1989 Luxfer cylinder VIP refusals

[PhilEllis]

Just for informational purposes, I should not have surprised anyone that 6351T6 aluminum alloys might be subject to sustained load cracking. Aluminum alloys with high silicon (SI) content (above 0.5%-0.7%) have always been known to be subject to SLC. 6351T6 alloy has almost 1% silicon. These alloys were widely used in aircraft construction, because of the good tensile strength and yield strength characteristics. Unfortunately, these same characteristics make them less than great for applications where a sustained load is exerted. Numerous aircraft accidents in the early 1965-1985 were thought to be caused by SLC with these alloys. Add to that the stress of cold thread cutting, and there might have been a pretty good idea that sustain loads might cause a problem at the cylinder neck area. They presumably continued to use this alloy because of the other very beneficial characteristics previously discussed.

Note: I am not a material engineer, so I could be wrong.

Phil Ellis

 

[KVDIVR]

Given the amount of retesters that are busted just "stamping" cylinders I would not be surprised if the hydro and VE were not really done. The hydro test is 5000psi so that should have opened an existing crack and been found by an Eddy Current test and visual inspection into the crown prior to its first re-filling. 6351 cylinders crack from sustained loads, storing cylinders full over long periods. The worst years for this problem is 1982 & 83 due to the amount of lead in the alloy those 2 years. The older cylinders need to be stored at <500 when not being used and should be Eddy tested every 12 months. People think only older alum cylinders get cracks. I have failed at least a half dozen 6061 cylinders for cracks, one was a 2005 with 5 cracks in the threads. If you question whether testing is being done, put some chalk dust in the crown with your finger before sending to hydro and check to see if it is still there when it comes back. Someone was busted in southern FL by a shop doing this.
Good Luck
Dive Safe

 

[PhilEllis]

.............The worst years for this problem is 1982 & 83 due to the amount of lead in the alloy those 2 years..........

Lead? There certainly could be some lead in the alloy in trace amounts. But, if lead was introduced as an alloy element, then it wouldn't be 6351 alloy. Now, I will admit that a manufacturer is free to put anything in the alloy that they choose, but having done so, they can no longer call it 6351 and be in compliance with the aluminum industry protocol for alloy designations.

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

 

[DA Aquamaster]

I think the point has largely been lost here.

1. The OP's original question was about a Luxfer S30 made in 1989 - a year after 6351-T6 alloy stopped being used in Luxfer S30 tanks - meaning the tank was made of 6061-T6 alloy.

Also,

2. Cracks can no doubt occur in any aluminum tank, but SLC crack propogation is slow, and the scuba industry standard requiring an annual eddy current, in addition to 5 year DOT requirement, gives ample opportunity to detect a crack once it becomes detectable and long before it will proress to the point of failure. The multiple tests also provides some insurance for test facilties that may not due it or for human error in testers who don't know what they are doing.

3. In the vast majority of cases, SLC cracks in scuba cylinders will leak before the tank can burst. If you look at reports of suspected SLC "failures" that have been reported since the implementation of eddy current/visual plus protocols, they have all been leaks, not catastrophic failures.

4. There has in fact been no catastropic failures of 6351-T6 tanks due to SLC since eddy current protocols were implemented in 2001.

Again, my sole concern with Eddy Current/Visual Plus testing is that it is almost always done before the hydro test on the assumption that it makes no sense to send a tank through the hydro test process if it cannot pass the VIP portion of the requalification. However, in my experience I have seen AL tanks that pass an Eddy Current/Visual Plus inspection but then either develop a leak during the hydro test or fail an eddy current inspection administered after the hydro test. This suggests that the hydro test itself speeds propogation of the crack or makes it more detectable.

Consequently, I think it is prudent for a dive shop to conduct an Eddy Current/Visual Plus inspection after the tank returns from a hydro test before they valve it and fill it - regardless of whether it is 6351-T6 or 6061-T6 alloy.

 

[rjack321]

Lead? There certainly could be some lead in the alloy in trace amounts. But, if lead was introduced as an alloy element, then it wouldn't be 6351 alloy. Now, I will admit that a manufacturer is free to put anything in the alloy that they choose, but having done so, they can no longer call it 6351 and be in compliance with the aluminum industry protocol for alloy designations.

Yes lead its a minor component of both alloys but 6351 had more. I would not personally know which years had the most lead or the worst SLC rates as I almost never see 6351 cylinders in service around here. KVDIVR is merely reiterating PSI's materials which do state that 82 and 83 were the worst years. And that warmer climates (expansion of gas beyond service pressures?) and hydros aggravate SLC.

 

[Leadking]

As this thread seems to be activated I will add the following comments;


The latest report I have seen states that "catastrophic failure" of 6351 cylinders are from cylinders that have had cracks for years and not from newly formed cracks. While I can appreciate how an air leak from a crack can scare the **** out of you, there does not appear that it is dangerous. I have discussed cracking in variety of metals with cylinder engineers and cracking without fragmenting is what a properly engineered cylinder is supposed to do.