Best aluminium alloy for the masts

[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/mat...ct03basics.htm

[Jimbo1490]

Thanks for the very informative posts!

Jimbo

[sailor2]

Originally Posted by 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.

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.

Originally Posted by Jimbo1490

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]

Originally Posted by 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...

Different poster, sorry