What are 6351 cylinders?

Aluminum 6351-T6 is an alloy that was previously used for SCUBA and SCBA cylinders. There is a problem specific to that alloy that makes them potentially very dangerous,so most fill station operators will not fill them. This is an old problem, but there are some of these cylinders still around and many divers are unaware of the dangers.

Several companies produced and distributed these, including Walter Kidde, Luxfer, and Cliff Impact.  Many millions of these high-pressure cylinders were produced from this alloy. It was discontinued for these by 1990, due to a metallurgical anomaly which can cause sustained load cracking (SLC) along the neck and shoulder areas.

SLC: an explosive problem

SLC cracks tend to develop slowly over several years, most frequently in tanks that have been stored full of air. SLC has caused the explosive rupture of many of these cylinders and multiple losses of life and limb.

Before the technology to electronically detect this problem was invented, we had one of these cylinders explode in our store during a fill. The force of the explosion tore a hole through the roof and knocked a chunk from the concrete slab floor. Damage to the steel frame of the basketball hoop hanging over our pool is still visible today. It was caused by just a small piece of the cylinder hitting the heavy gauge steel and shredding it apart. Luckily, no one was hurt.

This happened with many of these tanks, both in the US and abroad. In some cases, such as the incident in our store, the explosions caused damage but did not result in injury. In other cases, they caused serious injuries and even death.

The explosive problem occurred often enough to warrant a DOT investigation, which resulted in advisories and warnings but no recall. However, we know the problem has been understated. For example, the explosion at GCD has not been included in the known SLC explosion data reported by Luxfer, though we reported it when it occurred and then again later brought it to their attention that it was not included.

Luxfer, who produced most of these cylinders, switched to a proprietary 6061 alloy and phased out the use of 6351 completely in June 1988. They issued several bulletins regarding the SLC problems and offered replacement incentives for many years in an effort to get these older cylinders out of use.

Eddy Current Testing

Because of this significant issue, a new method of tank inspection, eddy current testing, was developed. You may have heard this referred to as VIP Plus, which is a specific brand name. The visual only method previously employed for tank inspection is no longer sufficient for testing, thanks to the metallurgical anomaly associated with the 6351 alloy. Eddy current machines use electronic waves and can detect cracks that are invisible to the eye.  DOT rules now require electronic eddy testing at the same time as hydrostatic requalification, but manufacturers recommend eddy current more often.

Dive Stores and fill stations are well aware of all this and have been for many years. Both the US DOT and NIOSH released multiple detailed safety advisories. Luxfer addressed the topic in several ways, including a seminar at DEMA, the largest annual diving industry trade show, and this may have been the most attended meeting in DEMA history. The room was packed full to standing-room only, and attendees overflowed into the hallway as well.

Do you have 6351 cylinders? Here’s what to do

Many individual divers, however, were not, and some still are not, aware of this significant problem. If you have aluminum SCUBA cylinders manufactured by Luxfer prior to July 1988, or any Walter Kidde tanks, they are likely 6351 cylinders. There are almost no fill stations who will agree to refill these now. While GCD does not have a store policy against this, we will not force our employees to do so. Due to their advanced age and the well known issue of SLC, we prefer not to fill them. As stated above, we have already had one of these tanks rupture and explode in our store.

In an effort to get these bottles out of use, we will issue a $40 credit toward the purchase of a new tank for each 6351 cylinder trade out.

*Image courtesy Extreme Watersports