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Is my cylinder made from the "bad" alloy aka AL6351?

Discussion in 'Tanks, Valves and Bands' started by Scared Silly, Aug 7, 2010.

  1. Scared Silly

    Scared Silly Regular of the Pub

    # of Dives: 500 - 999
    Location: on the path to perdition
    Hopefully, this thread will become a sticky as it is an attempt to help educate divers on cylinders that were made from the AL6351 alloy.

    Please do no add posts to this thread unless it is for factual information or corrections. I especially do not want discussions on what some dive shop policies are. I reserve the right to ask mods to delete superfluous posts as I would like to keep the thread as informational as possible.

    Also I will not go into the details of sustained load cracking. Other than to say some cylinders burst catastrophically rather than leak. Luxfer has some info:

    Luxfer: Sustained-load Cracking FAQ


    First, lets start off with the important part what are the affected cylinders? which are only those made from aluminum and specifically the alloy 6351 (steel cylinders are not affected).

    Here is the official advisory that was published by DOT:

    Notice No. 94-7; Safety Advisory; High Pressure Aluminum Seamless and Aluminum Composite Hoop-Wrapped Cylinders

    Here is the summary for scuba cylinders:

    All Walter Kidde DOT-3AL cylinders, of which production ceased in
    January 1990, are made of alloy 6351-T6. Cliff Impact DOT-3AL cylinders
    were made from alloy 6351-T6 before July 1990, at which time Cliff
    Impact changed to alloy 6061-T6. Catalina Cylinders did not produce any
    DOT-3AL cylinders from alloy 6351-T6; therefore, cylinders manufactured
    by Catalina are not subject to this notice.
    Until determined otherwise, any DOT-3AL or DOT-E 7235 cylinder
    should be assumed to be made of alloy 6351-T6, if it was:
    1. Manufactured by Luxfer USA before the applicable date listed in
    the chart below;
    2. Manufactured by Cliff-Impact before July 1990;
    3. Manufactured by any other company in the United States,
    excluding Catalina, before February 1990; or
    4. Manufactured outside the United States.

    Now lets focus on cylinders made by Luxfer as they are the most predominate scuba cylinder made with AL6351:

    30 and 63 cu. ft............................. S30, S63 .... mfg date 5-88
    40 cu. ft....................................... S40 ........... mfg date 6-88
    50 and 92 cu. ft............................. S50, S92 .... mfg date 4-88
    72 and 100 cu. ft........................... S72, S100 ... mfg date 8-87
    80 cu. ft....................................... S80 ........... mfg date 1-88
    80.8 cu. ft.................................... S80.8 ......... mfg date 5-87

    By quickly scanning one can see that by July of 1988 Luxfer had quit making cylinders using AL 6351 and had fully switched over to AL6061. As importantly, given that the vast majority of aluminum scuba cylinders in use are AL80 for the most part one could "safely" say that any Luxfer AL80 made with a hydro year of 1988 were made using AL6061. Though I have seen a 1-88 hydro date for a Luxfer cylinder.


    Where did the 1990 date cut off for not filling a cylinder that many shops use come from???

    In a nutshell it an arbitrary date. As noted, Luxfer quit using AL6351 18 months before hand. Yet Walter Kiddie and Cliff Impact made cylinders using AL6351 through Jan 1990 and July 1990 respectively. I suspect the date which is a nice round number came from the observation that Cliff Impact cylinders were few and far between and while Walter Kiddie were common that the chance of Jan 1990 sneaking in was remote.

    Commentary regarding the 1990 date. I have heard the reason for the 1990 date is that it easy to remember for those filling cylinders. I do not buy it. Those filling cylinders are suppose to be knowledgeable and trained in hazmat rules and CGA recommendations for their job. Which requires the ability to read. I do not believe that having a quick lookup table such the one above is so difficult that substituting an arbitrary date is reasonable.


    So now that you have determined that you have a cylinder made from AL6351 what needs to be done with it?

    The practical side. More and more shops are declining to fill cylinders made from AL6351. As such, before spending money on requalifying check to see if you can get it filled. Otherwise, it might just be a big wind chime.

    What needs to been at the time of requalification? (aka hydro).

    The hydro test is the same for any other cylinder except after Jan 2007 all cylinders made from AL6351 must also undergo an eddy current test at the same time. I.e. the hydro shop must perform the test. In addition, the cylinder must be mark with a "VE" after the hydro date to signify that the test was successfully passed. Here are the rules in all of their glory:

    Hazardous Materials: Aluminum Cylinders Manufactured of Aluminum Alloy 6351-T6 Used in SCUBA, SCBA, and Oxygen Services--Revised Requalification and Use Criteria | Federal Register Environmental Documents | USEPA


    What about the annual VIP for a AL6351 cylinder?

    While annual visual inspections for all scuba cylinders are recommended by the CGA and the scuba industry there is no law mandating them. That said they are a good idea.

    Before 2007, AL6351 cylinders were typically inspected like any other scuba cylinder with the exception of an eddy current test. Now that the eddy current test is part of the re-qualification process there is no need for the test to be performed as part of the annual visual inspection. However, most shops continue to do the test as part of the annual visual inspection as they have the eddy current equipment on hand.

    Historical note: before the 2007 rules went into affect Luxfer recommended that AL631 cylinders be eddy current tested at least every 2.5 years (Part 11):

    Luxfer: Sustained-load Cracking FAQ

    As a practical side the eddy current was incorporated into the annual inspection.

    Commentary regarding performing the eddy current test on AL6061 cylinders. Many shops perform the test on all aluminum cylinders regardless of the alloy. Some of the early testers gave erroneous results. Some testers work on all alloys. Regardless eddy current is not needed on AL6061 as they are not prone to Sustained-load Cracking. Check to make sure you are not paying for a test that is not needed.


    Hopefully this information will be helpful to those who have a cylinder made from AL6351. Personally, at this point I would advise against buying one and if given one I would pass and make it in a wind chime or turn it in for scrap.
    Last edited by a moderator: Jul 23, 2014
  2. akdeepdiver

    akdeepdiver Dive Shop

    # of Dives: 200 - 499
    Location: Iowa Park, TX
    Just a addition in reference to the tanks mentioned in the sticky that are SP7235/E7235. The Special permit issued from the DOT states the following that is good to know:

    1. These tanks are NOT to be used for underwater use.

    2. The Special Permit mandates that all cylinders manufactured after June 1989 must be made of 6061-T6 Alloy.

    3. These cylinders have a lifespan of 15 years from date of manufacturer. After 15 years they must be condemed. By doing the math, all cylinders made with 6351 alloy should be out of the system, or they should be condemed by the hydrotester next time they go in. Finnally, FYI, these cylinders require a hydrotest every 3 years.
    couv likes this.
  3. DA Aquamaster

    DA Aquamaster Directional Toast ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: NC
    I don't know many divers who'd actually use an SCBA tank underwater due to valve compatibility issues.

    Even if you get past that, they are really floaty.

    The intent of restricting them from underwater use more over was in regard to any hoop wrapped composite tank being very vulnerable to damage to the coating over the wrap allowing water to penetrate under the wrap - an issue that is totally separate from the aluminum alloy used in the tanks.
  4. zimmo

    zimmo Angel Fish

    # of Dives: 100 - 199
    Location: puget sound

    jIM STEELE Dive Shop

    # of Dives: I just don't log dives
    Location: ALAMO CALIF
    6498 is what I look at a lot of those had neck cracks most all seam to pass today i think they have got the ones that were not insepted right out of service if they pass i fill Itrust the DOT and CTC
  6. kduffy

    kduffy Angel Fish

    # of Dives: 50 - 99
    Location: ECKS USA
    I lost 4 of the 6498 alloy tanks to visual after always passing hydro and VE in the past. Since I often fill from my compressor without a fill cage in remote locations I don't mind the shop being a little over zealous and taking them out on what may have been cracks, there were plenty in each. I knew when I got them that it was a matter of time and was even a gamble when they went to hydro before use. Just a way to outfit the whole family quickly and inexpensively at the time.
  7. Glenn08

    Glenn08 Barracuda

    My twin 50's are 1974 vintage. I found a shop that hydro tested and filled them. Their comment: many shops would rather refuse to hydro or fill these than do the research and be informed. Also note that the failures are most common with tanks that were over-filled or abused. Shops used to slow-fill tanks in a water tank. The explanation at the time was that it was safer: no over-pressuring, and the water tank supposedly contained some of the explosion of a bad tank, aluminum or steel. So does this mean the shop that refuses to fill these tanks has a habit of hot-filling and/or over-filling tanks ? I wondered if I should ask if they quit diving because of the odds of being attacked by a shark ?
  8. GrainDane

    GrainDane Angel Fish

    Aren't VIP's required annually? I was always under the impression that its a requirement. Even carbon fiber tanks require an informal inspection at time of fill.

    Also, if your dive shop if condemning cylinders make sure they are certified thru DOT. They must have a RIN number (Requalifier Identifier Number). You can search your dive shop on PHMSA's website or just ask the shop. If they look at you funny, they don't have a RIN. It should be a letter followed by 3 number (for example X999). DOT certified hydrostatic testing shops are the only facilities allowed to condemn any DOT cylinder. I'm not assuming your shop is doing anything malicious. Most shops are safety oriented. It may just save you a tank (and purchase of a new one) down the road.
  9. akdeepdiver

    akdeepdiver Dive Shop

    # of Dives: 200 - 499
    Location: Iowa Park, TX
    Graindane , to answer your question no clarify your statement,
    By law, visual inspections are only required at time of hydro for dot specification cylinders. Some special permit/exemption cylinders require visual inspections at shorter intervals.

    The annual VIP is a dive industry standard but not law.

    As for condemning cylinders, a cylinder out of hand may be condemned by an individual/company that holds a RIN, a representTive of the DOT, or the manufacturer. A cylinder my be condemned by anyone performing a VIP (I.e. Dive shop) as long as they have prior permission by the owner of the cylinder.
  10. GrainDane

    GrainDane Angel Fish

    akdeepkiver, Just like running into a tree isn't necessarily illegal. Too bad visuals aren't required by law annually.

    Anyway, not just any dive shop can condemn cylinders. By regulation (you can find the entire CFR - Code of Federal Regs online for FREE at www.ecfr.gov - 49 CFR applies to this) no person or entity can "Condemn" a DOT cylinder. This must be done by a facility, not a subcontractor, with a current Requalification Idenitification Number (RIN). Here is an interpretation from DOT, which is incorporated by reference into 49 CFR.

    Near the bottom of the page, Click on "Can just ANYONE condemn a cylinder"
    Updates and Interpretations that affect Cylinder testing

    I'm sure the University of Michigan won any lawsuit that may have ensued.

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