The Statue of Liberty does not stand stock still as might become a statue. She instead sways in response to Aeolian and seismic forces while keeping her watch on New York Harbor. She has a central pylon that is tied to a winding, or armature, a skeleton of strap about 1 ◊ 4 inches in cross section. The winding matches the external contour of the statue. The hidden parts of the statue resemble the guts of a huge automobile starter, stood on end. The skin of the statue is riveted to saddles that are free to slide over the armature. In this way the statue may flex. The skin and saddles are copper. The armature is wrought iron, which is basically mild steel.
The Statue's designers realized that electrical contact between copper and steel in the presence of water constituted a galvanic cell, hence they coated the armature with an insulating layer. The sliding of the saddles soon wore through the layer. Galvanic corrosion then set in and over the hundred or so years of the statue's life reduced the saddle thickness by up to half. Even worse the resulting rust deposit between the steel and copper caused a force that popped many of the rivets. The statue thus lost structural integrity to a degree that demanded remediation. The arm holding the torch was in particularly bad shape.
A repair project was begun about 20 years ago (Engineering News Record, 07.05.84) and choice of a non-rusting replacement for the wrought iron in the armature was a key consideration. Non-ferrous materials were unsuitable for reasons of strength or weight, and type 316L stainless steel was selected. This steel is so close electrically to copper that galvanic corrosion will not occur. The "L" indicates low carbon, which should eliminate nasty sensitization effects, that can sometimes render stainless steel non-stainless.
The repair project was a joint French-American project. One French task was forming 3,500 lbs of stainless steel into the sometimes highly convoluted configurations needed to match the contours of the skin. Engineers were concerned that cold working the steel might result in brittleness or corrosion susceptibility. They were worried even though a wealth of experience on the behavior of type 316L stainless steel showed that such degradation had not been a problem.
Annealing at red heat would remove the effects of cold work and put all the armature material in the pre-deformation annealed state. However, the armature was bent into large space curves so that a huge annealing furnace was required. Part of my job as metallurgical consultant on the American side was to find a suitable furnace. I managed to track down a used furnace with a 3-ft diameter ◊ 25-ft long hot zone. This behemoth drew some 2 MW of power. I left the project without learning whether or not the furnace was ever employed.
At the time I thought the French were just being anal in worrying about annealing, but my views have since mellowed. Stainless steel entered general use only about 80 years ago, and type 316L is much a more recent alloy, so that long-term service behavior is unknown. Metallurgical history is full of stories of nasty surprises that occurred only in the long term: Fatigue and corrosion are notable causes of such failures, and the armature is subject to both.
For example, the de Havilland Comet jet airliner was well ahead of the competing Boeing 707 until three horrific, fatigue-induced crashes set the program back by years. With this setback went the British hope of pioneering commercial jet transport. Corrosion failure also takes time to occur. Earlier columns described how corrosion of sensitized stainless resulted in badly burned buttocks in one case and a severed Achilles tendon in another.
Some nasty surprises could await the armature steel in either the annealed or cold-worked state, though the latter is the more likely suspect. Having part of the armature cold worked and part annealed would have presented two targets for trouble. Annealing reduces the possibilities.
The Lady was in danger of dropping her torch 20 years ago. Erring on the side of caution to prevent a similar disaster sometime in the next 100 years just may have been a good idea.