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.
I ALWAYS make it a habit to thoroughly blow out any moisture left in the valve, especially in DIN valves. It's amazing the amount of water that can be trapped in those threads...
