Objects are falling once again in the Big Dig tunnels. Boston Herald.com reported that a 110 – pound light fixture fell in the travel lanes near the entrance of the I-93 northbound tunnel on February 8, 2011 but that it took several weeks before the event was made public by officials. Compounding the concern raised by this latest event in the tunnels was learning that transportation workers have found and temporarily repaired almost 350 corroded light fixtures. Only one fixture, the fallen one, has been replaced.
Adding confusion to the review of the problem, it was learned that in 2009 the chief of the Massachusetts Turnpike Authority (MTA) ordered an inspection of all 23,000 fixtures in the tunnels to be carried out each year with the results to be recorded and entered into a database. While MTA has stated the inspections have been performed, local critics are questioning the record keeping and asking why several weeks went by before the falling light fixture was made public.
Response to the Failure
In response to problems with the lighting fixture mountings, local administration officials have sent a letter to the fixture manufacturer discussing two key points:
- There are adhesion problems with the paint on the aluminum wireways resulting in flaking. Bare aluminum is being exposed to the tunnel environment causing excessive corrosion and premature failure.
- Due to the loss of paint, stainless steel mounting clips for the lights are coming in contact with the aluminum wireway resulting in galvanic corrosion and failure.
At a press conference, local officials indicated that the powder coating on the aluminum wireway track is the problem area. Each fixture is held up by 10 stainless steel clips. On the fallen fixture, all 10 clips demonstrated corrosion. Subsiquent inspections of the other fixtures have indicated problems with only a few clips per fixture. Temporary repairs are being made by moving the stainless steel clips around so that they are anchored at locations without current corrosion. Approximately 3000 clips have been moved in the initial inspection / response. It is expected that it will be several months before a permanent fix for the situation is developed. There are close to 230,00 clips present in the Big Dig tunnel system making this a massive undertaking to perform a hands-on physical inspection of each of the 23,000 fixtures. Preliminary indications are that most of the failures are located near the portals, an indication that the harsh environment is playing a role in the corrosion.
All the light fixtures involved were noted to have come with a warranty but it is not known at this time if the warranties are still in place. The tunnels opened at various times from 1995 to 2003 and it will require extensive review of construction documentation to determine the warranty situation. Unfortunately, the company that manufactured the light fixtures is no longer in business having been absorbed by another entity who has indicated they have no responsibility in the matter. Responsibility in this case will likely be determined by legal means if the current company owners remain uncommitted to cooperating in a repair resolution.
In a more recent development, Jeffrey Mullan, the Massachusetts Transportation Secretary confirmed the resignation of one highway official who was responsible for letting weeks go by prior to informing Mullan about the light fixture that had fallen in the tunnel. The state transportation department also released an investigative report which included an email from one highway official who warned that the failing fixtures were a “big deal” and that many were no longer safe. The video below is from a press conference by transportation officials that describes the problem and their initial response.
Estimates reported so far indicate that from 1.5 to 5 percent of the fixtures have evidence of serious corrosion deterioration at at least some of the clips. While galvanic corrosion (in a sea air- road salt - moist tunnel environment) has been blamed, no definitive answer on the root cause of why the paint coating on the aluminum has failed has been reported. In cases like this, possibilities for the flaking paint could include poor surface preparation or cleaning before painting, incompatible paint, improper thickness of coating, or a variety of other material related issues. Investigators will also have to determine if there were any contributing factors such as damage during initial installation, movement or vibration inplace resulting in paint damage and exposure of the aluminum or any number of other factors or combination of factors that occurred during the life of the fixtures. It is likely that a detailed inspection of the fixtures including laboratory testing of the materials involved will be necessary to fully determine nature and overall extent of the problem. The larger question of on-going damage and rate of deterioration does not appear to have been fully addressed to date. If it is determined that all the fixtures are eventually susceptible to galvanic corrosion to the point of failure and have to be replaced, the cost could be as much as $200 million according to transportation officials.
The Light was Not the First Object to Fall
On July 10, 2006, approximately 26 tons of concrete ceiling panels and suspension hardware within the Interstate 90 Connector Tunnel below D Street in Boston collapsed onto the roadway killing one person and injuring another. The ensuing investigation determined that the reason for the collapse was directly linked to the inappropriate substitution and use of a type of fast set epoxy for the anchor that supported the heavy concrete ceiling panels. A detailed case study of the I-90 Ceiling Panel Failure can be found at Failures Wiki.
What is Galvanic Corrosion?
Galvanic corrosion is an electrochemical process resulting in oxidation or corrosion of two dissimilar metals in contact in the presence of an electrolyte. The further the two metals are apart on the galvanic scale, the more potential for corrosion although the total amount of corrosion depends on a number of other factors as well. As the process progresses, the “less noble” metal gives up ions to the “more noble” one. In the tunnels, that would mean the aluminum would corrode as opposed to the stainless steel.
There are several common examples of galvanic corrosion problems that occur in the construction industry. One is when copper roof flashing is allowed to come in contact with aluminum or steel rain gutters resulting in a failure of the less noble metal, in this example, the aluminum or steel. As can be seen from the abbreviated Galvanic chart below, copper is at the opposite end of the scale from aluminum thus indicating any direct contact would likely result in deterioration of the aluminum.
* Source: Copper Development Association
A comprehensive discussion of dissimilar metal corrosion relative to copper titled Fundamentals: Architectural Considerations can be found at the Copper.org site maintained by the Copper Development Association.
Another galvanic corrosion concern in the industry relates to the use of the newer formulations of pressure treated (PT) lumber in contact with non-approved or unprotected fasteners. PT lumber contains copper as one of the preservative agents. In a moist or wet condition of use, the copper can galvanically corrode conventional unprotected steel fasteners or brackets. Copper is more noble resulting in damage and deterioration of the fasteners which are often steel nails and bolts.
An excellent article that came out on this topic when the new formulations of PT wood were being introduced is titled “The Next Generation – Pressure Treated Wood” , published by Fine Homebuilding magazine. Additional informaiton on this topic is available from the companies that manufacture pressure treated wood as well as those that supply fasteners and metal connections. One good source is the Simpson Strong-Tie Technical Bulletin on Pressure Treated Wood.
One of the most famous examples of galvanic corrosion in the US is the case of the Statue of Liberty. The wrought iron armature of the statue in contact with the copper skin and copper saddle clips in a salt / ocean air environment resulted in severe galvanic corrosion that was destroying the landmark. A detailed discussion of Liberty’s corrosion process can be found here. For a comprehensive story on the statue’s life and care, see this copper.org article discussing the Centennial Celebration Repair Program.
Galvanic Corrosion: Understanding galvanic corrosion can help reduce the risks of corrosion on steel structures by D.J. Bartlett, CTI Consultants Pty Ltd