Specification:Metal identification and failures

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=GENERAL=

DESCRIPTION
 This specification provides guidance for identifying metals and failure mechanisms. This specification has been developed for use on historic properties (defined as any district, site, building, structure, or object that is listed in or eligible for listing in the National Register of Historic Places) and provides an overview of accepted practices. The Architect will provide site-specific specifications, when appropriate. All work described herein and related work must conform to the Secretary of the Interior’s Standards for the Treatment of Historic Properties. The Contractor shall provide all labor, material, equipment, and operations required to complete the rehabilitation work indicated herein. All work described herein and related work must have the approval of a Cultural Resources Manager, Conservator, Historic Architect, or other professional who meets the standards outlined in the Secretary of the Interior’s Standards – Professional Qualifications Standards pursuant to 36 CFR 61. Such person is referred to in this document as the Architect. 

SECTION INCLUDES
 Metal Identification Metal Deterioration and Failure</li> </ol>

RELATED SECTIONS
<ol type="A"> 05010.02 – General Cleaning of Metal</li> </ol>

QUALITY ASSURANCE
<ol type="A"> Work Experience: The Contractor shall have a minimum of seven years experience in metal conservation with an emphasis on architectural metals. </li> The Contractor shall also demonstrate a working knowledge of The Secretary of the Interior's Standards for the Treatment of Historic Properties with Guidelines for Preserving, Rehabilitating, Restoring & Reconstructing Historic Buildings, including experience in historic metal conservation. </li> </ol>

SUBMITTALS
Not used.

DELIVERY, STORAGE AND HANDLING
Not used.

PROJECT/SITE CONDITIONS
Not used. =PRODUCTS=

METAL IDENTIFICATION
<ol type="A"> General: Steel and iron are the most common metals used for both structural and decorative purposes in architecture. Aluminum alloy, copper and copper alloys (e.g. brass, bronze), zinc, lead, and nickel metals are also used, though less frequently, in architecture. While cast and wrought iron were both used extensively for structural applications in the nineteenth and early twentieth centuries, their use is fairly rare today.

Metals serve a variety of functions, and their relative usefulness depends on their particular combination of properties including the following:  Chemical composition (influencing corrosion resistance and mechanical properties)</li> Carbon content (influencing weldability)</li> Ductility</li> Elongation</li> Fatigue properties</li> Fracture properties</li> Modulus of elasticity</li> Reduction of area</li> Tensile strength</li> Yield strength</li> </ol> </li>

The following is a list of metals found in historic buildings, along with their characteristics and uses:

Aluminum <ol> <li>Types of aluminum: <li>Nonfinished: A bare aluminum surface. Upon exposure to the air, bare aluminum develops a thin layer of natural oxide, a patina which protects the aluminum from corrosion. Non-finished aluminum is the most common type of interior and exterior finish found on historic buildings (1920 -1950).</li> <li>Anodized: An oxide coating is applied by passing an electrical current through the aluminum, providing greater resistance to atmospheric corrosion. Anodizing aluminum was invented in 1923 and began to be used for architectural elements in the 1950s.</li> <li>Aluminum alloys: A number of aluminum alloys have been developed to improve certain properties. <li>Nonheat-treatable alloys, which include 1-1/4 percent manganese and two to seven percent magnesium, are of relatively high strength and are used for cladding and corrugated roofing.</li> <li>Heat-treatable alloys contain varying proportions of aluminum, magnesium, silicon, and sometimes copper. These have high strength and are used for fasteners and for light structural members.</li> <li>Aluminum alloys used for casting usually contain silicon, silicon and copper, or silicon and magnesium. </li> </ol> </li></ol></li><li>Characteristics of aluminum: <li>Lightweight</li> <li>Corrosion-resistant</li> <li>Nonmagnetic</li> <li>Low melting point</li> <li>Moderately high coefficient of expansion</li> <li>Good thermal and electrical conductivity</li> <li>Malleable</li> <li>Very soft</li> <li>Ductile (can be drawn or &quot;stretched&quot;)</li> <li>Good corrosion resistance to atmospheric gases, moisture and soil</li> <li>Poor corrosion resistance to alkalis, hydrochloric acid, lead-based paints, some wood preservatives, and chlorides.</li> </ol> </li> <li>Uses of aluminum: <li>In the late nineteenth century, aluminum was typically limited to interior applications such as stairs, elevators and grilles.</li> <li>In the early twentieth century, uses of aluminum included decorative detailing; roofing, wall panels and spandrels; window mullions and frames, storefront surrounds, doors and door trims (as it could be extruded into lengths of specialized profiles or cross sections).</li> </ol> </li> </ol> </li> <li>Brass: Traditionally, a copper alloy that contains zinc is a &quot;brass.&quot; A copper alloy that contains tin (not exceeding 11 percent) is a &quot;bronze.&quot; Architectural use of brass is generally limited to interior applications such as window hardware and lighting fixtures.</li> <li>Bronze: Bronze is an alloy of copper that can vary widely in its composition. Like cast iron, bronze is a manufactured product. Copper is extracted from natural ores and usually alloyed with tin to create a metal which does not exist in nature. Many of its inherent problems relate to the normal physical process of the bronze &quot;returning to nature,&quot; i.e. to the most stable state of its components. Contemporary bronzes are typically copper alloys that may contain silicon, manganese, aluminum, zinc and other elements, with or without tin. Bronze in its rarely seen &quot;raw&quot; state is a pinkish, salmon colored metal. It usually exhibits some patination or corrosion so that its color normally ranges from lime green to dark brown. The variations in bronze (both in proportion and elemental composition) can significantly affect its weathering characteristics. <ol> <li>Types of bronze: &quot;True&quot; bronze is a combination of approximately 90 percent copper and 10 precent tin. There are three major classes or types of &quot;bronzes&quot; used in sculpture and construction: <li>Statuary Bronze: Used for casting statues and other ornamental objects, and may contain approximately 97 percent copper, two percent tin and one percent zinc.</li> <li>Architectural Bronze: Used for extruded moldings and forgings, commonly composed of approximately 57 percent copper, 40 percent zinc and three percent lead. This alloy is actually more of a &quot;leaded brass.&quot;</li> <li>Commercial Bronze: Used for weather stripping, composed of approximately 90 percent copper and 10 percent zinc.</li> </ol> </li> <li>Characteristics of bronze: Like copper, bronze is highly resistant to corrosion in</li> unpolluted environments. Bronze has good resistance to industrial, rural and marine atmospheres as well as weak acids, but poor resistance to ammonia, ferric and ammonia compounds, and cyanides.

<li>Uses of bronze: Its durability has long made it a choice for commemorative applications such as monuments and statues. Its forms are almost limitless since it may be cast in any shape for which a mold can be fabricated. For architectural applications it is used where a material harder than copper is required, where strength and corrosion resistance is required, and for ornamental purposes. Typical uses for architectural bronze are door and window frames and hardware, mailboxes and chutes, trim or rails, and furniture hardware.</li> </ol> </li> <li>Copper: Copper’s corrosion-resistant patina has long made it a building material of choice for exterior roofing, flashing and cladding applications. It is initially bright reddish-brown in color, but when exposed to the atmosphere it acquires its trademark patina that turns from brown to black to green over an eight- to ten-year period. This patina is a copper carbonate or copper sulfate formed on the surface of the metal when hydrogen sulfide combines with oxygen or sulfur dioxide. Though copper does corrode, this patina makes it a very corrosion-resistant material. Its high capacity for thermal and electrical conductivity also makes it suitable for electrical wiring and heating applications. <ol> <li>Characteristics of copper: <li>Durable</li> <li>Corrosion-resistant</li> <li>Strong</li> <li>Ductile</li> <li>Malleable</li> </ol> </li> <li>Uses of copper: <li>Historical Uses: <li>Sheathing for ships               </li> <li>Roofing and flashing. Sheet copper is light and easily formed</li> <li>Ornamental/decorative detailing. Examples include weathervanes, finials, moldings, cornices               </li> <li> Statues. Sections of sheet copper were often hammered over wooden or other forms to create ornaments or statues. Once the copper sheets had taken the shape of the form, they were removed and soldered together over a wooden or metal framework. The most famous example is the Statue of Liberty, which consists of copper sheeting over a steel framework. </li> </ol> </li> <li>Contemporary Uses: <li>Decorative detailing (limited due to the high cost of copper)</li> <li>Flashing, gutters and downspouts</li> <li>Piping systems</li> <li>Electrical wiring, telephone wiring, and heating and air conditioning systems </li> </ol> </li> </ol></li></ol> </li> <li>Iron <ol> <li>Cast Iron: Cast iron is one of the oldest ferrous metals used in construction and outdoor ornament. It is primarily composed of iron, carbon and silicon, but may also contain traces of sulphur, manganese and phosphorus. It has a relatively high carbon content of between two and five percent. The most common traditional form is grey cast iron, which is easily cast but cannot be forged or worked mechanically either hot or cold. In grey cast iron the carbon content is in the form of flakes distributed throughout the metal. In white cast iron the carbon content is combined chemically as carbide of iron. White cast iron has superior tensile strength and malleability. <li>Characteristics of cast iron: hard, brittle, nonmalleable (i.e. it cannot be bent, stretched or hammered into shape) and more fusible than steel. Its structure is crystalline and relatively brittle and weak in tension. Cast iron members fracture under excessive tensile loading with little prior distortion. Cast iron is, however, very good in compression. The composition of cast iron and the method of manufacture are critical in determining its characteristics.</li> <li>Uses of cast iron: Cast iron is used in a wide variety of structural and decorative applications because it is relatively inexpensive, durable and easily cast into a variety of shapes. Cast iron was also used for building fronts which became known as cast iron facades. Typical uses include: <li>Historical markers and plaques</li> <li> Hardware (hinges, latches)                 </li> <li> Columns, balusters                 </li> <li>Stairs </li> <li>Structural connectors in buildings and monuments</li> <li>Decorative features</li> <li>Fences </li> <li> Tools and utensils</li> <li>Ordnance </li> <li>Stoves and firebacks</li> <li> Piping </li> </ol> </li></ol></li><li>Wrought Iron: Wrought iron differs from cast iron and steel in its carbon content. It contains very little carbon (approximately .035 percent). Wrought iron is suitable for members in tension or compression, whereas cast iron is suitable for members in compression only. Although wrought iron was used extensively as a structural building material in the nineteenth century, it was superseded by steel by the turn of the twentieth century. <li>Characteristics of wrought iron: <li>Soft</li> <li>Ductile               </li> <li>Magnetic               </li> <li>Strong - high elasticity and tensile strength </li> <li>Malleable (Can be heated and reheated and worked into various shapes. Becomes stronger the more it is worked).</li> </ol> </li> <li>Uses of wrought iron: <li>In the seventeenth and eighteenth centuries, wrought iron was typically used for decorative applications including fences, gates and railings; balconies, porches, verandas and canopies; roof cresting; lamps; grilles; and hardware.</li> <li> In the middle of the nineteenth century, uses became more structural and included nails, iron cramps (to secure masonry veneer building frames), and structural members in tension such as tie rods, bulb-Ts and I-beams. </li> </ol> </li></ol></li></ol></li> <li>Steel: The most common alloy of iron. Contains a moderate amount of carbon (between .06 percent and two percent), which is the most cost-effective alloying material for iron. Carbon steel, composed simply of iron and carbon, accounts for 90 percent of steel production. Steel may also be alloyed with or contain traces of several other elements (including nickel, aluminum, copper, titanium, manganese, and tungsten) that can influence characteristics such as corrosion resistance, hardness or toughness. <ol> <li>Characteristics of steel: With the invention of the Bessemer process in the mid-nineteenth century, steel became a relatively inexpensive mass-produced good. Its strength, durability and widespread availability quickly made it the most widely used architectural metal. Modern steels are made with varying combinations of alloy metals to fulfill many purposes. Stainless steels contain a minimum of 10 percent chromium, often combined with nickel, to resist corrosion. Though not an alloy, galvanized steel is a commonly used variety of steel which has been hot-dipped or electroplated with zinc for protection against rust.</li> <li>Steel grading and standards: The more commonly used steel alloys are categorized into various grades by standards organizations. The American Iron and Steel Institute (AISI) has a series of grades defining many types of steel ranging from standard carbon steel to stainless steel. The American Society for Testing and Materials (ASTM) has a separate set of standards which define alloys such as A36 steel, the most commonly used structural steel in the United States.</li> <li>Uses of steel: Steel is one of the most common materials in the world today and is a major component in buildings, having largely replaced iron. Most large modern structures are supported by steel frames. Even those with a concrete structure use steel for reinforcement. It is also used for bolts, nails, screws and other fasteners in construction, as well as shipbuilding, oil and gas pipelines, and tools.</li> </ol> </li> <li>Tin: Tin is typically used in alloying with other metals (e.g. alloying tin with copper to form bronze). It is also used to coat harder metals such as iron and steel. Before the twentieth century, sheets of iron and steel were hand-dipped in molten tin or a combination of tin and lead to make tinplate and terneplate. In the twentieth century, electroplating (the process of coating a base metal with tin using an electric current) became popular. <ol> <li>Characteristics of tin: <li>Silvery-white metal</li> <li>Non-magnetic</li> <li>Fairly resistant to corrosion</li> <li>Non-combustible</li> <li>Lightweight</li> <li>Durable</li> <li>Soft</li> <li>Ductile</li> <li>Malleable</li> <li>Expensive, but long-lasting when properly maintained</li> <li>Low maintenance</li> </ol> </li> <li>Tin products: <li>Tinplate: Sheet iron or steel that has been coated with pure tin. The tin offers a lightweight, corrosion-resistant finish highly suitable for a roofing (and cladding) material.</li> <li>Terneplate: Sheet iron or steel which has been coated with a mixture of lead (75-90 percent) and tin (10-25 percent). The addition of the lead provides more durability.</li> </ol> </li> <li>Uses of tin: <li>Historical uses of pure tin include lighting devices such as perforated lanterns, candle shields, wall sconces, and mirror frames.</li> <li>Historical uses for tinplate and terneplate: <li>Roofing</li> <li>Decorative machine-pressed shingles</li> <li>Sheetmetal wall covering (formed to imitate masonry or other building materials)</li> <li>Flashing, gutters and downspouts </li> <li>Dormers</li> <li>Fire protection on wood doors and shutters</li> <li>Ornamental elements (door and window heads, balusters, urns, roof ornaments) </li> </ol> </li> </ol> </li> </ol> </li> </ol>

METAL DETERIORATION AND FAILURE
<ol type="A"> <li>General: Every metal has its own particular set of characteristics and responses to the environment. Humidity, temperature and condensation can greatly affect metal deterioration, and different finishes may react differently with the environment and result in different corrosion types and rates. Rather than go into great detail for each metal type, this section will address several mechanisms of deterioration that are common to most metals. These can be broken into two broad categories: chemical deterioration and physical deterioration.</li> <li>Chemical Deterioration <ol> <li>Corrosion: In a general sense applicable to all metals, corrosion is a chemical process through which metals (including “man-made” metals and alloys) gradually revert to their stable mineral components. Corrosion may also be referred to as mineralization. Corrosion of one form or another is the chief cause of the deterioration of metals. The degree of corrosion which occurs, and the corrosion by-products which result, are affected by several factors including composition, environmental conditions and adjacent materials.</li> <li>Categories of corrosion include natural corrosion, chemical corrosion, and galvanic (or electrochemical) corrosion. Natural corrosion is simply the result of heavy soiling on surfaces that have been neglected or are not cleaned regularly while chemical corrosion can take a variety of forms: <li>Electrolytic/oxygen cell corrosion: Oxidation of metal is due to chemical reaction between metal and oxygen in the environment. Oxidation is the most common cause of deterioration (rust) of unprotected iron and steel products. Exposed bronze notably undergoes continuous change and progresses through several predictable "stages" of oxidation and corrosion.</li> <li>Atmospheric corrosion: The most common form of corrosion. Moisture containing environmental gases (carbon dioxide, oxygen, sulfur compounds, etc.) produces chemical corrosion on the metal. With bronze, the most significant cause of deterioration is atmospheric sulphur and chlorine in the presence of moisture.</li> <li>Deterioration of metals by attack of chemical solutions with either low or high acidity (pH): For example, acids from unseasoned wood, damp oak, cedar, and redwood may attack aluminum. Corrosion will result from direct contact between wet wood and aluminum, as well as water draining off a roof of unweathered wood. Corrosion may be accelerated on an aluminum roof where condensation develops on the underside of the roof, much like a terneplated or tinplated roof. If standing water is acidic, corrosion cells will develop on the aluminum.</li> <li>Uniform corrosion: Attacks the metal surface evenly. Often called “patina,” which is considered a desirable, and even attractive protective surface layer. The stability of an initial corrosion layer of even thickness protects the metal against further corrosion.</li> <li>Galvanic corrosion: Occurs as an electrochemical action between two dissimilar metals, either through direct contact or via an electrolyte. Galvanic corrosion causes extensive deterioration to the attacked metal(s), and in turn the corrosion products stain and streak the adjacent surfaces. With aluminum, galvanic corrosion will occur if the aluminum comes in contact with other metals such as tin, iron and steel (if they are not painted), and especially copper. Aluminum is compatible with zinc, cadmium, lead, galvanized steel, monel, magnesium, and usually nonmagnetic stainless steel (nonmagnetic stainless steel is sometimes corrosive to aluminum when the two metals come into contact in industrial environments).</li> <li>Pitting: Corrosion that attacks the metal surface in localized areas. It is not evenly distributed, but occurs in holes in the surface. Frequently the result of galvanic corrosion due to impurities in metals.</li> <li>Selective corrosion: Occurs when one metal is selectively removed from an alloy in corrosive conditions. Dezincification (the selective removal of zinc from brass) is a classic example.</li> <li>Stress corrosion: Occurs where metals are under stress and exposed to localized corrosive environments.</li> </ol> </li> </ol> </li> <li>Physical Deterioration <ol> <li>Erosion: The slow process of abrasion which can eventually wear the metal substance completely away. Occurs when a corrosion-resistant oxide layer is removed and the bare metal beneath corrodes. Aluminum features are extremely vulnerable to erosion because this metal is so soft. When exposed to abrasive agents, erosion of aluminum can be a critical problem.</li> <li>Fatigue: Fatigue cracking occurs when metals are subjected to cyclic stresses. These cracks develop slowly but if left untreated can lead to brittle fracture. Related forms of physical deterioration include hardening, embrittlement, or outright failure as a result of repeated flexing, bending, or other stresses due to loading or working. Fatigue is one of the most common failures resulting from the stresses associated with expansion and contraction.</li> <li>Fracture cracking: Brittle cracks that take place with little or no preceding deformation, often triggered by an impact or sudden increase in load-bearing.</li> <li>Deformation: May be caused by out-of-plumbness, settlement or failure of supports, inadequate bracing, removed members or missing connectors, etc.</li> <li>Creep: Occurs when metal undergoes slow deformation when subjected to loads and/or high temperatures. A lead sheet on a steep roof slope is a classic example.</li> <li>Combination of physical and chemical attack: <li>Weathering</li> <li>Stress corrosion cracking: Attacks areas in a metal that were stressed during metal working.</li> </ol> </li> </ol> </li> </ol>