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14 AUGUST 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 8 The Emerald Princess. Photo by Bahnfrend/Wikimedia Commons. Corrosion Sparks Cylinder Explosion on New Zealand Cruise Ship T he corrosion behind a gas cylinder explosion that caused a fatality earlier this year on the massive Emerald Princess cruise vessel raises questions as to whether current inspection requirements are adequate, according to an interim investigative report 1 issued by the Trans- port Accident Investigation Commission (TAIC) (Wellington, New Zealand). The ship was docked at Port Chalmers in Dunedin, New Zealand, and scheduled to depart on February 9, 2017, when a nitro- gen cylinder exploded while being refilled. Built in 2007, the 287-m long, 113,561- ton ship has the capacity to host 3,113 pas- sengers and 1,173 crew members. The ves- sel is fitted with 18 lifeboats, and each has an emergency launching and recovery sys- tem so the lifeboats can be launched even if the diesel-electric ship suffers a total power failure. To supply the energy for such an operation, each lifeboat requires a piston accumulator and a bank of four high-pressure nitrogen cylinders for a pneumatic launch, TAIC investigators explain. Each bank of cylinders sits within a steel frame unit, and the cylinders are secured by a steel band. These banks are fitted on a covered but open deck, thus exposing them to salt spray and sea air. According to the TAIC report, crew members noticed on February 9 that the pressure of the nitrogen system for one of the lifeboats had dropped to ~2,393 psi (17 MPa)—below the normal working pressure range of 2,611 to 3,046 psi (18 to 21 MPa). "The four nitrogen cylinders were con- nected by f ixed pipes to a pneumatic dis- tribution block," TAIC investigators say in the report. "The crew checked all of the connections for leaks, but were unable to f ind any. They did not check the nitrogen cylinders, because they thought it highly unlikely that the pressurized cylinders had been structurally compromised." The crew then decided to replace the entire manifold in case there were leaks within the unit. To do this, they depressur- ized the system and exchanged the pneu- matic distribution block with a new one before following the standard procedure to repressurize the system, the TAIC says. During the f inal stage of the repres- surizing operation, one of the four nitro- gen cylinders burst and fatally injured a worker who was standing close by. The cylinder exploded with such force that the top of it was never found. A ll of the cylinders for the affected lifeboat were taken to the TAIC's techni- cal facility in Wellington, where an exam- ination was made by an independent met- allurgical contractor. The metallurgist found that corrosion had reduced the thickness of the cylinder wall by 75% to only 1.5 mm at the point of failure, com- pared to the designed thickness of 6 mm. "The burst nitrogen cylinder had suf- fered signif icant corrosion at the point of failure, which was situated approximately 150 mm below where the steel securing band had clamped the cylinder within its frame," TAIC investigators write. "The corrosion had developed on the outside of the cylinder. The internal surface was clear of corrosion." A second area of corrosion was found near the bottom of the cylinder, as well as a third area of concern where the protec- tive coating had been scraped away to observe the cylinder's identif ication details, according to the report. "The metallurgist's initial assessment was that the failure had occurred as a result of overload caused by corrosion thinning," TAIC investigators say. Moreover, a num- ber of other cylinders on the ship and the piston accumulator were also observed to have corrosion-related damage. The nitrogen cylinders on the ship are inspected annually by the manufacturer's authorized representative, and the most recent inspection was only two weeks prior to the accident, the TAIC says. On each occasion, the cylinders were found to be in satisfactory working condition, although the manufacturer says the MATERIAL MATTERS