Armature Bar †Replacement


PREPARATION OF THE METAL†


STATEMENT OF INTENT†


P.A. Fiebiger. Inc. was asked to prepare prototype pieces for replacement armature elements in the Statue of Liberty. The treatments and recommendation outlined below are meant only as suggestions based on our present understanding of the problem.†


Four pieces of mild steel were hand forged to specific configurations. based on patterns provided by the Committee. Stresses that developed in working the steel were relieved by annealing. Select pieces were drilled. tapped, chamfered, and micro-blasted by sand (1/4 grade) and aluminum oxide. These steps were intended to duplicate the kinds of operations that would be carried out on actual armature and support elements used in the Statue of Liberty.†


ELECTRO-PLATING: THE PRINCIPLE†


The electro-plating of iron is a standard technique for protection against corrosion under exposure to a hostile environment. Iron electroplated with successive layers of copper and a tin-nickel alloy represent a state of the art treatment. It has been well documented in extensive tests carried out by the Tin Institute and others, and is accepted as a standard technique for corrosion prevention in many industrial applications.†


The principle of the treatment is based on the noncorrosive nature of the tin-nickel alloy. Iron itself is readily susceptible to oxidation. and converts to an iron oxide (rust). A tin-nickel alloy will remain stable in corrosive environments such as the conditions anticipated in the Statue of Liberty. If the iron is plated with a tinnickel alloy, the outer skin will protect the iron. However, in the event of any micro-porosity or mechanical damage to the tin-nickel, the iron would rapidly corrode at those points, especially in the presence of the more noble tinnickel alloy. The purpose of the copper inter-layer is to prevent the chance of such corrosion. A thick. ductile layer of copper effectively plugs up any micro-porosity. pre-†



venting migration of water to the iron substrate. It also provides protection against mechanical abrasion of the tinnickel. In terms of electrolytic corrosive activity, both the tin-nickel and iron will be sacrificed in preference to the more noble copper metal. Since the tin-nickel alloy is stable. it prevents the copper from changing its electrical state and protects the copper. The iron is totally covered with copper and is effectively sealed from oxygen and cannot convert to rust. This, in brief. explains the long term stability of the iron, copper, tin-nickel system. It protects the steel from corroding. without altering the mechanical properties of the steel in terms of its elasticity. flex~ etc.†


ELECTRO-PLATING: THE TREATMENT†


The four pieces were first plated with a thick layer of copper. This assures complete coverage of the steel surface. Also, it provides some ductility to the protective skin as the steel goes through the dimensional changes and deformations anticipated at the Statue of Liberty. The second layer is a tin-nickel alloy, which acts as the noncorrosive skin. as explained above.†


A final layer of copper is plated on top of the tin-nickel alloy. The purpose of this skin is to protect the copper plates in the event of contact with an armature element. An exposed tin-nickel surface might accelerate corrosion of a copper surface with which it is in contact. By plating with a final layer of copper, all points of contact between the copper plates and the armature elements are in the most compatible and least corrosive states, since no dissimilar metals are present. Only copper is in contact †with copper.†


If another metal (ie: Ferralium) is used in place of iron, there may be concern for the long term characteristics of this metal while in contact with copper. Further consideration must be given to its relationship with other metal elements, i.e. mechanical fasteners. The necessary equilibrium can be assured via electo-plating the Ferralium with copper.†


RECOMMENDATION ON THE USE OF INDIUM†


It is recommended that an inter-layer of Indium, either in sheet or plated on the surface, be used between all bolts and overlapping sections of metal. Indium has a large inherent plastic flow under compression. even at the temperature conditions anticipated at the Statue of Liberty. As such, it will tend to fill all voids and afford complete coverage between the two surfaces in which it is applied. Even as the metal sections move. and the bolts expand and contract in place, the Indium will continue to 'flow' under compression. filling all voids. It assures intimate contact between surfaces at all times. Indium is an excellent oxygen scavenger. In this capacity, it will preferentially take up any oxygen in preference to the other metals present, thus preventing corrosion of the copper in which it is in contact.†


Because of the cold flow and oxygen scavenging properties of Indium. it will afford protection at those points most susceptible to corrosion. Without Induim. exposed copper surfaces will convert to copper sulfate and iron to rust. The constant expansion and contraction of the metal support pieces and bolts will create voids which, if not filled. will permit corrosion at these points. Indium acts to fill these voids. As a double protection. it will scavenge any oxygen that is present in preference to the other metals present.†


January 13. 1984 Submitted by:†


Steven Weintraub, Associate Conservator. Objects Conservation, The Metropolitan Museum of Art, New York. NY.†


Richard Smith, President, †Group Research, Inc. New York, †NY