CS35 – Ferralium 255 – Statue of Liberty

The Statue of Liberty is not just an iconic structure, but also a significant engineering challenge.

The statue was constructed by forming a grid structure, against which independent sections of the copper cladding could be fixed by ‘saddles’ – allowing a degree of movement. Failure of these saddles around the torch area resulted from a combination of entrapped moisture and galvanic corrosion, as water ran down the arm. Shellacked asbestos cloth was originally used between the iron grid and the copper saddles, but over times it had dried, making the asbestos porous. In turn, it absorbed moisture and actually promoted electrolyte contact and galvanic corrosion. The accelerated corrosion of the iron caused it to swell and distort. That, in turn, caused the saddle rivets to be pulled through the copper skin in many places.

The condition and subsequent refurbishment of the statue became a national cause. Many companies donated both materials and services to the project, including Cabot Haynes, who were producing Ferralium^® 255 under licence at that time. The use of Ferralium^® 255 followed an in-depth investigation of suitable materials by the National Park Service’s North Atlantic Historic Preservation Center. Crucially, when used for the flat bar structure and associated bolting, it exhibited minimal reaction with the copper cladding; it offered thermal expansion and elasticity similar to that of the wrought iron, but was far stronger. At that time, it was deemed that Ferralium^® 255 would have been too difficult to shape into the complex forms of the armature bars. The metal selected for them was the extra-low-carbon, Alloy 316L stainless steel. It also exhibited minimal reactivity with the copper, and had a modulus of elasticity similar to the wrought iron.

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