When Well-Intentioned Repairs Go Awry

In July 2007, Thomas Point Shoal Lighthouse in Maryland reopened to the public for tours after a challenging restoration. Challenging partly because of the location in Chesapeake Bay, but also challenging because of well-intentioned but misguided repairs while under the management of the Coast Guard. Adrienne DeRan, of Structural Preservation Services in Maryland, has written an article detailing the challenges of repairing the repairs in a recent issue of Associated Construction Publications.

An inspection by the Coast Guard in 2001 found that previous attempts to repair problems at the light had caused galvanic corrosion in the area of the splash zone, or waterline. Anne Arundel County of Maryland, the Annapolis Maritime Museum, the United States Lighthouse Society (San Francisco, Calif.) and its Chesapeake chapter began raising funds to start repairing the repairs. A second assessment revealed a section loss of about 10 percent for steel WF-beams and cast iron knuckles, as well as a loss of about 30 percent of tierods and turnbuckles.

Although the lighthouse foundation originally consisted mostly of wrought iron and cast iron, it had been replaced at some point with standard A36 carbon steel as part of a repair attempt. Unfortunately though, the introduction of these dissimilar metals to the marine environment had resulted in galvanic corrosion.

What Is Galvanic Corrosion?

It’s the electrochemical process when two dissimilar metals (like copper and steel) come in contact with each other while submersed in an electrolyte (like sea water), which will cause the erosion of the weaker metal. A “galvanic couple” is set up because different metals have different electrode potentials. The electrolyte provides a means for ion migration. This leads to the anodic metal corroding more quickly than it otherwise would; the corrosion of the cathodic metal is retarded even to the point of stopping. The presence of electrolyte and a conducting path between the metals may cause corrosion where otherwise neither metal alone would have corroded.

An example occurred in the Royal Navy frigate HMS Alarm. The wooden hull of the vessel had been sheathed in copper to prevent attack by barnacles. It was soon discovered that the sheathing had become detached from the hull in many places because the iron nails which had been used to fasten the copper to the timbers had been rotted. Closer inspection revealed that some nails, which were less corroded, were insulated from the copper by brown paper which was trapped under the nail head. The copper had been delivered to the dockyard wrapped in the paper which was not removed before the sheets were nailed to the hull. The obvious conclusion therefore, and the one which was contained in a report to the Admiralty of 1763, was that iron should not be allowed direct contact with copper in a sea water environment if severe corrosion of the iron was to be avoided. Later ships were designed with this in mind. Not only is sea water a very good electrolyte owing to its high salt concentration, but the “attack of the nails” was encouraged by their very small exposed area compared with that of the copper-sheathed hull.

(Source: Wikipedia)

Other Restoration Challenges

Rotting fascia boards also had to be replaced, and all the non-original replacement parts (tierods, clevis ends and turnbuckles) were also removed and replaced with non-galvanically reactive metals. Shims were installed between the metals to prevent any possibility of problems down the road. Liquid wood consolidant was applied to the sagging wood joists. They also had to be jacked up to their original elevation to prevent collapse in the future. Replacement tructural wooden beams were made to fit into the usable existing ones.

All this had to be performed at sea, using a small boat to access the lightstation. A larger vessel couldn’t be used due to the shallow waters around the light. Dock space was limited, and weather conditions had to be monitored closely to ensure the workers’ safety. Work areas were often submerged by tides and waves.

With all these challenges, the company managed to repair the structure, with an eye to preventing future corrosion. Working under the Secretary of the Interior’s Standards for Historic Preservation may seem like overly technical guidelines to some, but they are in place for a reason. What was built 150 years ago used different processes than today, and you cannot slap a coat of today’s paint on yesterday’s building and call it good.

Coming Soon…

Painting Pemaquid (another repair gone awry)

Photo Credits:

• Thomas Point Shoal Lighthouse by Eric Brian. Some rights reserved.
• US Frigate Essex. Public Domain photo

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