Bumpy Ride for Submerged Subway Cars
We recently released a story about New York subway cars being disposed of at sea. We it seems now that idea isn’t panning out the way they planned.
The state has halted a program to create an artificial fishing reef off Atlantic City by sinking old New York City subway cars underwater. State officials say an initial batch of about 100 cars that have been sunk so far are showing “unusual damage.” Divers from the NJ Marine Fisheries Administration have observed significant damage to several of the 35,000-pound, stainless steel cars which were supposed to maintain 90 percent of their structural integrity after 30 years. Some have apparently collapsed, despite being in the water less than a year.
Aquabio, a natural resource and environmental consulting firm based out of Arlington, Massachusetts has suggested that a failure mode analysis might provide some answers on why these cars are deteriorating at a faster rate than anticipated. Aquabio also notes that if loose material from the subway cars subsequently moves off the reef site, collateral damage to fishing gear and potential permit problems could emerge.
Although heavy fishing gear impact, severe storm surge, or unusual bottom impact may eventually be identified as a cause, careful attention should be directed toward the components within the subway cars themselves – specifically, toward the connection between components.
The reduced effective life expectancy of the subway car artificial reef may not be due to the stainless material but lie in how the material is fastened or connected to form the car structure.
Corrosion may have occurred where stainless steel components are fastened either to carbon steel or other stainless steel components, or are no longer exposed to oxygenated sea water. If the subway car bodies are constructed with a stainless steel skin on a carbon steel frame, corrosion between dissimilar metals may result in fasteners failing. However, if the subway car bodies are all stainless, the welded seams may fail, particularly if the welds are not passivated. (“Passivation” is the formation of a hard non-reactive surface film that inhibits further corrosion, a process normally used when exposure to sea water is anticipated during design.)
Under certain conditions, some types of stainless steel, including the 300 series alloys the cars are fabricated from, may be vulnerable to corrosion in sea water. This corrosion may be further enhanced by biofouling, sand scour, or flexing caused by wave forces. Corrugated surfaces will likely speed up corrosion, especially if low oxygen areas are created by sediment, encrusting biota, or biofilms.
Stainless steel oxidizes and forms a protective layer, but if oxygen is not available due to coverage by biofilms, biota, or sediment, corrosion is likely. Once a seam opens up, the impact of wave forces would become greater and could account for subsequence collapse.
Constant cyclic applied tensile stresses from wave forces may also be an interacting factor, especially along the welded joints (corrosion of metal is accelerated if it is exposed to tensile stress). The tensile stress may even be greater on the stainless cars. Although they are apparently about the same weight as Redbird carbon steel cars placed earlier, the stainless cars are larger with more door and window openings and may be less stable than the earlier carbon steel cars.
This type of failure, where corrosion and/or fatigue contributes to weakening the structural integrity, has been observed on other types of metal reef structures in sea water. Such rapid corrosion of stainless steel has been reported in some desalinization or power plants and offshore platforms.
Since detailed observational data are currently lacking, it’s premature to make any conclusions about what actually caused the damage. However, if other explanations don’t pan out, corrosion is something to consider, particularly if this level of damage is observed on other stainless cars, but not on the Redbirds with a similar or greater sea water exposure period.
Whether corrosion, fatigue, or some combination of factors, is identified as the failure mode, reef structural degradation is the effect and this consequence critically impacts reef performance and return-on-investment.
Furthermore, if loose material from the subway cars subsequently moves off the reef site, collateral damage to fishing gear and potential permit problems are also possible.