main difference is on their chemical composition. 6061 has greater yield strength and hardness, but the tensile strength of 6082 is more than 6061. Which of these properties are most important?
What about stiffness? Which property directly effect on stiffness?

[Ad Hoc]

Whoosh
6082 has 0.1% copper, 6061 generally has 0.3% copper. (Can be as high as 0.4%).
Both DNV and LR no longer accept 6061 owing to its poor corrosion properties compare to 6082. It has been well documented. DNV have not outright banded it, just prefer you don't use it. LR have removed it from its approval list. Yet in both cases if there is an application that is identical to one which is proposed and has demonstrated little or no corrosion effects, then class will allow a dispensation. To my knowledge LR have no allowed 6061 for around 5~7 years. I wouldn't recommend it either, 6082 is far superior for the aforesaid reasons.

Copper over time weakens the protective oxide layer. Hence 7000 series ally which have added copper and those in the 2000 series are best avoided.

Clearly LR USA has different ideas to the rest of the world.

[deucalion]

How can I provide documents of LR or DNV rules about these issues (aluminium masts) ? Any website?

[Ad Hoc]

Sorry, don't understand the question?

[robherc]

Ad Hoc, he's asking for a link to research the LR and DNV rules on aluminum masts for himself. ... and so he can have the documentation if anyone asks.

[Ad Hoc]

If that is all he is after, just looking at DNV rules: Pt.2Ch.2 sec.9 A1300 table A3, will give the mechanical properties, or LR Pt.2. Ch.8 Sec1. table 8.1.4 also.

As for masts. It is like anything in design. You establish the loads, select a suitable geometry/structure design, then see what stresses are involved then select the material to suit the computed stress and predicted fatigue. Without establishing the loads and its in-service conditions and method of construction, bit hard to advise....there is not a "This is a Mast, please select one" book. All masts are unique to each vessel.

[alex folen]

Hello, forgive my ignorance. But in airplane experimental design we think of the lightest material with the greatest strength. I have looked at newer designed sailboats (for speed) and the lighter craft the faster in same design. Is this correct? I would think of aircraft material and its strength and go from there, with all the calculations thereafter. May cut out a lot of guess work as to the type of material needed? Perhaps I’m still ignorant of sailboats, heck I’m new this new found love!

[Jimbo1490]

Depends if the mast is to be welded or unwelded when used.
6061 is popular in the US because the aircraft industry requires shed loads of the stuff, so it is always in stock. It does however have 3 times the copper content of 6082, which is the reason why most Class societies no longer allow 6061 to be used. 6061 has poor corrosion properties.

If you think 6061 has poor corrosion resistance, you would do well to stay clear of 7075. That stuff is a corrosion nightmare! I have seen bare spots on aircraft where minor surface corrosion was dressed in preparation for a paint job, then left bare for a month, and these spots 'blew out'; went to intergranular exfoliated corrosion. In one month! The reasons this does not normally happen in service on aircraft are that 1) aircraft aluminum is 'Alclad' (plated with pure aluminum) and 2) aircraft skins are painted and 3) aircraft are virtually always subject to a rigorous inspection program.

Of course when corrosion spots are dressed out, the Alclad coating gets removed, thus the rapid progression of new corrosion. I would consider this alloy wholly unsuited to the marine environment due to a propensity for intergranular exfoliation type of corrosion, which is always structurally significant.

Jimbo

[Ad Hoc]

Jimbo1490
Exactly 7075 has some 1.2~2.0% copper, which is way over than of 0.1% of 6082. Even 6061 seems better in comparrison at roughly 0.4%. And yet you see some boat builders, like the one above, think it isn't a problem to be concerned with!

Copper, the enemy of aluminum...

[Jimbo1490]

Copper cannot be considered the enemy of aluminum in that without copper, there is no such thing as hardened aluminum. Without 'hardenable' aluminum, you are left with only the weaker, 'non-structural' aluminum alloys. Free crystals of copper intertwine into the crytalline lattice of aluminum molecules, thus preventing adjacent 'sheets' of aluminum lattice from sliding past each other. The aluminum lattice usually forms large common interface planes between 'sheets'. These interface planes become weak points in the crystalline structure since they slide past each other easily. During rapid cooling after heating, the copper crystals become embedded in adjacent sheets. In order for the 'sheets' to slide thereafter, they must either shear the copper crystals or tear the aluminum lattice within the same sheet, where it is the strongest. This makes the alloy MUCH stiffer and a bit stronger as well.

Jimbo

[Ad Hoc]

Ahh...you misunderstand me. I'm not referring to it metallurgical composition. I am well aware of the role of copper in this aspect of making the alloys. I am referring to the finished product, ie when used on a boat.

Bimetallic corrosion of aluminium by copper in solution, ie sea water, is a killer.

This is why LR no longer allow 6061, too much copper..

Aluminium has a nominal electrical potential of -750mV, whereas copper has -360mV. For bimetallic corrosion to take place a there must be, in general, a potential greater than 100mV. Clearly copper is way way over this...and as such is a killer when in solution, to copper.

That's why copper (pipping, paint etc etc) when in direct contact or in solution of any form, must be avoided at all costs, with aluminium, when used on a boat.

[sailor2]

Copper cannot be considered the enemy of aluminum in that without copper, there is no such thing as hardened aluminum.
JimboThat is simply a wrong statement. Copper is just one out of many metals to make aluminium hardenable.

Mg alone will make alu work hardenable, but not allow hardening by heat treatment. It is the best alloying for corrosion resistance properties for marine use. When welded, heat will soften it again in areas that heat up.
Using Si + Mg alloying makes alu both work hardenable and heat treatable. This is so even with absolute zero copper content. And still has very good corrosion resistance, althoug slightly less so than using only Mg. Thats the most recomendable grade for marine use when heat treatable grade is needed. 6082 grade has max copper content as stated earlier, but no minimum content. Special orders with less copper is possible from some manufacturers with same structural mechanical properties but even beter corrosion resistance.

Pure Zn alloying also allows hardening, but is prone to corrosion far too much for marine use without very carefull sealing and regular inspection. It is still possible for masts with good coating by Al-Oxid, but not for anything under waterline.
Include both Zn and Copper and result is the strongest Alu there is, but a nightmare regarding corrosion. Still used on aircrafts with regular inspection.

[Jimbo1490]

It's not a 'wrong' statement; your example of work hardening is invalid as metallurgically, this type of hardening is inferior and degrades strength properties. None of these alloys you mention is considered a 'high-strength' hardenable alloy. Only the alloys containing copper can get you this mix of properties. Only copper hardens aluminum by imbedding between adjacent 'plates' of aluminum's crystalline lattice, preventing those plate from sliding, the mechanism of pure aluminum's malleability. This is why only alloys with copper can be 'precipitation hardened', the type of hardening of most interest in a structural metal. The other alloying agents harden aluminum a different (inferior) way.

I stand by the statement.

Jimbo

[hunny06]

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[Ad Hoc]

Jimbo1490

I think there is cross lines going on here with terminology.

Work "hardening", ie using mechanical means to increase the strength of an alloy from say O-temper to say H116-temper, is one definition. This is the terminology used by many, but is correctly termed strain-hardening in the metullurgical sense. (To "work a metal", is where the common usage comes from)

The addition of copper brings about many changes to increase the mechanical properties and also changes the alloy designation from non-heat treatable to heat treatable. These alloys can then be age-"hardened" further to increase its properties, thus confusing the layman with respect to the word "hardening", even further!

The term "hardening" is the confusing bit as it relates to many different disciplines all at the same time, with subsequent different meanings. Since hardness testing is a relatively quick and easy way to assess the strength of a material without the need to prepare tensile test samples.

As such, the term ‘hardness’ is a comparitively important for engineers, but, it is not strictly spekaing considered to be a fundamental property of matter. Even though it is refered to as such. The index of hardness is a manifestation of several related properties of the metal, which may well include a combined effect of yield point, tensile strength, ductility, work-hardening characteristics and resistance to abrasion.

It is however, most commonly used when referring to the weld quality since it is directly proportional to the UTS and as such directly related to the quality of the weld. Which is why hardness testing is done right across the weldments from parent metal to parent metal across the HAZ for class certs. Sorry, slight digression...but you get the point.

[sailor2]

It's not a 'wrong' statement; your example of work hardening is invalid as metallurgically, this type of hardening is inferior and degrades strength properties. None of these alloys you mention is considered a 'high-strength' hardenable alloy. Only the alloys containing copper can get you this mix of properties. Only copper hardens aluminum by imbedding between adjacent 'plates' of aluminum's crystalline lattice, preventing those plate from sliding, the mechanism of pure aluminum's malleability. This is why only alloys with copper can be 'precipitation hardened', the type of hardening of most interest in a structural metal. The other alloying agents harden aluminum a different (inferior) way.

I stand by the statement.

Jimbo
Si + Mg type of alu with zero copper content is precipitation hardened !!!
That's a metallurgical fact. And the reason that type of aluminium alloy is hardenable by heat treatment, like T6. That's artificially age hardened as you propably know or at least should.

"None of these alloys you mention is considered a 'high-strength' hardenable alloy" I didn't claim so, but Zn alloyed types can be classified like that even with zero copper content. A high-strength classification was not a requirement in this thread anyway.

[sailor2]

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=214&pageid=2144417047
Make sure to press all these 3 buttons that shows on that link to see all the content there !!!
* Solute Mg effects
* Stress and strain
* Corrosion and strength

Suitable info for some aluminium alloys discussed in this thread :
http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=214&pageid=2144417081
http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=214&pageid=2144417085
http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=214&pageid=2144417086
http://aluminium.matter.org.uk/aluselect/01_applications.asp?AlloyID=23
http://aluminium.matter.org.uk/aluselect/01_applications.asp?AlloyID=25
http://aluminium.matter.org.uk/aluselect/01_applications.asp?AlloyID=26
http://aluminium.matter.org.uk/aluselect/01_applications.asp?AlloyID=27
Some relevant quotes from those links :
EN AW-2024 / ISO: Al Cu4Mg1 Composition: Al 4.4Cu 1.5Mg 0.6Mn
Characteristic Properties: Heat treatable alloy. Very good machining characteristics. High strength alloy with a strength slightly higher than 2014(A) and 2017A and 2030. High fatigue strength. Suitable for welding only by resistance welding. Corrosion resistance only with cladding or other protection.

EN AW-6082 / ISO: Al Si1MgMn Composition: Al 0.9Mg 1.0Si 0.7 Mn
Characteristic Properties: Very good corrosion resistance. Very good weldability (lowered strength values in the zone of welding). Good machinability. Good cold formability in T4 temper after a stabilizing heat treatment. Heat treatable medium high strength construction. Alloy with a strength somewhat higher than 6061. Medium high fatigue strength. Not suitable for complex sections.

EN AW-7010 / ISO: Al Zn6MgCu Composition: Al 6.2Zn 2.4Mg 1.8Cu Zr
Characteristic Properties: Heat treatable very high strength alloy with a strength slightly higher than 7075. Very high fatigue strength. Stress corrosion resistant. Not suitable for high temperature applications or fusion welding. Corrosion protection is recommended also in atmosphere.

EN AW-7020 / ISO: Al Zn4,5Mg1 Composition: Al 4.5Zn 1.2Mg Mn Cr Zr
Characteristic Properties: Heat treatable high strength weldable alloy. High fatigue strength. High strength in welded structures. Suitable precautions (aging after welding) must be taken against stress corrosion cracking and exfoliation.

Note that 7020 doesn't have to contain any copper, yet classified as high strength alloy unlike 6061 or 6082, as I said earlier. don't miss the statement on 7010 that even with atmostheric conditions instead of marine ones, corrosion is an issue !

[Jimbo1490]

Note that 7020 is also prone to exfoliation, a severe and always structurally significant form of corrosion wherein the metal basically disintegrates.

Jimbo

[apex1]

Very informative and interesting statements above.
A laymans question:
Will they answer the question of the topic Best aluminium alloy for the masts eventually?
Regards
Richard

[sailor2]

Note that 7020 is also prone to exfoliation, a severe and always structurally significant form of corrosion wherein the metal basically disintegrates.
Jimbo
Which is why I didn't recomment it for mast production. Just provided the evidence contrary to your statement that "copper is mandatory to acheave high strength aluminium alloy". Just as I did on the other statement of yours that "This is why only alloys with copper can be 'precipitation hardened"

I just don't like incorrect statements no matter who writes them, that's all there is to it. Don't take it personally, I treat others just the same way when I find claims to be incorrect instead of just being different opinions.

[sailor2]

Very informative and interesting statements above.
A laymans question:
Will they answer the question of the topic Best aluminium alloy for the masts eventually?
Regards
Richard
Depends on the mast. On most cases stiffness of the stayed mast is the critical issue for dimensioning, which means a lot of alternatives are strong enough and a mast designer can from theoretical point of view consentrate on what is most corrosion resistant or in practise, what is most cost effective and easily available.
Most used alloys are 6082, 6061 and even 6063 or 6060 in small boats.

Any alloy from 7000-series is a rare exception, but those have been made.
I would guess AC boats used those before carbon took over, as longetivity was no issue for them.

Haven't heard anybody having any 2000-series alloy masts, which doesn't prove there isn't any.

There are no facts about what's best for average joe. Only opinions and mine is go with most used 6000-series alloys, easy to maintain as riggers have a lot of practical experience of those.

[Ad Hoc]

Sailor2
You state that 7020 doesn't "have to" contain copper, that in itself is misleading. Since all commercially available 7020 contains 0.2% copper.

JImbo1490.
Not sure how a heat-treatable high strength alloy as 7020 is prone to exfoliation corrosion? Since 7020 is high strength, there is no requirement to roll it (strain hardening) from its final heat treated temper, ie mechanically. The heat treatment gives greater properties. To my knowledge, I've never seen 7020 offered as rolled plate, strain hardened at that. In fact is it extremely rare to find 7000 series as rolled and strain hardened plate. Since strain hardening is generally used with alloys containing Mg/Mn to improve its mechanical properties. This is not necessary with 7000 series as it is already high strength.

Exfoliation is caused by exposed edges (of the many layers in the alloy micro structure) when an alloy has too much strain hardening, the layers just peel away. It can also be caused by cold roll process when strain hardening the alloy to a different temper, such as H321, instead of a hot rolled process, when strain hardening the alloy.

[Jimbo1490]

Exfoliation is caused (fundamentally) by different alloying agents that don't play well together. It can be kicked off by atmospheric exposure or any other cause for corrosion, but it sometimes happens without any apparent external cause, just the 'fundamental' one. I have removed new surplus Lockheed wing planks from factory crates, still bathed in cosmoline, only to find that they had disastrous exfoliated corrosion. The alloy was 7078. As far as I'm aware, all the 2000 and 7000 series alloys contain copper, which is exactly why they can be precipitation hardened. So in the end you have to decide if the loss of strength (by alloying without copper) is worth the extra corrosion resistance. But copper is not "the enemy of aluminum" even in this context. It's just another engineering trade-off.

Jimbo

[Ad Hoc]

Exfoliation is simply the delamination of the thin layers of the aluminum, from strain hardening the alloy, with white corrosion products between the layers from the environment, ie sea water and other elements in solution.

You're probably referring to the result of exfoliation corrosion owing to poor maintenance, which in reality is bimetallic corrosion. Such as often found next to fasteners where an electrically insulating sealant or a sacrificial cadmium plating has broken down, permitting a galvanic action between the dissimilar metals. This is common on poorly maintained aircraft structures.

[Jimbo1490]

The exfoliation I'm referring to happens without any dissimilar metals in contact with the structure at all (other than those dissimilar alloying agents), only other aluminum structure and aluminum fasteners. This need not be caused by strain hardening. Heat treated aluminum is always fully annealed as part of the heat treating process, so that would not be a factor.You don't see very much of this type of corrosion on aluminum in the marine industry because most boat builders know better than to use these alloys (2xxx and 7xxx) in a marine environment, but spontaneous exfoliation is quite common in aircraft made from these alloys. But as they are as strong as mild steel, they are quite attractive to aircraft designers.

Jimbo

[sailor2]

Sailor2
You state that 7020 doesn't "have to" contain copper, that in itself is misleading. Since all commercially available 7020 contains 0.2% copper. From the standard the 0.2% is the max allowed to fit under 7020 designation. The standard does not specify minimum copper content. Which means the properties of 7020 can be accheaved without any copper content.
There is allowable range for Zn content as well as Mg content as these are necessary for properties to be what the standard requires.

What commercially available 7020 contains depends what is ordered if you order enough quantities. Quite many section manufacturers on this planet, are you sure you know them all that well ?


EN AW-7020
Zn 4.0 - 5.0
Mg 1.0 - 1.4
Mn 0.05 -0.50
Fe <= 0.40
Cr 0.10 -0.35
Si <= 0.35
Zr+Ti 0.08 -0.25
Zr 0.08 -0.20
Cu <= 0.20
Total Other <= 0.15
Other Elem <= 0.05
Al Remainder

All compositions in wt%. Jimbo wrote :As far as I'm aware, all the 2000 and 7000 series alloys contain copper, which is exactly why they can be precipitation hardened. so how do you explain the following then ? Are those folks at European Aluminium Association just lying or what ?As with some of the 2xxx series, alloys in the 7xxx series also have additions of magnesium to maximise their age-hardening potential where the precipitating phases are typically of the type MgZn2. Such alloys give medium strength, but are relatively easily welded. Aluminium-zinc-magnesium alloys have a greater response to heat treatment than the binary aluminium-zinc alloys resulting in higher possible strengths. The additions of zinc and magnesium however decrease the corrosion resistance. Possible precipitates in aluminium alloys include at least : Mg2Si, Al2Cu, MgZn2, LiAl3.
Any of these allow precipitation hardening by heat treatment.
Jimbo, the site I gave in the link for is very informative and intended to be educational. Also in metallurgy. If you really think copper based Al2Cu is only possible precipitate you should educate yourself a little more. http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=61&pageid=989848382

[Ad Hoc]

Jimbo
That's interesting. You're quite right 2000 and 7000 series are rarely used in the marine industry, for good reasons too. But i cant get my head around exfoliation corrosion occurring if the alloy has no strain hardening. Since where are all the layers of atoms in the microstructure, to delaminate? Perhaps you're referring to intergranular corrosion, which i can well understand in your situation is probably the mechanism. Since, if an alloy is not strain hardened, i find it hard to understand how exfoliation corrosion can take place.

Sailor
Again, interesting. I have limited knowledge of 7020, but each time i have referred to it, or been asked to, in the past, I've always been quoted 0.2%, but never as a max, always as an absolute value.

[Jimbo1490]

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

[Ad Hoc]

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?

[Ad Hoc]

Jimbo

this website is nicely explained on the subject:

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

[Jimbo1490]

Thanks for the very informative posts!

Jimbo

[sailor2]

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.
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.

Copper is hardly "the enemy of aluminum" as you stated.
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.

[Jimbo1490]

Jimbo1490
Exactly 7075 has some 1.2~2.0% copper, which is way over than of 0.1% of 6082. Even 6061 seems better in comparrison at roughly 0.4%. And yet you see some boat builders, like the one above, think it isn't a problem to be concerned with!

Copper, the enemy of aluminum...

Different poster, sorry:(