Support Items

SLC is a thoroughly researched and well-documented metallurgical phenomenon that occasionally develops in cylinders made from 6351 alloy, as well as in other types of pressure vessels and structural components under stress for sustained periods of time. SLC has occurred in cylinders manufactured by various companies, including Luxfer. Cylinders that have been mechanically damaged, over-filled or otherwise abused are more susceptible to SLC, which is not a manufacturing defect, but a phenomenon inherent in the metal itself.

Research has consistently shown that sustained-load cracks grow very slowly, typically taking eight or more years to grow large enough to cause a cylinder leak or, in extremely rare cases, a rupture. Because SLC growth is so slow, properly trained inspectors have adequate opportunity to detect cracks and remove cylinders from service during normal annual inspections or as part of the DOT-required requalification process every five years. Both Luxfer and DOT have long maintained that proper, diligent inspection by correctly trained personnel is the best way to avoid cylinder-related accidents.

It is important to note that SLC has occurred only in a small number of older cylinders made from 6351 aluminum alloy. Cylinders made from 6061 aluminum alloy, the metal currently used by all U.S. manufacturers, have not exhibited SLC.

Luxfer discontinued use of 6351 alloy in June 1988 and since that time has only manufactured scuba cylinders from Luxfer’s proprietary L6X® formula for 6061 aluminum alloy. Out of more than 40 million cylinders, no Luxfer cylinder made from L6X® (6061) alloy has ever exhibited a sustained-load crack.

For more detailed information about SLC, visit the Luxfer website at www.luxfercylinders.com, click “Support” on the top menu bar, click “Frequently Asked Questions” on the left-hand menu and then click the “Sustained-load Cracking (SLC)” heading. For specific dates when Luxfer switched different cylinder models from 6351 to 6061 alloy, click “Support,” then “Technical Bulletins” and scroll down to Dates when Luxfer changed its aluminum alloy from 6351 to 6061 (posted November 12, 2003).

If you have additional questions, call Luxfer’s toll-free customer service line: 800-764-0366.

1See Department of Transportation: HM-220F, Federal Register, Vol. 71, No. 167, pages 51122-51129.

All-metal Luxfer 6061 aluminum alloy (L6X®) cylinders1—To remove paint from an all-metal cylinder and prepare the surface for repainting, blast with plastic, glass or other soft media just long enough to remove most of the old paint. Do not blast with sand, steel, grit or aluminum oxide media, which are too abrasive. Heavy or improper blasting with hard or overly abrasive media removes metal as well as paint from the cylinder, which can thin the cylinder wall and make the cylinder unsafe to fill or use. Improper blasting also obliterates stamped markings and damages the outer surface of the cylinder.

Never use corrosive, caustic or acidic paint strippers or solvents to remove paint. Never use burning or torching techniques. Do not use knives or other sharp instruments to remove labels.

Once paint has been removed, use a neutral, mild, water-based detergent to wash the cylinder. Rinse the cylinder well and thoroughly dry it at a metal temperature not exceeding 160 degrees F (70 degrees C). Carefully inspect the cylinder surface for damage and corrosion in accordance with all applicable regulations. If the cylinder passes the inspection, it is ready for repainting.

Composite cylinders—Do not attempt to remove intact old paint from composite cylinders by blasting or any other means that may damage the cylinder exterior. Lightly rub the surface with fine-grit emery paper to prepare it for repainting. If the surface is dirty, clean it with a neutral, mild, water-based detergent, then rinse it well and dry it thoroughly.

Never use corrosive, caustic or acidic paint strippers or solvents to remove paint or to clean composite cylinders. Never use burning or torching techniques. Do not use knives or other sharp instruments to remove labels.

Once the outer surface of the cylinder is clean, inspect it for damage in accordance with all applicable regulations. If the cylinder passes the inspection, it is ready for repainting.

All cylinder types—Do not use the above preparatory processes on the internal surface and thread area of any Luxfer cylinder. Different processes (not covered by this document) should be used to clean threads and internal surfaces.

While preparing a cylinder exterior for painting, take care not to damage the valve connection area or cylinder threads. Do not allow any process materials (e.g., cleaning agents, blast media, grit, etc.) to contaminate the threads and inside of the cylinder.

All-metal Luxfer 6061 aluminum alloy (L6X®) cylinders—Do not use acidic or caustic paints that may damage the cylinder exterior. You may use any neutral, air-drying or aliphatic, water-based or solvent-based liquid paint that is compatible with aluminum. You may also use neutral, aluminum-compatible powder coatings as long as curing temperatures and exposure times do not exceed those recommended by Luxfer. Please note that different aluminum alloys have different maximum-temperature specifications; for example, for Luxfer 6061-alloy (L6X®) cylinders, the maximum metal temperature must not exceed 350ºF (175ºC). Exposure times at various temperatures vary according to alloy. Please contact Luxfer for time/temperature information about the particular cylinder and alloy that you intend to repaint using powder coatings. Cylinders subjected to temperatures and exposure times exceeding Luxfer recommendations or heated to unknown temperatures must pass hydrostatic tests before being filled or used.

Composite cylinders—Use flame-retardant, air-drying or aliphatic, water-based or solvent-based epoxy or polyurethane liquid paints on Luxfer composite cylinders. Do not heat cylinders to speed up curing or drying of paint. Do not use powder coatings or other coatings that must be heated or baked. Never expose composite cylinders to heat exceeding 180ºF (82ºC).

Do not paint over cylinder labels. Mask labels before painting and make sure that they remain legible. (Note: Color coding cylinders to identify gas contents or usage does not eliminate the regulatory requirement for legible, properly configured labels.)

All cylinder types—Do not allow paint to contaminate the threads, valve connection area or internal surface of any Luxfer cylinder.

1Luxfer markets its proprietary version of 6061 aluminum alloy under the registered trade name L6X

Luxfer Gas Cylinders reçoit occasionnellement des demandes d’extension de durée de vie pour les bouteilles bobinées en composite de Luxfer.

L’ensemble des bouteilles bobinées en composite de Luxfer a été conçu pour une durée de vie spécifique, généralement limitée à 15 ans. Actuellement, aucune autorisation légale ne nous permet de prolonger la durée de vie de ces bouteilles. En outre, si ces produits enregistrent un niveau de sécurité exemplaire, c’est en partie grâce aux limites réglementant leur durée de vie.

C’est la raison pour laquelle la politique de la société Luxfer Gas Cylinders ne prévoit aucune extension de durée de vie des bouteilles en composite.

Gelegentlich erhält Luxfer Gas Cylinders Anfragen über die Möglichkeit, die Nutzungsdauer seiner vollumwickelten Composite-Flaschen zu verlängern.

Alle vollumwickelten Verbundflaschen von Luxfer sind für eine beschränkte Nutzungsdauer ausgelegt, in den meisten Fällen auf 15 Jahre. Derzeit gibt es keine Ausführungsbestimmungen für die Verlängerung der Nutzungsdauer von solchen Flaschen. Überdies hat die gewerberechtlich vorgeschriebene Beschränkungen der Nutzungsdauer zu der beispiellosen Sicherheit dieser Produkte beigetragen.

Aus diesem Grund besteht Luxfer Gas Cylinders auf seinen Unternehmensgrundsätzen und verlängert die Nutzungsdauer seiner Verbundflaschen nicht.

Luxfer Gas Cylinders has received an increasing number of inquiries about the difference between “bowed” and “bulged” aluminum cylinders. The pictures and information below are intended to clarify this important difference.

Bowed Cylinder A bow is a slight curve in the sidewall of an aluminum cylinder (see image above). A fairly common cosmetic feature, a bow is not dangerous and does not affect cylinder performance. A bow is an occasional by-product of the manufacturing process, not a manufacturing defect. Bows occur most often in taller cylinders, including scuba cylinders with a capacity of 80 cubic feet or more. A typical bowed cylinder has one convex side curving slightly outward and an opposite concave side (180° degrees away) curving slightly inward. The curves are long and gradual, covering much of the length of the cylinder sidewall. Most bows are barely discernible with the naked eye, but they sometimes become noticeable when you hold a straight edge against the cylinder wall and rotate the cylinder to reveal both the convex and concave sides. More severely bowed cylinders are sometimes called “banana shaped,” and even these cylinders are safe to use. Infrequently a cylinder will have one straight side and one slightly convex (outward curving) side. Such a condition, called an unparallel sidewall or a plano-convexity, is also a type of bow. It does not affect cylinder performance. Inspectors should be careful not to misidentify the harmless, slightly convex side of a bowed cylinder as a bulge, which is completely different and a very rare condition.A bow is often so subtle that the naked eye has difficulty detecting it. (In the picture above, the bow has been exaggerated somewhat to make it clearly visible.) Using a straight edge will usually reveal convexity and concavity. Bear in mind that the convex (outward curving) side is sometimes more pronounced than the concave side. Do not mistake the convexity of a bow for a bulge! (For more information about bulges, see the following section.)

Bulged CylinderA bulge is an extremely rare, dangerous condition. Bulged cylinders must be immediately condemned and removed from service. Unlike a bow, which is usually slight and sometimes difficult to see, a bulge is generally very pronounced and obvious, even to the naked eye. Bulges occur in cylinders that have been overheated or in cylinders with sidewalls thinned by severe corrosion. There are two basic types of bulges. A long, convex bulge protrudes outward noticeably on one side or around the entire circumference of the cylinder; such a bulge occurs when an entire cylinder has been exposed to high heat. A smaller, “goose-egg-shaped” bulge usually indicates localized overheating, which is the result of “spot annealing.”

If you are uncertain whether a cylinder is bulged, remove it from service and contact Luxfer Customer Service before condemning the cylinder.