Scuba Cylinder Long-Term Storage: Fact and Fiction

[Leadking]

slag report: http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Files/05H010_Final_Report_12_17_06.pdf
cylinder failure; http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Files/dot3al_rupture.pdf
DOT 6351 analysis; http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Files/3al_mainreport.pdf
DOT 3AL rupture;http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Files/3al3000.pdf
DOT Biennal report (see pages 18/55/62);http://www.phmsa.dot.gov/staticfiles/PHMSA/DownloadableFiles/Files/3al3000.pdf

Have fun reading!

 

[oxyhacker]

If you look at the early reports of the SLA problem, 6351 is often referred to as being leaded. The 1987 notice of a purposed rule change, in Federal Register vol. 52 no.172, makes several references to low lead (>100 ppm) and high lead (over >100 ppm) "varieties of 6351".

According to ASTM standards upto .05% by weight can be another element with a .15% by weight total. I agree with the statement that lead was added to help in forming process as a way to help lubricate the process.

 

[PhilEllis]

Just as a point of reference, many aluminum alloys allow for very small percentages of various different metals WHEN THEY OCCUR AS A RESULT OF NATURAL TRACE IMPURITIES. However, the official registration of the allow 6351 does not permit the addition of lead. Now, anyone is free to add whatever they want to any product they produce. However, the protocol is to never add a metal that is not specifically specified as any alloying material for the designated number. If you INTENTIONALLY add lead to an aluminum alloy that does not specify it, you are supposed to renumber the alloy and register the number and contents with the aluminum association.

If Luxfer intentionally added lead as an alloy element to their 6351, regardless of the purpose or benefit of the use, then it actually ceases to be alloy 6351.

Phil Ellis
www.divesports.com

 

[Leadking]

100ppm by weight =.0001x(2.6989/11.35)=.00042 well below .05% by weight allowed under ASTM standards.

2.6989 specific gravity of aluminum
11.35 specific gravity of lead

 

[PhilEllis]

100ppm by weight =.0001x(2.6989/11.35)=.00042 well below .05% by weight allowed under ASTM standards.

2.6989 specific gravity of aluminum
11.35 specific gravity of lead

That does clearly address maximum contents for analysis purposes. However, the INTENTIONAL addition of none alloying elements is not permitted by alloy numbering protocol. The only metals allowed to be INTENTIONALLY added to alloy 6351 are Silicon, Manganese, and Magnesium.

Added Note: When I get home tonight, I will try to dig up an Aluminum Association reference to this. Please note, what they (reportedly) did is not illegal and doesn't violate any federal statutes of which I am aware. It just doesn't conform to protocol when creating this numbered alloy.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

 

[Leadking]

Phil,

can you provide source material?

Let's all learn something!

 

[PhilEllis]

Phil,

can you provide source material?

Let's all learn something!

Lee, I think I can tonight when I get home. I have an Aluminum Association alloying guide at home. For a big part of my early "metals" life, I subscribed to the online version. However, since I no longer work in the metals business, I only have the printed text.

Just as it might interest some, the reason that alloys are "registered" in the first place and given a number based on the standard registration system, is to allow users of that product to anticipate certain performance standards that might result from use. In the "old" days before alloying standards, an aluminum mixture might have all sorts of things in it. This presented a very mixed and confusing picture for users, engineers, and manufacturers. As a result, extensive testing had to be performed on EVERY product. This was expensive and wasteful. As a post WWII aerospace and industrial economy exploded, users were demanding that there be some "control" on what you could put in aluminum alloys. The official alloy system, developed by Alcoa, ended this uncertainty by developing the alloy number system.

Beer Cans. Sometime, I will relate the real story as to why the top of a beer can is made from a different alloy from the body. It has to do with remelting the allow for recycling.

Phil Ellis
www.divesports.com

 

[WD8CDH]

Just as a point of reference, many aluminum alloys allow for very small percentages of various different metals WHEN THEY OCCUR AS A RESULT OF NATURAL TRACE IMPURITIES. However, the official registration of the allow 6351 does not permit the addition of lead. Now, anyone is free to add whatever they want to any product they produce. However, the protocol is to never add a metal that is not specifically specified as any alloying material for the designated number. If you INTENTIONALLY add lead to an aluminum alloy that does not specify it, you are supposed to renumber the alloy and register the number and contents with the aluminum association.

If Luxfer intentionally added lead as an alloy element to their 6351, regardless of the purpose or benefit of the use, then it actually ceases to be alloy 6351.

Phil Ellis
Discount Scuba Gear at DiveSports.com - Buy Scuba Diving Equipment & Snorkeling Equipment

FWIW Luxfer states that their current SCUBA tanks are made from a Proprietary 6061 alloy. What exactly is "proprietary" about it I have no idea, but the tanks meet DOT specs.

 

[PhilEllis]

Phil,

can you provide source material?

Let's all learn something!

Lee, I tried and tried to find an internet reference for the proper practice of aluminum alloy construction, but was unable to find what I was searching. However, I have a short quote from Aluminum Industry Association manual for construction of alloys. This quote is from an Alcoa developed document that was incorporated in the Aluminum Industry Standards for Construction of Materials (1977). The quote is as follows.........

"Designated alloys constructed under this numbering system allow for the trace presence of various earth elements. However, the engineering design of specific registered alloys permits the intentional introduction only of those alloying elements specified for the registered alloy target. Intentional introduction of other elements, even those allowed in trace amounts, is not permitted under this section. This applies even when such introduction results in the presence of a specific earth element well within the trace allowances. When specific earth elements are intentionally added to an alloy which otherwise does not permit the addition of that element, the newly created alloy should be registered and the chemistry should be submitted for inclusion under this standard as a specific and new earth element combination or alloy."

"Nothing in this standard shall prohibit or otherwise discourage the addition of the various earth elements into an aluminum alloy. However, registration of the resulting alloy is essential to maintain the ability of users to rely upon and act upon the predictable performance requirements and known failure modes of specific alloys."

I clearly understand what this means, and I think it would be obvious to most.....if you add something to alloy 6351 that isn't intended to be added, it is no longer 6351. Now, it may well be something amazingly close to 6351, but it needs a new name.

Phil Ellis
www.divesports.com

 

[PhilEllis]

FWIW Luxfer states that their current SCUBA tanks are made from a Proprietary 6061 alloy. What exactly is "proprietary" about it I have no idea, but the tanks meet DOT specs.

The term "proprietary" likely indicates something in the processing of their specific melts of 6061 alloy. However, it could also be some certain chemical "doctoring" of the alloy. But again, any chemical "doctoring" must still be accomplished using only the permitted introduction of elements. "Proprietary" could also indicate a unique combination of post-melt processes. These post-melt processes would include conditioning, hardening, or working. These are typically indicated as one or more "dash" designations after the original alloy number. This is where the -Tx or -Hx business would be indicated.

Phil Ellis
www.divesports.com