Scuba Cylinder Long-Term Storage: Fact and Fiction

[Doc Harry]

TAKE THIS QUIZ BEFORE YOU READ FURTHER.

1. When storing a scuba cylinder for long periods of time, should the tank be stored full or almost empty?

a. Full
b. Almost empty
c. It does not matter

2. When storing a scuba cylinder for long periods of time, in what position should it stored?

a. Upright
b. On its side
c. It does not matter

3. When storing a scuba cylinder for long periods of time, does the breathing gas (i.e., air, Nitrox, etc.) affect the tank?

a. Yes
b. No
c. It does not matter

4. Conversely, when storing a scuba cylinder for long periods of time, does the cylinder affect the breathing gas?

a. Yes
b. No
c. It does not matter

5. When storing a scuba cylinder for long periods of time, does a little bit of moisture affect the tank?

a. Yes
b. No
c. It does not matter

6. When we talk about long-term storage of scuba cylinders, what time period constitutes long term?

a. Three months
b. Six months
c. One year
d. Five years
e. It does not matter

INTRODUCTION

The proper procedures for the long-term storage of scuba cylinders are apparently controversial. There are many recommendations from government agencies and industry leaders on the subject that many people view as plain common sense. However, many others argue that scuba cylinders are so robust that it just does not matter how they are stored long-term.

In order to better understand for myself the consequences, if any, of improper storage, I decided to review the original research and literature behind all of the recommendations. If you are not interested in reviewing the original research and literature, then I suggest that you skip to the end. Otherwise, what follows is my detailed review and interpretation of the following:

a. Compressed Gas Association publication C-1 (hydrostatic testing)
b. Compressed Gas Association publication C-5 (wall stress requalification criteria)
c. Title 49 of the Code of Federal Regulations
d. The Battelle Memorial Institute study on scuba cylinder corrosion
e. The University of Rhode study of corrosion of steel and aluminum scuba cylinders
f. Department of Transportation (DOT) Cylinder Failure Analysis Reports
g. Scuba Cylinder Internal Corrosion: An Engineering Safety Study
h. United States Underwater Fatality Statistics

VOLUMETRIC EXPANSION & METAL FATIGUE OF SCUBA CYLINDERS

In normal use, a scuba cylinder expands when it is pressurized and contracts when the pressure is relieved. This is called elastic expansion. However, the cylinder does not return to its pre-pressurization volume because it has been permanently stretched to a slight degree. This slight increase in volume is called permanent expansion.

When pressure is applied to a dangerously weakened cylinder, it will expand more than expected for the given pressure. This increase in the elastic expansion indicates a reduction in the cylinder wall thickness and/or yield strength of the cylinder material. Elastic expansion will progress at an exponential rate with each expansion/contraction cycle until failure occurs.

The purpose of hydrostatic testing every 5 years is to identify cylinders that no can longer safely tolerate expansion-contraction cycles due to excessive metal weakness. Each manufacturer determines maximum allowable elastic expansion limits for their cylinders through material testing. These Rejection Elastic Expansion (REE) values are often stamped on the tanks.

Dangerously weakened tanks can also demonstrate progressive permanent expansion. A cylinder must be condemned if its permanent expansion exceeds 10% of the total expansion.

The REE is often stamped on the tank

Hydrostatic Testing

Hydro Water Jacket diagram

The Compressed Gas Associated (GCA) Publication C-1 promulgates the standards for conducting hydrostatic testing for compressed gas cylinders: GCA C-1 Methods for Hydrostatic Testing of Compressed Gas Cylinders.

The Compressed Gas Associated (GCA) Publication C-5 promulgates the testing criteria (such as Rejection Elastic Expansion (REE) criteria) for hydrostatic testing: GCA C-5 Wall Stress Requalification Criteria for High Pressure Seamless Steel Cylinders.

The Code of Federal Regulations mandates the criteria for cylinder condemnation: Title 49 of the Code of Federal Regulations, section 173.34(e)(6).

Testing criteria are derived from material studies conducted by the manufacturer and consultants. Here is a sample engineering report describing methods used in determining the hydraulic limits of a cylinder: Hydraulic Pressure Cycling and Performance Evaluation of a DOT-3AA 2265 Cylinder, M9501

Discussion of Volumetric Expansion & Metal Fatigue

Luxfer advertises that their aluminum scuba cylinders have been test cycled in excess of 100,000 times to service pressure.

Thus I can expect that a properly-maintained scuba cylinder should last me a lifetime. However, factors such as over-pressurization, heat, damage, corrosion and manufacturing anomalies can significantly shorten the life of any given cylinder. Hydrostatic pressure testing is necessary to identify cylinders that are in danger of failing before they explode.

An understanding of elastic and permanent expansion has helped me to understand that my scuba cylinders should be stored empty to minimize permanent expansion. Storing a tank at full service pressure increases permanent expansion. However, practical considerations require that cylinders be stored with some pressure (e.g., 300 psig) to prevent moisture from entering.

SEE PART 2 NEXT

 

[Doc Harry]

THE BATTELLE MEMORIAL INSITUTE STUDY ON SCUBA CYLINDER CORROSION

Henderson, N.C., W.E. Berry, R.J. Eiber, and D.W. Frink, Final Report on Phase I Investigation of Scuba Cylinder Corrosion, report prepared for the U.S. Navy Supervisor of Diving, Naval Ship Systems Command, by the Battelle Memorial Institute, Columbus Laboratories, Columbus, Ohio, September 1970, 118pp.

The Battelle study authors are hereafter referred to collectively as Battelle.

Background

In the late 1960s a gelatinous corrosion product was found in a number of U.S. Navy aluminum scuba cylinders at a facility in Maryland. Concerned, the Navy inspected at total 1,336 aluminum cylinders. The inspection revealed that 16% of the cylinders were corroded, many with severe corrosion. Did cylinder corrosion degrade cylinder strength? The U.S. Navy consulted the Battelle Memorial Institute to investigate this, and other, questions about scuba cylinders.

Battelle examined and tested 68 of the Navy's corroded aluminum cylinders and 10 new aluminum cylinders. Battelle performed rupture experiments on 3 new aluminum cylinders and 3 of the most severely corroded Navy cylinders. They also reviewed data from 29 aluminum cylinder rupture tests performed by Pressed Steel Tank Company. Battelle also analyzed corrosion and non-corrosion material from a total of 11 corroded Navy cylinders.

The Battelle study is one of the best sources of information I have on the long-term storage of scuba cylinders. It is so important because it investigated the effects of long-term corrosion of scuba cylinders that were actually in use. It was not entirely a theoretical experiment. However, the Battelle study was not a randomized, case-controlled study that specifically addressed storage issues. Nonetheless, I can draw some specific conclusions from the study results.

Battelle's Rupture Tests of New Aluminum Cylinders

Comparing the calculated rupture strengths of new cylinders with those of corroded cylinders would provide a rough indicator of whether or not corrosion had degraded cylinder strength. However, Battelle had to control for differences in cylinder composition and dimensions. Therefore, they conducted detailed material and physical analysis of all of the cylinders included in the rupture tests. These measurements included material tensile strengths, gross cylinder dimensions and volume, as well as 20 measurements of wall thickness at specified locations.

Battelle performed rupture tests by filling 3 new, uncorroded aluminum cylinders with water until they ruptured. Pressures at rupture ranged from 7,025 to 7,740 psig. The rupture data from these 3 new aluminum cylinders was found to be representative of the data that was provided by Pressed Steel from rupture tests of 29 other new aluminum cylinders.

Battelle's Measurement of Tensile Strength of New Aluminum Cylinders

Battelle then tested the uniaxial tensile strength of the 3 new (but recently ruptured) aluminum cylinders. Ultimate yield stress ranged from 47,200 to 47,700 psi.

Battelle's Rupture Tests of Corroded Aluminum Cylinders

The 3 most corroded aluminum cylinders were selected for rupture tests. The designation of severe corrosion was entirely subjective as determined by visual inspection with an automobile light bulb that was inserted into the tank. Battelle conducted the rupture tests as described above. Pressures at rupture ranged from 7,225 to 7,950 psig.

Battelle's Measurement of Tensile Strength of Corroded Aluminum Cylinders

The uniaxial tensile strength of the 3 corroded/ruptured aluminum cylinders ranged from 47,000 to 49,800 psi.

Battelle's Evaluation of Cylinder Corrosion and Degradation

Battelle used the actual rupture pressures and tensile strength to calculate the rupture strength of the cylinders. Then, in order to determine if corrosion had affected rupture strength, Battelle compared the ratios of the rupture stresses to the ultimate tensile stresses for the new cylinders and corroded cylinders. This method was thought to provide a more accurate measure of the effect of corrosion on cylinder rupture strength by taking into account differences in material properties and cylinder dimensions.

Battelle found that these calculations revealed a 4% reduction in the calculated rupture stress of the corroded cylinders. However, Battelle stated that the small sample size in their investigation precluded any statistical significance. Furthermore, Battelle stated that the strength reduction is not believed to be significant because of the margin of safety that must be provided in the cylinder design.

Battelle's Discovery of a Critical Flaw Causing Degradation of Cylinder Rupture Strength

During the evaluation of cylinder corrosion, Battelle discovered several cylinders that had internal side wall corrosion in a longitudinal pattern. This corrosion pattern resulted from moisture inside of the tank accumulating on the internal side wall when a tank is stored on its side. Battelle conducted an investigation to determine if this type of corrosion would degrade cylinder rupture strength.

First a new aluminum cylinder was artificially flawed. A longitudinal pit was engraved into the internal side wall, 6.74 inches long, 0.357 inches deep and 1/16-inch wide. The artificial flaw had rounded margins. Ultrasonic measurements confirmed the dimensions of the artificial flaw. This pit would have easily condemned the cylinder by today's standards (standard: >6 inches and >0.030 inches deep).

The cylinder was then pressurized with nitrogen until it exploded at 3,430 psig. Battelle stated, this experiment confirmed that a ruptured, pneumatically pressurized scuba cylinder represents a serious potential personnel hazard. However, Battelle also concluded that it would take a large flaw to produce rupture at the 3000-psig operating pressure of the cylinder.

Evaluation of the 3 corroded cylinders used in the rupture tests did not reveal any longitudinal flaws that would suggest that such a flaw was the cause of the cylinder failures. Instead, those cylinders were diffusely corroded.

However, in one of the test cylinders the fracture line was noted to be guided from one corrosion pit to another. This particular cylinder had, on average, the shallowest (0.012-0.026 inches) but longest (0.266-0.360-inches) corrosion pits. (There was one other particularly deep pit measuring 0.047 inches deep x 0.390 inches long.) The other two cylinders had corrosion pits ranging from 0.024-0.036 inches deep to 0.125-0.312 inches long. These pits would have condemned all of the cylinders by today's standards (standard: >15% and >0.030 inches deep).

Battelle concluded that deep pits arranged longitudinally probably presented the greatest hazard to personnel and that some schedule of cylinder examination is mandatory.

Battelle's Conclusions

Battelle concluded that the observed corrosion had slightly reduced the calculated rupture strength of the aluminum cylinders. Their conclusions were not statistically significant due to small sample size. The reduction in rupture strength was deemed not significant due to strength margins afforded by the cylinder manufacturing process.

Battelle also recommended changes in cylinder cleaning procedures and cylinder inspection procedures to prevent corrosion from progressing to the point of degrading the rupture strength of aluminum scuba cylinders.

Battelle did not make any comments about scuba cylinder storage.

Discussion of the Battelle Study

The Battelle study addressed a variety of issues regarding scuba tanks (such as the manufacturing process) that were not applicable to my investigation. Those issues were excluded from the above discussion and will be excluded here, too.

Is the Battelle data applicable to today's scuba cylinders? Perhaps, perhaps not. The Battelle study was not a statistically-significant, randomized, case-controlled study. Nonetheless, I can use the Battelle data to better understand the nature and time course of corrosion and its effects on scuba cylinders. From that understanding I can make informed choices about how I stow my tanks.

One major concern about the applicability of the Battelle study is that the cylinder alloy was not specified for any of the rupture tests. The oldest cylinder used in the rupture test was manufactured in 1956, the year that the 6061 aluminum alloy was making its way into military service. So were these cylinders a 6061 alloy? No one will never really know because the chain of documentation has been broken. Given that the manufacturing process has been updated and that technology has improved quality control, it is difficult to determine whether or not rupture tests conducted in 1970 (on a cylinder manufactured in 1956) have any meaning today.

The service histories of the test cylinders are also not know. Had the cylinders been in heavy or light use? Had they been rode hard and put away wet? Or had they been in constant use? One of the cylinders was manufactured in 1956, fourteen years before the rupture tests. The other two corroded cylinders had been manufactured only 6 to 7 years before the rupture tests. It is difficult to draw any conclusions about the degree of corrosion without knowledge of the cylinder's service history.

Would I ever see such severe corrosion in scuba cylinders today given that we now have annual cylinder inspections? The Battelle study was conducted in 1970. Cylinders manufactured in 1967 and 1965 showed only slight corrosion at worst. The severe corrosion was observed in cylinders manufactured in 1956, 1961, 1963 and 1964. These cylinders were in service 14, 9, 7 and 6 years, respectively, prior to the study. Based upon the pit depth measurements of five severely corroded cylinders, all of them would have been condemned by today's standards. So at least in this particular study, aluminum cylinders in the Navy became severely corroded in as little as 6 years.

One might be tempted to conclude that the Battelle study demonstrated that even severe corrosion that would condemn a cylinder had no significant impact on cylinder rupture strength. However, remember that the Battelle study had a sample size too small to definitively prove or disprove anything at all, except what happened to those 78 specific cylinders used in the study.

So what conclusions can I draw from the Battelle study?

Even though scuba tanks are not supposed to have water in them, water can and does inadvertently enter scuba tanks during the course of normal use. This water causes corrosion.

Diffuse corrosion does impact aluminum cylinder rupture strength, but whether or not this impact is significant during the lifetime of a modern scuba cylinder for a recreational diver has never been determined in a properly designed study.

Large, deep artificial pits engraved into the internal side wall of an aluminum scuba cylinder can cause the cylinder to explode at pressures typically found during recreational scuba fill operations. Natural corrosion pits of similar size can be expected to have similar consequences. Frequent inspection of scuba cylinders is necessary to find corrosion pits before they get large enough to impact cylinder strength.

Deep, longitudinal pits that result from storing a cylinder on its side with moisture in the tank probably presents the greatest risk of explosion. Since my tanks can become contaminated with moisture without my knowledge, it is probably prudent to store my tanks standing upright so that corrosion with be focused at the base. The thick base of the tank will tolerate deeper pits than the side walls.

One issue that has not been addressed is that of the corrosion timetable. If I store my cylinder in the garage over the winter, can anything bad happen in such a short period of time? The Rhode Island Study may give us insight into this issue.

 

[Doc Harry]

THE RHODE ISLAND STUDY ON SCUBA CYLINDER CORROSION

Cichy, Francis, Hilbert Schenk, and John J. McAniff. Corrosion of Steel and Aluminum Scuba Tanks, University of Rhode Island Technical Report 62, 1978.

The University of Rhode Island study authors are hereafter referred to collectively as URI.

Background of the URI Study

The University of Rhode Island (URI) tested the corrosion effects of fresh and salt water on aluminum and steel scuba tanks. This study is probably the best source of information I have on the long-term storage of scuba cylinders. This study is so important because it actually tested the effects of long-term corrosion of scuba cylinders. It was not entirely a theoretical experiment. However, the URI study was not a statistically-significant, randomized, case-controlled study. Nonetheless, I can draw some very specific conclusions from the study results.

Water has been known to enter scuba tanks through the second and first stages of the regulator. URI took an empty diving cylinder with an attached two-stage regulator to a depth of only 10 feet in a swimming pool. They pushed the purge button ten times. Later, after removing the cylinder's valve, they found between 195 and 211 milliliters of water in the tank. URI therefore decided to use 500 milliliters of water in their tests described below.

URI Steel Cylinder Corrosion Test

URI used 6 new DOT-3AA uncoated 4130 high-pressure steel cylinders for the test. They put 500 milliliters of either fresh or salt water in the cylinders. The fresh water was local tap water. The salt water was a standardized commercial salt solution. The cylinders were then pressurized with air (20.9% oxygen) and stored at about 102-105 degrees Fahrenheit in an ammunition bunker for 100 days. Each cylinder had an attached thermocouple to monitor actual temperature. The cylinders were stored either upright (vertical) or lying on their side (horizontal). A summary of the test conditions is shown below:

Four test tanks contained salt water; two were stored on their side at full pressure and one was stored vertically upright at full pressure. The fourth cylinder was stored vertically but had almost no pressure (100 psig). All were kept at about 103 degrees F.

Two test tanks contained fresh water; one stored on its side at full pressure and one was stored vertically upright at full pressure.

After 100 days at about 104 degrees F, the cylinders were removed from the bunker, the water was dumped and the cylinders were examined.

Results:

One cylinder that was stored horizontally with salt water at full pressure was so badly corroded internally that only 30 ml of salt water remained. The rest of the salt water had been consumed in the corrosion process. Corrosion had reduced the cylinder wall to only 1/3 of its original thickness (wall thickness before: 0.179 inches after: 0.055 inches. After only 100 days, this cylinder was corroded so badly that it was in real danger of exploding. It was not subjected to hydrostatic testing.

The other matched cylinder (salt water, horizontal, full pressure) also demonstrated substantial internal corrosion. Only 110 ml of salt water was still present in the cylinder. Corrosion had reduced the cylinder wall to about 1/2 of its original thickness (wall thickness before: 0.179 inches after: 0.096 inches. This cylinder was not subjected to hydrostatic testing because of the wall corrosion.

The cylinder that had been stored vertically (upright) with salt water at full pressure had less overall internal corrosion because the water was not spread out across the side wall. A total of 280 ml of salt water was still present in the cylinder. Corrosion had reduced the cylinder wall by only 15% (wall thickness before: 0.217 inches after: 0.181inches. This cylinder was not subjected to hydrostatic testing.

The fourth salt water cylinder that had been stored almost empty (100 psig) had the least overall internal corrosion, compared to all of the other cylinders that contained salt water. Corrosion had reduced the cylinder wall thickness by only 7% (wall thickness before: 0.217 inches after: 0.202 inches. The cylinder was not subjected to hydrostatic testing.

The two cylinders that contained fresh demonstrated only minor internal corrosion and both passed hydrostatic testing.

Discussion of the URI Steel Cylinder Corrosion Test Results

It is quite clear from this study that corrosion is significantly accelerated in the presence of salt water compared to fresh water. Although 500 ml of salt water is a lot, the cylinder had lost 2/3 of it wall thickness in only 3 months. One could easily imagine similar results from storing a slightly-contaminated steel cylinder in a Florida shed over the summer. In fact, there is one documented death related to corrosion that took place over a 3-month period, as will be discussed later. While such a scenario might not reduce the wall thickness by 2/3 as it did on the URI study, it could easily result in enough corrosion to condemn the tank or cause it to explode if overfilled.

If we compare the corrosion of the salt-water cylinder that was stored vertically but almost empty with that of the salt-water cylinder that was stored vertically but pressurized, it is clear that corrosion is accelerated in the presence of elevated pressure (and salt). This difference is the result of elevated oxygen partial pressure (pO2) and oxygen's role in corrosion. Thus, steel cylinders should be stored with minimal pressure to minimize the pO2 if they are being used in a salt water environment.

Finally, if we compare the corrosion of the salt-water cylinders that were stored upright with that of cylinders that were stored horizontally, it is also clear that the position of the cylinder greatly influences the degree of corrosion in the presence of salt water. Cylinders stored horizontally demonstrated large areas of significant corrosion since the salt water was spread over a large surface area. Therefore, steel cylinders should be stored upright to minimize the extent of salt water corrosion.

One might ask why you would need to take such precautions since water is not supposed to be in a cylinder. The answer is that unless you remove the valve and inspect the cylinder prior to storage, you simply do not know what is or is not inside of your tank.

URI felt that in addition to the explosion risk, there was also significant danger of having the valve suddenly plugged by the large amounts of rust in the cylinders.

 

[Doc Harry]

URI Aluminum Cylinder Corrosion Test

By 1975, the problem of bimetal (galvanic) corrosion was becoming readily apparent. The combination of dissimilar metals (aluminum cylinder and brass valves) along with an electrolyte solution (seawater) was causing a galvanic current to flow between the aluminum (the anode) and the brass valve (cathode). This was causing cylinder neck threads to degrade quite severely and become fixed to the valve threads (galling). In some cases, the galling was so severe that cylinders threads were completely stripped when the valves were forcefully wrenched off the cylinders.

Galvanic Corrosion

In light of the problem of galvanic corrosion, URI decided to test galvanic corrosion in the aluminum cylinder study. Some of the aluminum cylinders were stored inverted so that the salt solution was in contact with the brass valves.

URI used seven new 72-cubic foot aluminum cylinders (DOT-3AL) for this test. The alloy was not specified but 6351 is inferred. Test conditions were similar to that of the steel cylinder test. Some of the tanks were inverted to immerse the valves for the galvanic corrosion evaluation. One such cylinder had only 250 ml of salt solution so that the valve snorkel was above water when it was inverted. The remaining inverted cylinders had 500 ml and the valve snorkel was completely submerged. All cylinders were pressurized with air (20.9% oxygen) and the residual gas was analyzed after 100 days.

There was also one inverted steel tank included in the series to test the effects of galvanic corrosion of steel tanks. The steel cylinder was pressurized with air (20.9% oxygen) the residual gas was analyzed after 100 days.

A summary of the test conditions is shown below. All cylinders contained 500 ml of either fresh or salt water (except as noted).

One steel cylinder contained salt water; it was stored vertically (inverted) at full pressure

Six aluminum cylinders contained salt water. Two were stored on their side at full pressure. One was stored vertically (upright) at full pressure. Two were stored vertically but inverted; one of these had only 250 ml of salt water. The sixth cylinder was stored almost empty (100 psig) vertically but inverted,

One cylinder contained fresh water; it was stored vertically but inverted at full pressure.

After 100 days at about 104 degrees F, the cylinders were removed from the bunker, the water was dumped and the cylinders were examined.

Results

The salt-water steel cylinder that was stored vertically but inverted at full pressure was very badly corroded internally with large sheets of corrosion hanging on the walls. This finding of severe corrosion mirrored the findings of the previous steel cylinder study. Corrosion had reduced this cylinder's wall to less than 1/2 of its original thickness (wall thickness before: 0.151 inches after: 0.070 inches. In contrast, the brass valve suffered very little corrosion.

The most surprising finding, however, was the residual gas analysis. The gas in the steel cylinder had very abnormal values: oxygen was significantly reduced (15.0%), carbon monoxide was elevated (10 ppm) but carbon dioxide was normal (0.01%). The Law of Thermodynamics predicts that such a drop in oxygen content would be associated with the production of 1.5 pounds of rust, which agrees well with what was found inside of the cylinder. The cylinder should have also lost 150 psig to oxidation, but the cylinder dropped only 80 psig. This discrepancy was felt to be within the error limits of the small-faced pressure gauges that were used. For comparison, the gas analyses in two matched aluminum cylinders were normal (20.9% oxygen, 3.0-3.5 ppm carbon monoxide and 0.03% carbon dioxide).

The two salt-water aluminum cylinders that were stored horizontally at full pressure showed negligible internal corrosion. One cylinder did not have any pits at all. The other cylinder had pits no deeper then 0.020 inches. These cylinders passed hydrostatic testing with only 1.48% and 0.32% permanent expansion, respectively. Residual oxygen content in one of the cylinders was measured at 20.9 percent.

The salt-water aluminum cylinder that was stored vertically but inverted at full pressure showed the greatest internal corrosion despite having the least amount of salt solution (250 ml), compared to the other cylinders. Wall thickness was preserved but pits were as deep as 0.084 inches. Despite have the worst corrosion, this cylinder was not in any danger of failing hydro.

The two salt-water aluminum cylinders that were stored vertically but inverted had locked valves. The cylinder threads had to be stripped to remove the valves. The cylinders themselves showed negligible internal corrosion without any pitting. No other valves were locked on any other cylinders.

The salt-water aluminum cylinder was stored with the lowest pressure (100 psig) had the least internal corrosion despite the fact that it contained salt water. There was no pitting. The cylinder passed hydrostatic testing with only 1.54% permanent expansion.

The fresh-water aluminum cylinder that was stored vertically but inverted showed substantial internal corrosion despite having fresh water, not salt water. Wall thickness was preserved but pits were as deep as 0.047 inches. Residual oxygen was measured at 20.9 percent.

Discussion of the URI Aluminum Cylinder Corrosion Test Results

I believe that the most surprising finding in this study was the reduction of oxygen in the steel cylinder gas due to corrosion. After only one hundred days, 500 ml salt water caused so much oxidation and corrosion that oxygen content was reduced to only 15.0 percent. It is unfortunate that residual gas was not measured during the previous steel cylinder corrosion test. I feel that this study demonstrates that steel tanks that are stored for long periods of time must be either (1) reanalyzed for oxygen and reanalyzed for carbon monoxide before use, or, better yet, (2) completely drained and refilled with fresh gas.

There is one documented death from breathing a corrosion-induced hypoxic mixture. This case is discussed in another section.

Unlike the previous steel cylinder study that established the highly corrosive influence of salt water, this study demonstrated that salt water had an inconsistent influence on aluminum cylinders. Only two aluminum cylinders were substantially corroded, one of which was the cylinder with fresh (tap) water. URI blamed this unexpected finding on high levels of copper ions (0.18 ppm) in the tap water. Copper ions are known to promoted galvanic corrosion. However, URI failed to explain why the steel cylinders with the same fresh (tap) water did not suffer substantial galvanic corrosion like the aluminum cylinders. The combination of brass valves and copper ions should have produced galvanic corrosion in the steel tanks just as it did in the aluminum tanks.

On aluminum cylinder was thought to be so badly corroded because the valve snorkel protruded above the water line. The valve snorkels in the other test cylinders were completely submerged. Complete submersion of the cathode (brass valve) limited the galvanic corrosion process to the rate at which oxygen diffused through water. However, since the valve snorkel in the cylinder in question was not submerged, galvanic corrosion was able to proceed unimpeded. Furthermore, galvanic corrosion accelerated as copper ions entered the water from the corroding brass valve snorkel.

Also, unlike the previous steel cylinder study that established accelerated corrosion in the presence of a high partial pressure of oxygen (pO2), I found this aluminum cylinder study to be inconclusive. One aluminum cylinder developed only negligible corrosion in the presence of a low pO2 (100 psig) even in the presence of salt water. But four other aluminum cylinders also developed only negligible corrosion at high pO2 (full pressure) even in the presence of salt water.

The effect of the positioning of the tanks (vertical versus horizontal) was also rather inconclusive. This may because the corrosion due to physical positioning was confounded by the effects of galvanic corrosion. In contrast to the steel cylinder study, the two horizontal aluminum tanks showed only negligible corrosion in the presence of salt water and high pO2. The aluminum cylinder that was stored upright also showed only negligible corrosion in the presence of salt water and high pO2. It appears that the only conclusion I can draw is that it does not seem to matter if aluminum cylinders are stored upright or horizontally, they just should not be stored inverted.

Finally, it appears that aluminum cylinders are much more susceptible to galvanic corrosion that steel cylinders. One aluminum cylinder had a valve that was locked tight even though it was stored upright. Another aluminum cylinder showed substantial corrosion despite containing a fresh water solution instead of a salt solution. The steel cylinders from the previous study that contained a fresh water solution had only minor corrosion without any pitting. As a result, I conclude that aluminum cylinders must be kept as dry as possible, especially in a seawater environment.

 

[Doc Harry]

URI Galling Experiment

The galvanic corrosion and neck thread galling discovered in the URI Aluminum Cylinder Corrosion Test prompted URI to conduct a follow-up investigation. The purpose of the follow-up study was to determine if anything could be done to prevent galling that causes aluminum cylinder neck threads to be stripped when the valve is removed.

The corrosion products found on the neck threads of aluminum cylinders from the previous study were analyzed. The material was found to be a combination of aluminum hydroxide and zinc hydroxide, both known products of galvanic corrosion. (Brass valves are coated with zinc.)

URI used five 50 cubic-foot aluminum cylinders (DOT-3AL) and one 80 cubic-foot aluminum cylinder. The cylinders alloys were not specified. Test conditions were similar to that of the previous aluminum cylinder test. The tanks were inverted to immerse the valves. All cylinders had 250 ml of standardized salt solution so that the valve snorkel was above water. All cylinders were pressurized with air (20.9% oxygen) to full pressure. Two cylinders had their metal valve snorkels replaced with plastic ones. Two other cylinders had aluminum valves and snorkels instead of brass valves. Another cylinder had two o-rings installed on the brass snorkel to prevent water from reaching the snorkel threads.

All valves threads and cylinder neck threads were coated with a lubricant to test whether or not galvanic corrosion and galling could be prevented. These lubricants were either: (1) High Purity Goop, (2) Molycote 557 or (3) Dow Corning 111.

A summary of the test conditions is shown below. All cylinders contained 250 ml of salt water and were stored vertically but inverted at full pressure.

Two cylinders had chrome-plated brass valves/snorkels
Two cylinders had plain brass valves with plastic snorkels
Two cylinders had aluminum valves and aluminum snorkels

After 100 days at about 104 degrees F, the cylinders were removed from the bunker, the water was dumped and the cylinders were examined.

The two cylinders with chrome-plated brass valves had substantial internal corrosion with pits as deep as 0.118 inches. The valves were easily removed. The cylinders passed hydrostatic testing.

The two cylinders with plain brass valves but plastic snorkels had negligible internal corrosion. One cylinder had no pits at all and the other cylinder had pits as deep as 0.020 inches. The valves were easily removed. The cylinders passed hydrostatic testing.

The two cylinders with aluminum valves had negligible internal corrosion with pits as deep as 0.020 inches. The valves were easily removed. The cylinders passed hydrostatic testing.

Discussion of the URI Galling Experiment

All of the valves in this experiment were easily removed after 100 days. Therefore I conclude that, from the perspective of galling, it does not matter what kind of lubricant is used, but a thread lubricant should be used to prevent galling.

Did thread lubricant reduce corrosion? We need to compare the test cylinders in the galling study with matched cylinders from the aluminum cylinder study. Unfortunately, only one cylinder from each study can be matched. A salt-water aluminum cylinder from the Aluminum Corrosion Study and a salt-water aluminum cylinder from the Galling Study can be compared; both were stored vertically but inverted at full pressure. Both cylinders demonstrated the most substantial internal corrosion in their respective study. The pits were as deep as 0.084 inches in one cylinder and 0.118 inches in the other cylinder.

It is difficult to draw definite conclusions based on pit depth comparisons between only two tanks, especially when the pit depths are measured in thousandths of an inch. What is clear to me, however, is that thread lubricant does not substantially reduce overall corrosion.

Finally, the one result that jumps out at me is that the two cylinders with standard chrome-plated brass snorkels demonstrated the most substantial internal corrosion. The other cylinders with aluminum and plastic snorkels had negligible internal corrosion. This reaffirms that submerging the cathode entirely limits galvanic corrosion to the rate at which oxygen diffuses through water.

A FATALITY FROM BREATHING A CORROSION-INDUCED HYPOXIC MIXTURE

Schench, Hilbert V., and McAniff, John J. United States Underwater Fatality Statistics-1974. NOAA Report URI-SSR-75-10

In 1974 there was one documented case of a death that was caused by breathing a corrosion-induced hypoxic mixture.

In this case, the diver took a steel tank to a depth of 12 feet to search for an outboard motor. The victim had last used this tank three months previously and it only had 300 psig remaining. Five minutes into the dive his bubbles were noted to cease and his body was later recovered.

Analysis of this accident revealed severe corrosion of the tank with large amounts of rust. There was 200 psig remaining but the oxygen content of the gas was measured to be only 2% to 3%.

The steel tank in this accident had neither a current hydro nor a current visual inspection. (The last documented visual inspection was in 1964.)

Discussion

This case reinforces the fact that steel tanks that are stored for long periods of time (greater than 90 days) must be either (1) reanalyzed for oxygen and reanalyzed for carbon monoxide before use, or, better yet, (2) completely drained and refilled with fresh gas.

In this case, the cylinder had been used without incident three months prior to the accident. Then, in only three months, corrosion had reduced the oxygen content to a level that did not support life.

CORROSION-INDUCED SCUBA CYLINDER EXPLOSIONS

Peyser, Richard. Scuba Cylinder Internal Corrosion An Engineering Safety Study. Master's Thesis, University of Rhode Island, 1970

In 1963 one person was seriously injured when a scuba cylinder exploded while being filled. The cylinder had a current hydro. Evaluation of the cylinder revealed the cause of the cylinder failure was severe internal corrosion.

Schench, Hilbert V., and McAniff, John J. United States Underwater Fatality Statistics-1972. NOAA Report URI-73-8

In 1972 there was one fatality from a corrosion-induced cylinder explosion. In this case, a cylinder was being filled and it exploded at 2,900 psig, killing one person. Subsequent examination of the cylinder revealed significant corrosion that reduced the wall thickness by 50 percent. The cylinder had neither a current hydro nor a current visual inspection.

 

[Doc Harry]

Scuba Cylinder Long-Term Storage: Fact and Fiction

So what is the bottom line? Fatal outcomes are rare but I would not want to be that rare case. Corrosion is common. Less common is corrosion that is significant enough to warrant tumbling or other service. Even less common is corrosion or wear that is significant enough to cause a cylinder to fail hydro. If I paid $500 for a steel cylinder, I would want it to last forever.

So based on the best evidence, here are my answers to the quiz. But do not follow my advice. Either follow the recommendations of experts or read the literature and make your own informed decision.

QUIZ.

1. When storing a scuba cylinder for long periods of time, should the tank be stored full or almost empty?

b. Almost empty

Storing a cylinder almost empty helps to reduce corrosion because of the reduced oxygen partial pressure. While this is not so important with an aluminum cylinder, it can make a big difference with a steel cylinder. Also, storing a cylinder almost empty just makes sense from the perspective of reducing permanent expansion although the effects are minimal.

2. When storing a scuba cylinder for long periods of time, in what position should it stored?

a. Upright

Steel cylinder should be stored vertically in an upright position to minimize the effects of corrosion. If a steel cylinder is stored on its side and there is water present in the cylinder, the thin side walls and the large contact area can significantly corrode the cylinder side wall in as little time as 3 months. In the Battelle study, corrosion reduced side wall thickness to less than 1/3 of the orginal wall thickness in some cases. The Battelle study concluded that corrosion that results from lying a cylinder on its side probably represents the greatest hazard to personnel. It seems that aluminum cylinders can be stored in any position (except inverted) because aluminum cylinders are much less prone to severe corrosion.

3. When storing a scuba cylinder for long periods of time, does the breathing gas (i.e., air, Nitrox, etc.) affect the tank?

a. Yes

Higher partial pressures of oxygen cause higher rates of corrosion. It appears that it is the partial pressure of oxygen that is the major factor, not the percentage of oxygen in the gas per se. Obviously the fraction of oxygen affects the partial pressure but the total pressure has a relatively greater effect. The presence of water is a prerequisite for corrosion, though. A dry breathing gas does not corrode.

4. Conversely, when storing a scuba cylinder for long periods of time, does the cylinder affect the breathing gas?

a. Yes

The URI corrosion study demonstrated definitively that oxygen is consumed and carbon monoxide is generated during the corrosion process in steel cylinders. No such effect has ever been demonstrated with aluminum cylinders. There was one documented death from a corrosion-induced hypoxic mix in a cylinder that apparently developed over a period of only 3 months. Before using a steel cylinder that has been stored long-term, either dump the gas and refill or analyze the gas for oxygen and carbon monoxide content.

5. When storing a scuba cylinder for long periods of time, does a little bit of moisture affect the tank?

a. Yes

Moisture is the enemy. More water equals more corrosion. It is a self-limiting process because the water is consumed during the corrosion process. Steel tanks are more susceptible to corrosion than aluminum tanks, but aluminum tanks are more susceptible to galvanic corrosion of the neck threads in a salt water environment.

6. When we talk about long-term storage of scuba cylinders, what time period constitutes long term?

a. Three months

A lot can happen in three months.

 

[Luis H]

Nice write up and gathering of information about Scuba cylinders.


PM sent.

Thanks

 

[pescador775]

The Navy aluminum cylinders were 6061 alloy but for some reason were more susceptible to corrosion than the proprietary alloy used by Luxfer. That is, the Luxfer tank made of "6061" or 6351 appears less susceptible to the jello type corrosion than that of the spun cylinders used by the Navy. The Navy cylinders were also more susceptible to thread corrosion when used with the beryllium-copper valve manifolds.

Increased corrosion may occur if the tank is stored full. This is due to the increased partial pressure of water vapor which disappears as pressure is released. Anecdotal evidence seems to confirm that very high oxygen content MAY aggravate corrosion. However, cylinders stored with pure oxygen do not seem to be affected adversely if they are dry. As you say, it is the moisture.

Anecdotal evidence suggests that high pressure cylinders stored with low gas press are most in need of hydrostatic "rounding" before proof testing. IOW, an HP cyl stored with full press is less likely to fail.

I stored three, steel 3000 psi cylinders full, upright for 15 years. That is to say, they were in active use in the summer and stored full during the off season (six months). One failed hydro when tested at the 15 year mark. All were slightly to moderately rusty inside. I sent the failed tank to PST for evaluation but they lost it. I also took steps to improve filtration on my compressor.

I stored two aluminum 80's full, upright for 20 years and they passed hydro. There was little or no corrosion internally.

 

[Luis H]

An understanding of elastic and permanent expansion has helped me to understand that my scuba cylinders should be stored empty to minimize metal fatigue. Storing a tank at full service pressure increases permanent expansion. However, practical considerations require that cylinders be stored with some pressure (e.g., 300 psig) to prevent moisture from entering.

Note: fatigue life and static load are not related. Steel does not suffer from time dependent static stress issues such as creep or "sustain load cracking". As we all know aluminum can suffer from sustain load cracking.

Long term high pressure storage of a steel cylinder that is not subject to corrosion is not a problem.To avoid corrosion very dry gas is the first step (reduced O2 content does help, but is secondary).

I will expand more on this after I finish reading all the articles.

 

[pescador775]

I don't think much of the sustained load theory. Personally, I believe the publicized failures were due to cycling alloys from defective batches, and which was aggravated by hydro testing. That is, if a small crack starts as the result of a hydrostatic overpressure it can only get worse from there. I bought aluminum tanks in 1972, used them for a year and decided to relegate them to air tool use. They were kept at full press and hydroed for the first time in 1994. Both tanks passed hydro and visual. I continued to use them until 2004 when I discarded them due to bad vibes. They certainly didn't owe me anything and probably would have passed again.