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Paul, I am assuming the original rust contamination was "Surface only" and there was no deep pitting beyond standards. Basic principle ~ For every action, there will be a reaction, including removal of rust as well as embedded residual acid becoming trapped in wall pores. This could ruin a cylinder if not properly handled and poses a health hazard to anyone breathing the cylinder after filling. Hopefully a hot water [180F 200F] rinse with an acid neutralizer was done. The temperature of the water will expand the material molecules and thus help in the extraction and neutralization of any embedded residual acid. If not done, the subsequent emitted vapors and/or particles from the acid could contaminate the gas supply upon filling, as the walls expand, probably resulting in a bitter taste and possible respiratory damages. Final rinse should be with a rust inhibitor compound to help prevent future rust contamination ~ Oxygen safe rust inhibitor along with the usual requisite cleaning of cylinder and valve components if to be used in EANx or O2 service. Phosphoric acid at a 5-10% dilute is usually the industry preferred cleaner for such operations. Hydrochloric seems a bit aggressive in my opinion but will work. I'm not a chemist so perhaps someone else with expertise in that arena can comment regarding the various acid attributes and properties. Caution should be exercised not to allow acid to be exposed to any galvanized processes present on the cylinder as the zinc will react and dissolve partially or completely depending upon the concentration of acid in the dilute. While Hydrostatic [destructive testing] is the accepted industry standard in the U.S., ultrasonic is a vastly superior non-destructive testing method for analyzing metal fatigue. Often cylinders that pass a Hydro will subsequently fail a ultrasonic that reveals hidden metal fatigue. Some gas supply companies are beginning to offer it as an alternative service primarily to allow a way to test vessels presenting difficult size and configurations for Hydro equipment to handle properly. While the prices I've been quoted are competitive with Hydro, I would not invest this much trouble into a 72 ft3. The above considered and performed, there should be no safety issues since any remaining residual would be so infinitesimally microscopic to be worthy of concern from a structural point or for respiratory aspects. NEVER USE ACID CLEANERS ON ALUMINUM CYLINDERS! Acids will attack aluminum models and structurally weaken them.. The usual culprit of internal rust in steel cylinders is poorly maintained purification and filtration systems on dive shop compressors. If an operator isnt properly trained keeping up with the run hours and doesn't have a CO/Moisture indicator inline, the moisture level can easily and quickly become elevated beyond standards. This is even more true in shops where numerous shop monkeys are doing fill work as opposed to a trained and designated operator(s). The prudent diver is wise to query operators regarding their knowledge and qualifications and weed out those who only have the money to buy a pump. Limit where you purchase air/gas to 1-3 locations and visual inspect for accountability more often if you violate your select sources. If you find "Anything" in your cylinders [other than air/gas mix], someone is not doing their job correctly! They should be avoided at all costs. Comprehensive accident analysis should always include a through evaluation of all elements and compounds contained in victim cylinders. I have seen at least one fatal accident where a USD plastic dip tube disintegrated and allowed excessive charcoal filter media to obstruct the first stage filter when the diver inverted. As for doubling 72s up for 60-meter mix dives, Humm doesn't that speak for itself? ;- ) Regards, Jer -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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