Failure Knowledge Database / 100 Selected Cases

Bursting of a scuba cylinder made of an aluminum alloy
June 30th, 2000, Miyako-jima, Okinawa Prefecture
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
Scuba or aqualung is an automatic self-breathing system for underw ater diving, and it has been widelyused fo r l eisure di ving. A scub a cyli nder m ade o f an aluminum al loy su ddenly bu rst ri ght aft er it wasfilled with air. According to the investigation after the incident, it was identified that there were a numberof the cra cks along the a xis of t he s crew part of cy linder.
corrosion and stress corrosi on cracking (SCC).

The causes of t he crac ks were i nter-granularCountermeasures, such as the revision of th e regulations

for the safety of scuba cylinders, were carried out.
1. Component
A scuba cylinder m ade of an aluminum alloy (Capacity: 10. 3liter, E xternal diameter : 185 mm,

Thickness : 13.5mm, Length : 6 28mm, Ma terial : A6351-T6, M aximum fillin g pressure : 21.6MPa(220kgf/cm2), Manufacturer : Luxfer Gax Cylinders. (See Fig. 1)
2. Event
Air was fille d int o the scuba cyli nder at the a ir fil ling station.

After the pressure reached 20.1 MPa

2(205kgf/cm ) and the filling was finished, the cy linder suddenly burst when the scuba cylinder was beingchanged.

The fill ing t ube w hipped aro und, a nd the operator w as bruised o n the rig ht l eg. Figure 2

shows the fracture surface of the burst cylinder.

Although the cylinder burst into two portions and flew

across the room, the damage resulting from the accident was not serious because the filling station was in a
half basement.
The following related accidents were revealed after the accident.
○ Overseas
There were seven burst accidents of the A6351-T6 cylinder in the U. S. A., Australia and New Zealand.
○ Do mestic
About 53,000 A6351-T6 cylinders were imported from 1985 to 1990, and a t otal of 10,462 cylinders
were re-inspected over the last five years.
During the air filling of an A6351-T6 cylinder at the filling station of Hachijoujima, Tokyo on August
8th, 2000, leakage from the shoulder of the cylinder occurred.
The al uminum alloy material used f or scuba cylinders w as c hanged from A6351-T6 t o A6061-T6.
However, similar cracks were found in the A6061-T6 cylinders.
According to the results of inspection of cylinders at the inspection station of Okinawa Prefecture fromJanuary to April 2001, the cracks along the axis inside the screw were found in nineteen (eleven of Luxfer
Gas Cy linders an d e ight of Aluminum Precisi on Prod uct) o ut of 487 A 6061-T6 c ylinders.
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nineteen cylinders which had a lo ng crack were cl osely investigated, and it was found that the cracks had
lengths from eight to eleven mm and depths from one to eighteen mm.
The following results were obtained from the fault tree analysis.
○ Figure 6 : T he fau lt tre e diagram focus ed on the fra cture m ode, f racture m echanism and fract ure
process.
The s cuba cylinder burst i nto two p ieces with out showing LBB (Leak Before Burst).

The fracturesurface consists of a sym metrical old surface and a newly created surface in the screw part of t he cylinder.
The origin of the failure is the old fracture surface, and the final rapture was occurred in a ductile fracturemode. The old frac ture s urface was t he axial crack from the screw of cylinder , and the frac ture wa s
caused by int er-granular corrosion.

The crack did not propa gate fro m the screw surface t o t he oute r

surface of the cylinder, but from the screw surface to inside the cylinder through its shoulder. T he fracture
had the "thumb-nail shape" (Figure 4).
cracking.

The "thumb-nail shape" is a distinctive feature of stress corrosion

The thumb-nail cracking did not penetrate into the cylinder: however, the cracking propagatedremaining very thin layer of the ligament, and the scuba cylinder burst at last.
Figure 7: The fault tree diagram focused on the inadequacy and inferiority of the design and manufacturingof the scuba cylinder (1).
The aluminum alloy m aterial used for scuba cy linders w as ini tially A 6351-T6.
proven that A6351-T6 showed characteristics of Sustained Load Cracking (SLC).
was changed to A6061-T6.

However, it w as

Therefore, the material

However, a great many cylinders made of A6351-T6 are still being used.

Figure 8: The fault tree diagram focused on the inadequacy and inferiority of the design and manufacturingof the scuba cylinder (2).
Stress corrosion cracking, which induces thumb-nail cracking, resulted from inter-granular corrosion at
the surface of the screw part of the cyl inder.

Although there was som e general and localized corrosion at

the inside of t he shoulder, body and bottom part, de ep inter-granular corrosion was not found. Throughthe m etallographic observation of t he ve rtical se ction of t he cylinder, it was fo und t hat t he cry stal grainbecame course a nd crystal grain show ed anisotropic growth.

They we re c aused by the warm worki ng

during m anufacturing t he head and t he r ecrystallization d uring s olution trea tment.

The s usceptibility tointer-granular corrosion and stress corrosion cracking was increased by the coarseness and the aeolotropic
shape of the crystal grain.
Figure 9: The fault tree diagram focused on the load history, environment and material of the cylinder.
A corrosion environment is required to induce inter-granular corrosion.
tensile stress are both r equired t o i nduce stress corro sion cracking.

A corrosion environment and

The p ossibilities of th e existence o finterstitial wa ter an d sa lt were in vestigated. The fil ling of the c ylinder with air is us ually con ducted
through a mother cylinder.

Because of inevitable inadequacies in the compressor, the drain separator andthe activated carbon a bsorption equipment, if i nterstitial water a nd sal t e xists in t he m other cy linder,interstitial wa ter an d sa lt would n aturally occ ur in t he scu ba cylinder as wel l. Even if there were n ointerstitial water and salt in the mother cylinder, if some water an d salt remained in the valve of the scuba
cylinder, there would be interstitial water and salt in the scuba cylinder.

2

It is also possible that interstitial

Failure Knowledge Database / 100 Selected Cases

water and salt could occur in the cylinder because of i nsufficient pressure in the scuba cylinder before thefilling of the air.

Although the exact cause cannot be specified, it is possible that the cause is one or moreof those processes mentioned above.

In reality, it was found that there were traces of interstitial water andsalt together with corrosion in the inner surface of most of the scuba cylinders that were investigated.
Figure 10: The fault tree focused on the inspection of the cylinder.
It is regulated by law that scuba cylinders must be inspected once every five years.
had passed the inspection three times in the past.

The burst cylinder

However, it is unknown whether or not a detailed visualinspection of the screw part was performed in any of t he past inspections.

If a detailed visual inspection

had been conducted and if it had been determined that there was n o cracking, then we m ust conclude thatthe detection of the cracking was not possible and that the cracking existed without being observed.

It isdifficult to find inter-granular corrosion along the axis at the shallow surface of the scuba cylinder by visualinspection. Even if the cracking becomes the thumb-nail type, the mouth of the cracking at the screw partis very small, so considerable technique and experience is required for successful visual inspection.
Figure 11: The event tree of the bursting of the scuba cylinder caused by stress corrosion cracking.
The scuba cy linder w as made of the problematic A 6351-T6 aluminum alloy.

Furthermore, th e grainaround the part of the screw become coarse and the grain shape aeolotropic due to the warm working andthe s olution treatment duri ng th e m anufacturing of th e he ad par t of the cylinder.
appears that water and salt got into the cylinder during the filling of the air.

On t he o ther han d, i t

Therefore, the inter-granularcorrosion apparently had been accumulating for a long time, and the stress corrosion cracking by the hoop
tensile stress caused by the i nner press ure of t he cy linder changed t he in ter-granular c orrosion into thethumb-nail cracking form.

The thumb-nail cracking was not observed by visual inspection of the cylinder.

The cracking grew under the continuous use of the scuba cylinder. Finally, the scuba cylinder burst, startingat the cracking, during the filling of the air.
3. Course
On August 23rd and September 8th, 2000 and April 24th, 2001 an advisory bulletin was issued to eachprefecture for distri bution to t he owners of scu ba cylinders, a ir fill ing sta tions a nd cylinder i nspecting
stations by t he Safe ty Divi sion of t he N uclear and Industrial Safet y Agency (NISA) of t he Agency for
Natural Resources and Energy (ANRE) in the Ministry of Energy, Trade and Industry (METI).
After the cracking was found in the cylinders made of A6061-T6, the NISA informed each prefecture on
July 4th, 2001 that a committee had been established by the NISA in the High Pressure Gas Safety Instituteof Japan (HPGSI) to investigate the possibility of bursting of cylinders made of A6061-T6.
The c ommittee for th e i nvestigation of the sc uba cy linders made of alum inum all oys, cha ired byProfessor Hideo Kobayashi of the Tokyo Institute of Technology, completed its investigation and opened its
report to the public on the HPGSI website on October 19th, 2001.
4. Cause
This case had a typi cal complex set of causes.

Six causes are shown bel ow. If one of these causes

had not occurred, the burst accident would not have happened.
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Failure Knowledge Database / 100 Selected Cases(1) Mater ial (A6351-T6)
In proper sel ection of amaterial whi ch is sus ceptible to co ntinuous load crack ing, inter-granularcorrosion, and stress corrosion cracking(2) Manufacturing (coarse crystal grain)
Inadequate procedure for manufacturing and heat treatment of the head part of the cylinder(3) The occurrence of interstitial water and salt in the cylinder(4) The extreme difficulty of observing the cracking during visual inspection(5) No occurrence of LBB (Leak Before Burst)
It is re quired th at there sh ould be L BB at the m aximum filling pressure during the m anufacturing.
That is, even if the cra cking on the surface ca used and exte nded by the stres s corrosi on cracking, adetectable leakage should precede the burst by the penetration of cracking, so the safety can be maintained.
However, LBB is o nly required for the parts of the container that are thinnest.

The screw, shoulder andbottom of the container, which are t hicker than body, are not subjects of th e requirement.

It was foundthat the LBB was not enforced for the thumb-nail cracking at the screw part.
(6) The lack of other accident information
There were similar accidents in foreign countries, but the information regarding those accidents was notproperly conveyed.
5. Immediate Action
The m aterial used f or scu ba cy linders has be en changed fro m A 6351-T6 t o A 6061-T6.
cylinders that are made of A6351-T6 are still being widely used.

However,

Furthermore, cracking along the axis in

the screw part of cylinders that are made of A6061-T6 was also found.

Therefore the following practicalcountermeasures are suggested.
(1) Apart from the inspection conducted once every five years, the owner of the scuba cylinder should be
required to conduct a visual i nspection fo r fin ding crac king al ong th e a xis i n th e screw par t. This
inspection should be required by law.

It should be noted that special technique and experience are

required for proper visual inspection.
(2) Air filling operators should be warned of the importance of the competence and maintenance of the air
filling station.
(3) Owners of scuba cylinders should be warned of the importance of protecting cylinders from interstitialwater and salt.
(4) Manufacturers and importers should be required to improve the criteria for the manufacturing and heat
treatment of t he head p art of t he cylinder.

The re ntal system for scuba cylinders is wi dely u sed.

Therefore, the user of the scuba cylinder is not always the owner.

The relationship, processing chain

and resp onsibilities of t he user, t he ow ner, t he operator of the air f illing, the i nspection s tation, t he
manufacture, and the importer must be clarified.
(5) Research on stress corrosion cracking of the 6000 series aluminum alloy should be increased.

4Failure Knowledge Database / 100 Selected Cases

6. Countermeasure
Based o n t he results of t he study by th e committee for th e i nvestigation of scu ba cylinders m ade ofaluminum alloys, on October 19th and November 23rd, 2001, a meeting of the high pressure gas division of
the high pressure and gunpowder safety branch of the Research Committee for t he Natural Resources andEnergy was h eld.

In the meeting, a n article o n "t he concre te m ethod of the countermeasures for t hemaintaining s afety of sc uba cy linders m ade of al uminum alloy s" w as recom mended f or in clusion in th ereport.
Special pro posal w as made for the revis ion of t he safe ty regula tions for cy linders to req uire a visualinspection of the screw part once a year in addition to the full inspection conducted once every five years.
Thus, according to this proposal, the additional clause (the ministerial ordinance No. 84 issued on June
10th, 2002) o f the safety regulations for cylinders (the ministerial ordinance N o. 5 0) w as p ut in to effect.
This cl ause s pecifically establishes t hat the p eriod of i nspection of scu ba seam less cylinders m ade ofaluminum alloys be once a year in article 24 of the safety regulations for cylinders.

It was also establishes

that the inspector must certify that there is neither harmful damage such as the cracking along the axis ofcylinder nor malfunction a round the scr ew part i n art icle 3 N o. 5 of n otice N o. 1 50 (vis ual i nspection),
which establishes the various details such as markings as well as the inspection methods based on the safety
regulations.
The High Pressure Gas Safety Agency had voluntary standards, "the regulations for an inspection of thecylinder for the scuba (KHKS 004-1983) ", that were actu ally applied.

However, the standards were oldand in tended for cy linders made of st eel. Th erefore, ta king into ac count t he pro posals f rom theinvestigation of the accident, a revision of the standards was commenced with the intention of treating bothaluminum and steel cylinders.
The committee for the revision of the regulations for the inspection of cylinders for SCUBA, which waschaired by Pr ofessor H ideo K obayashi, Tokyo Inst itute of T echnology a nd est ablished i n the cy linder
division of t he te chnical m embers of th e H igh Pressu re Gas Safety Agency , held its first m eeting o nJanuary 24th, 2002.

The self-established regulations, "regulations for the inspection of seamless cylinders

for scuba", were published in July 2002.
7. Knowledge
Complex causes:
The most accidents are caused by the results of conflicts among a number of individual causes.
accidents having complex causes, the projecting cause will usually be regarded as the major cause.
are two ca uses of co mplex causes.
accelerate the major cause.

In

There

In the first case, there is one m ajor cause, and the other causes justIn the second case, there are a number of major causes, and if a ny one of the

major causes fails to occur, the accident will not happen.
Particularly i n the investigation and a nalysis of acci dents i nvolving the destruction of m achinery, thematerial is generally na med as t he m ajor caus e, and investigations and e xperiments are f ocused on t hematerial.

That is, the material is treated as being suspect from the beginning.

5This is natural, because theFailure Knowledge Database / 100 Selected Cases

destruction of the m achinery is the des truction of material. The problem is with the c onclusion of th einvestigation and analysis of t he accident.

In Japan, the number of major causes given for an accident isusually very l arge, a nd therefore the m aterial rel ated fact ors w ill certainly b e na med am ong t he major
causes.

The des ignation of a lar ge number of ca uses is based o n a n aspect o f Japa nese c ulture t hat

stipulates t hat the s ocial re sponsibility o f an ac cident should be vague and t he r esponsibility should be
divided between 1) the user and manufacturer of the machinery, and 2) the manufacturer of the material.
It is necess ary to st udy suf ficiently t he results from a com plex cause a nd to ca refully c onsider the
results.
8. On the SideThe English name for a pressure vessel is 'cylinder".
"bomb".

However, in Japanese, a pressure vessel is called

A bomb in English is an explosive, and a Japanese 'bomb' carries this meaning as something that

could explode, becoming a deadly weapon that flies around after the explosion.
○

Applications of the vessel
The applications of the vessel range from industrial use, to medical use, home use and leisure.

The gascontents vary with application, such as air, oxygen or nitrogen.
○

Shape of the vessel
The shape of the vessel is similar to that of a sake bottle in that it has a thin neck, a broad bottom and araised bottom.

The function of the thin neck is to serve the contents a little at a time.

broad bottom is for stability when standing upright.

The function of the

The function of the raised bottom is for increasing thestability in the upright position.
○

The material of the vessel
The gas is light but the vessel is heavy.

In the past, the vessels were constructed to be stout and heavy

like a safe because the gas was considered to be precious and dangerous.

However, mass consumption of

the gases has cha nged this perspective, and in accordance to the request for lower weight, the material has
been changed from steel to aluminum and further to composite materials.
○

Corrosion starts from the bottom of a vessel
When the vessel made of steel is exposed to rain, it starts to corrode from the outer surface and bottom.

Cases where a hole penetrates t he body and the vesse l ex plodes o ccur m ore often w hen c orrosion s tarts
from the outer surface than when corrosion starts from the inner surface.
○

How much corrosion can the vessel incur and still be safe ?
A vessel will be safe even if half of the body thickness is corrode. Vessels are built to be that strong.

○

The vessel itself is a projectile
When a vessel explodes, in addition to splinters of the vessel material, the vessel itself can become aprojectile. If the valve is ope ned abr uptly, t he vessel would f ly li ke a rocke t becaus e of t he pressur erelease. A light vessel will fly more easily.
○

The gas must leak before the explosion
It is dangerous if a vessel explodes suddenly.

Even if stress corrosion cracking occurs, if the gas leaks
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Failure Knowledge Database / 100 Selected Cases

before the explosion when the cracking penetrates the body, the potential for explosion would be detected,
and i t m ight be possible to av oid the e xplosion by le ssening the pressure.
Break/Burst, or LBB.

This is c alled Le ak Before

According to the manufacturing standards for vessels, design for L BB is required.

With LBB design, we can be confident that the gas will leak before the explosion.
9. Primary Scenario
01. Ignorance
02. Insufficient Knowledge
03. Prejudice
04. Production
05. Hardware Production
06. Manufacturing of machinery and equipment
07. Cylinder
08. Aluminum alloy
09. Inappropriateness of material decision
10. Inappropriateness of manufacturing and thermal treatment condition
11. Usage
12. Operation/Use
13. Use of equipment and material
14. Interstitial water and salt
15. Failure
16. Fracture/Damage
17. Stress Corrosion Cracking (SCC)
18. Usage
19. Maintenance/Repair
20. Inspection
21. Difficulty of crack detection
22. Failure of Leak Before Break (LBB)
23. Failure
24. Large-Scale Damage
25. Burst

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Failure Knowledge Database / 100 Selected Cases

Fig. 1 Major dimension of cylinder.

Fig. 2 Fracture surface of broken cylinder.

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Failure Knowledge Database / 100 Selected Cases

Fig. 3Fig. 4

Old fracture surface.

Thumbnail-shape fracture surface.

9Failure Knowledge Database / 100 Selected Cases

Fig. 5 Metallographic observation results.

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