On 28 October 1886 the Statue of Liberty was dedicated in New York Harbor: a 46.05-metre figure dressed in copper only 2.37 mm thick, 3/32 of an inch according to the National Park Service, hung on an iron frame by Eiffel and Company. Of the statue’s 254,000 kg, about 81,300 kg is that thin copper skin. Under the icon sits a rule that every facade detail still has to obey. Every metal has a voltage, and two different metals touching in salty moisture make a battery.

The statue turned 140 last week. On the first floor of the Eiffel Tower, the exhibition Lady Liberty, derrière l’icône, le génie d’Eiffel, run with MuAM, closes on 12 November after twelve panels on the frame that, according to the City of Paris, still lets the statue stand up to the winds of New York Bay. It is a good moment to learn the concept that frame taught the world: the galvanic series.

What it is

What it is: The galvanic series is a ranking of metals by nobility, meaning their tendency to stay metal rather than dissolve as ions. Gold and platinum sit at the noble end. Zinc and magnesium sit at the active end. Copper, stainless steel, carbon steel and aluminium fall in between. When two metals from different rungs touch, and a film of water carrying salt bridges them, the pair becomes a cell. The more active metal turns into the anode and corrodes. The more noble one becomes the cathode and is protected.

A battery needs three things, and so does galvanic corrosion: two metals at different potentials, an electrical contact between them, and an electrolyte that touches both. Take away any one of the three and the circuit opens. That one sentence is the whole design method.

Why it works

Why it works: Each metal in an electrolyte settles at its own electrode potential. Join two of them and electrons flow through the contact from the less noble metal to the more noble one, while ions carry the charge back through the water. At the anode, iron gives up its electrons and leaves as ions: Fe → Fe²⁺ + 2e⁻. At the cathode those electrons do no harm. They reduce dissolved oxygen instead: O₂ + 2H₂O + 4e⁻ → 4OH⁻. The cathode is not consumed. It only keeps the reaction running, and that is why it stays bright while its partner thins.

Designers work with the anodic index, a table of potentials in volts. Take copper at −0.35 V and wrought iron or carbon steel at −0.85 V. The gap is 0.50 V. The usual guidance tolerates about 0.15 V in harsh environments such as salt spray, coast and splash zones, about 0.25 V in normal outdoor conditions, and about 0.50 V only where humidity is controlled. Copper on iron in a harbour sits at the very top of the controlled-environment band, with no salt allowance left. For orientation: 18% chromium stainless sits at −0.50 V, exactly 0.15 V from copper; wrought aluminium sits at −0.90 V; and zinc at −1.25 V. Treat these numbers as design guidance, not as potentials measured on site, because the real value moves with temperature, chloride and surface film.

The second rule is geometric, and it is where an architect’s drawing decides the outcome. The current that leaves the anode has to match the current the cathode accepts, so the ratio of areas sets the damage. A large cathode wired to a small anode concentrates the whole attack on the small part. A large anode with a small cathode spreads it thin. Liberty is the hard case: square metres of copper skin acting as cathode on comparatively slender iron bars acting as anode.

Origins

Origins: In the 1780s in Bologna, Luigi Galvani watched dissected frog legs twitch when touched by two different metals and read it as animal electricity. Alessandro Volta disagreed, and on 20 March 1800 he wrote to the Royal Society describing a stack of dissimilar metal discs separated by brine-soaked card. That was the voltaic pile, and in it the dissimilar metals were the whole engine. Two decades later the Navy Board asked Humphry Davy why the copper sheathing on Royal Navy hulls was dissolving. His paper, read on 17 June 1824, reports copper sheets in the tidal flow of Portsmouth Harbour protected by zinc and iron at 1/40 to 1/1000 of their surface. Davy’s verdict: “When the metallic protector was from 1/40 to 1/150, there was no corrosion nor decay of the copper.” At 1/200 to 1/400 the copper lost weight, and the smaller the protector, the greater the loss.

Davy’s insight is the one to keep: you choose which metal is eaten. In ICorr’s “200 years on”, Ackland, Dylejko and Dr Markus Büchler, CEO of the Swiss Society for Corrosion Protection, report that the protection “worked unambiguously to prevent corrosion of the copper in all cases.” Fouling then took hold on some ships, and the Admiralty stopped the scheme on 19 July 1825. The method itself survived and became cathodic protection, which today guards pipelines, bridges and reinforcing steel. The essence was right in 1824. The ship’s hull simply asked for a second job the copper had been doing for free.

Sixty years later Eiffel’s engineers did what a careful frame designer does. They let the skin float, so it could expand and contract on 1,800 iron armature bars, and they insulated the copper-to-iron joint with shellac. A century of harbour weather wore that insulation out. As PBS NOVA reported, “through galvanic or electrolytic corrosion, some iron bars had eroded to as little as a third of their original thickness.” The statue was, in the documented assessment, far from unsafe. It had simply outlived its isolator.

←TODAY: Liberty’s copper still rides on stainless replicas of Eiffel’s bars, set apart by PTFE strips since the 1984–86 restoration.
→3012: In Zurich-3012 every metal part carries its potential as data, and the junction is checked before it is drawn.
Fulcrum: A 0.50 V gap from 1886 becomes a one-line rule only because we can read the old frame and the new script side by side.

The restoration is the most elegant answer in the story. Crews replaced about 10,000 linear feet of armature, 35,000 lb in total, along with 30,000 copper rivets. They worked at no more than 12 bars every 24 hours, so the skin was never left unsupported. Their names rarely make the plaque, but their pace is the reason the statue stood through its own surgery. The new bars were exact duplicates in stainless steel, which closes the gap with copper to about 0.15 V, and PTFE strips now sit where the shellac once did, breaking the circuit outright. Two of the three battery conditions were attacked at once. The statue reopened on 5 July 1986, and ten of the original puddled-iron bars remain in her right foot.

An isolator only works while it stays whole. The shellac held for about a century, and every gasket, sleeve or strip you specify has a service life too, so it belongs in the maintenance plan and not only in the detail.

In practice

In practice: Architects draw galvanic couples every week without naming them. Copper flashing laps onto a steel lintel. Aluminium cassettes hang on stainless brackets, a gap of about 0.40 V where the aluminium is the metal that gives way. Zinc-coated fixings go into steel, which is Davy’s choice made on purpose, because galvanising is a sacrificial anode you can buy by the kilogram. In a Swiss facade the electrolyte is rarely sea spray. It is winter de-icing salt splashed at plinth height, condensation trapped behind a ventilated cladding, or runoff from a copper roof onto a lower aluminium sill. The parts that matter are the small ones: the bracket, the screw, the rivet, the anode that is small next to a large noble cathode.

Atelier: For an office running Rhino and Grasshopper on its envelope packages, the galvanic series is a lookup table plus a contact test, and a model that knows each part’s metal can run that test on every detail rather than relying on a coordinator’s memory. The useful habit is to make the metal a property of the geometry and the check a property of the model. Monday move: add a metal User Text key to every exterior metal object in the current facade model, using Rhino 8’s Attribute User Text panel or Grasshopper’s user-text component, before the next detail set is issued.

Hack

Hack: Read each exterior part’s metal from its User Text, find the pairs that actually touch, and flag every pair whose potential gap passes 0.15 V. The domain is physics, an electrochemical potential difference evaluated on real contacts. In a GhPython component with a list input ids of Rhino object GUIDs, six lines do it:

import rhinoscriptsyntax as rs, scriptcontext as sc, Rhino, itertools
sc.doc = Rhino.RhinoDoc.ActiveDoc  # read User Text from the Rhino model
IDX = {"copper": -0.35, "stainless18cr": -0.50, "steel": -0.85, "aluminium": -0.90, "zinc": -1.25}
for a, b in itertools.combinations(ids, 2):
    dv = abs(IDX[rs.GetUserText(a, "metal")] - IDX[rs.GetUserText(b, "metal")])
    if dv > 0.15 and len(Rhino.Geometry.Intersection.Intersection.BrepBrep(rs.coercebrep(a), rs.coercebrep(b), 0.5)[1]): print(a, b, round(dv, 2))

The loop checks every pair, not every object, because a bracket can touch a sill and a panel at once, and each contact is its own battery. Model touching parts with a hair of overlap, or raise the tolerance, so coincident faces return an intersection. Raise the limit to 0.25 V for sheltered details, and keep the table in a single shared file so every project reads the same numbers. Where a pair flags, draw the isolator: a PTFE or EPDM strip, an insulating sleeve, or a closer metal.

The material outlasts the file

From the late 2070s, the failures I remember were rarely the ones nobody could calculate. They were the junctions where the reason for a gasket lived in one engineer’s head and left with them. Liberty survived because her frame was legible enough for a 1980s crew to read the battery inside it and rebuild it bar by bar. Write the potential into the part, keep the table in plain text, and the next team can do the same.

Open your current facade model, tag every exterior metal with its metal key, and run the ΔV check before the detail leaves the office.

Sources & Further Reading

• Primary: ICorr — 200 years on: Sir Humphry Davy and cathodic protection (Ackland, Dylejko, Büchler)
• Reinforcing: PBS NOVA — Saving the Statue of Liberty