Cast Iron: Characteristics, Uses and Problems
- CSI Division:
- Division 5 - Metals
- Metal Materials
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This set of guidelines provides general information on the characteristics and common uses of cast iron and identifies typical problems associated with the material. See also: "Checklist for Inspecting Cast Iron Failures".
Cast iron is one of the oldest ferrous metals used in construction and outdoor ornament. It is primarily composed of iron (Fe), carbon (C) and silicon (Si), but may also contain traces of sulphur (S), manganese (Mn) and phosphorus (P). It has a relatively high carbon content of 2% to 5%. It is hard, brittle, nonmalleable (i.e. it cannot be bent, stretched or hammered into shape) and more fusible than steel. Its structure is crystalline and it fractures 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.
The most common traditional form is grey cast iron. Common or grey cast iron is easily cast but it 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. It is also known as 'malleable' or 'spheroidal graphite' iron.
Cast iron is still manufactured by much the same process as it was produced historically. Iron ore is heated in a blast furnace with coke and limestone. This process "deoxidizes" the ore and drives off impurities, producing molten iron. The molten iron is poured into molds of the desired shape and allowed to cool and crystallize.
Upon manufacture, cast iron develops a protective film or scale on the surface which makes it initially more resistant to corrosion than wrought iron or mild steel. Finishing may include bituminous coatings, waxes, paints, galvanizing and plating. In addition, there are a variety of treatments that can reduce rusting and corrosion caused by environmental factors. Factory preservative treatments are typically barrier coatings intended to prevent the castings from oxidizing (rusting) in the presence of humidity and oxygen in the air.
Margot Gayle, David W. Look, John G. Waite. Metals in America's Historic Buildings. Washington, DC: National Park Service, 1992. (USGPO 1992-332-360)
L. William Zahner. Architectural Metals: A Guide to Selection, Specification, and Performance. New York City: John Wiley & Sons, 1995.
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. Most of the typical uses include:
Historic markers and plaques
Hardware: hinges, latches
Structural connectors in buildings and monuments
Tools and utensils
Stoves and firebacks
The basic cast iron material in all of these applications may appear to be the same, or very similar. However, the component size, composition, use, condition, relationship to adjacent materials, exposure and other factors may dictate that different treatments be used to correct similar problems. Any material in question should be evaluated as a part of a larger system and treatment plans should be based upon consideration of all relevant factors.
Problems and Deterioration
Cast iron is extremely strong and durable when used appropriately and protected from adverse exposure. It is much stronger in compression than in tension, therefore it is commonly found in columns, but not in structural beams. It is, however, highly susceptible to corrosion (rusting) when exposed to moisture and it has several typical problems which usually can be identified by visual inspection. The following sections will identify and discuss the most common problems encountered with cast iron. For general guidance on inspecting for cast iron failures, see 05010-01-G.
Natural or Inherent Problems
The typical deterioration or corrosion process for cast iron is a one-step straight line process of oxidation (or rusting) which begins on exposure to air and moisture and will continue (unless interrupted) until the metal is gone. This process is described in the following section.
Rusting, or oxidation, is the most frequent and easily recognizable form of cast iron deterioration. Cast iron is highly susceptible to rusting when the humidity is higher than 65%. Iron (Fe) combines with oxygen (O) in the presence of water vapor (H2O) to become rust (Fe2O3). This process can take place at significantly different rates depending on the material composition, protective treatments applied and severity of exposure. If rusting occurs at a rapid rate, it can result in severe damage or total loss of a component in a short time; therefore, the presence of any rust on a cast iron artifact should alert the observer to the presence of a serious problem. Rusting can occur when the humidity is as low as 58% in the presence of certain pollutants, especially sulfur dioxide, ammonia sulfates or even the presence of body oils from touching. Reducing the humidity to 30% or below has been found to be effective in preventing rusting, however this is not a practical solution for outdoor cast iron.
Rusting is such a common problem that it is quite easily recognizable. Rust (Ferrous Oxide, Fe2O3, and Ferric Oxide, Fe3O4) is an orange colored surface coating, ranging in texture from scaly to powdery. It is loosely bound and the outer layers will usually come off when rubbed by hand or brushed against. It is not a deposit on the surface. Rust is the result of the combination of the iron (Fe) with oxygen (O) in the air, in the presence of moisture. The presence of rust means that some original iron material has been converted to iron oxide and irreversibly lost from the cast iron piece.
The probability of rust occurring is generally dependent upon two factors:
The degree of protection (usually a protective coating) provided to keep moisture from contact with the metal, and
The degree of moisture present in the air.
Protective coatings used on iron include bituminous coatings (such as tars), waxes, paints and sophisticated metallic coatings. Effective coatings, well maintained, provide the most reliable protection against rust and corrosion of cast iron, however, there are a wide variety of coatings available, and these can be confusing to users not thoroughly versed in the technical data for each type.
Humidity is the second factor affecting the rate of oxidation (rusting) of iron. It is generally accepted that rusting cannot begin unless the relative humidity is at or above 65% (this figure can be lower, however, in the presence of pollutants). Relative humidity is, however, not the only factor to be considered. Once rusting has started, at least two other phenomena may occur:
Some rust or ferrous oxide can become hydrated, i.e. it can contain moisture within its chemical structure, thereby exposing the iron to additional moisture, and
The porous rust may act as a reservoir for liquid water, keeping it in contact with the iron and perpetuating the rusting process.
Both of these conditions are microscopic in nature and invisible to casual inspection. Maintenance staff and trained personnel, however, should be aware of the processes, and the potential for the processes to damage the cast iron. The presence of visible rust is the symptom indicating that a problem exists. Appropriate action should be taken to prevent rusting, and where it does occur, to correct it with an appropriate treatment. See individual repair or preventive maintenance procedures for specific guidance as needed.
Many other factors can affect both corrosion and the rate of corrosion. Sea water, salt air, cements, plasters, ashes, sulphur, soils and acids can accelerate the corrosion of iron. Corrosion rates can also be accelerated where the detailing of the cast iron provides pockets which can collect and hold moisture and corrosive agents. Preventive maintenance plans should consider detailing, such as crevices and recessed areas, in establishing routine inspection techniques and frequency of inspection.
Cast iron contains carbon, in the form of graphite, in its molecular structure. It is composed of a crystalline structure as are all metals; i.e. it is a heterogeneous mass of crystals of its major elements (Iron, Manganese, Carbon, Sulphur and Silicon). One condition which can occur in the presence of acid rain and/or sea water is "graphitization." The stable graphite crystals remain in place, but the less stable iron becomes converted to insoluble iron oxide (rust). The result is that the cast iron piece retains its shape and appearance but becomes weaker mechanically because of the loss of iron. Graphitization is not, however, a common problem. It generally will occur only after bare metal is left exposed for extended periods, or where failed joints allow the penetration of acidic rainwater to interior surfaces.
This corrosion process is galvanic, with the carbon present acting as the most noble (least corrosive) element and the iron acting as the least noble (most corrosive) element. The composition or microstructure of the iron affects the durability of the object because the rate of corrosion is dependent upon the amount and structure of the graphite present in the iron.
Barrier coatings are the most commonly used protective mechanisms for cast iron. Some type of coating (such as a wax, paint or metallic coating) should probably be considered an integral feature of cast iron in service. The absence of such a coating, or a failure in an existing coating should be corrected. Inspection should include a visual examination of all surfaces to determine if a coating exists, a fact which may be very apparent for opaque paints and coatings but substantially less apparent for clear lacquers, waxes or oils. Surfaces having the appearance of raw metal should be carefully examined for signs of rusting. Absence of a coating should be considered a major problem and corrective action should be undertaken. See individual repair or preventive maintenance procedures for specific guidance as needed.
Failure of a coating should also be identified and corrected. Coatings can wear away, crack, flake, blister, or peel away, indicating that the coating has failed and is no longer protecting the cast iron from moisture. Failed coatings can, in fact, trap moisture beneath the film and accelerate corrosion at certain points on the surface. Inspection of the surface should include a careful check for all of these types of coating failures. A record should be made of any coating failures observed so that corrective action may be taken.
Mechanical failures of cast iron are typically of two types and are relatively common problems.
- Structural Failure:
Cast iron may contain various imperfections due to the manufacturing process. These may occur due to air holes, interrupted pouring, uneven cooling (cold sheets), cracks and cinders. Where such imperfections occur, the piece may be weakened mechanically, sometimes severely. These manufacturing problems are not generally visible upon inspection; however, there are several non-destructive techniques of identifying these types of problems, such as the use of fluorescent fluids and ultraviolet lamps, or x-ray. These non- destructive techniques require specialized knowledge and equipment, and are not generally feasible for use by maintenance staff. They should be undertaken by specialists with experience.
Visible inspection may, however, enable detection of mechanical failures after the failure has occurred or begun to occur. Stress cracks in paint or metal may be symptomatic of this problem. Failures may begin as gradual separations which are visible upon inspection, and may be detected and corrected prior to a total, catastrophic failure of the piece. Linear cracks in paint film or metal should be investigated and/or monitored to determine if they are active. Non-destructive techniques may be used if symptoms exist, but the Regional Historic Preservation Officer (RHPO) should be consulted in the solicitation of professionals who are experienced in use of these techniques.
- Mechanical Failure of Connections:
Larger cast iron pieces are generally systems composed of smaller castings, mechanically connected. This can even be the case for a simple baluster or historical marker. One of the most common failures that occurs with such systems is the failure of the connectors or joints. Loose, missing or broken screws, clamps or bolts may result in loose, failed or missing components. Visual inspection should include examination of cast iron pieces for sections which are loose and/or disoriented, and which have loose or missing screws or bolts. Further manipulation by hand, with probes, may indicate whether a casting is a discrete piece, mechanically attached, and whether or not it is in the early stages of working loose. It is especially important to detect connectors which are in danger of imminent failure if not corrected. Corrective action should be undertaken in either case, but the treatment plan should take into account the severity of the problem, consequences of failure and nature of the intervention required to correct the problem. See individual repair procedures for specific guidance as needed.
Another mechanical problem can be caused by inappropriate mechanical repairs to broken pieces. Some repairs may create openings that allow water penetration and "pockets" that collect water, both of which can cause problems. Castings which have been filled with concrete are also a potential problem since they may promote "crevice corrosion" due to entrapped water. Visual inspections should check for such conditions and where they exist, maintenance staff should plan to correct the problems and/or be vigilant for signs of deterioration.
Cast Iron Alloys for Replacements
Cast iron problems, especially corrosion problems, may be reduced or eliminated in cast iron that is an alloy of silicon, nickel, chromium and/or copper. For example, silicon is often present in cast iron to some degree, but it is not considered an alloy until the percentage exceeds the 3% upper range of non-alloy cast iron. Where silicon is present, a protective surface film develops during oxidation.
There are three main categories of cast iron alloys:
High nickel (frequently containing copper or chromium)
All of these alloys, plus copper alloys, have been tested and found to have increased corrosion resistance. The degree of increased resistance is dependent on many factors, primarily the alloying metal and the percentage of alloy relative to the carbon content of the cast iron. While a discussion of alloy durability and formulation is beyond the scope of this standard, users should be aware of the effect of alloying and consider the implications when ordering new cast iron replacement objects. Such consideration may involve experienced metallurgists, foundrymen, conservators, and historical architects.
Maintenance of Cast Iron
The maintenance principles for cast iron are, in order of appearance:
Prevent rust and corrosion.
Paint and plug holes.
Maintain structural soundness.
keep it together with binding and bolts, welding, etc., and brace loose elements by resetting.
Recreate missing pieces using casting replacement parts (iron, aluminum, fiberglass, or epoxy), or wooden replacements, with appropriate composition and/or coatings to provide for color blending.
Cast iron requires continual maintenance. Check periodically for water collection spots and dry as necessary. Signs of corrosion are when rusty looking stain marks appear on the metal. If these areas are rubbed the metal surface is revealed as well as traces of perforation. Check for small chips in the coating surface and peeling of the coating surface.
Replace or repair as necessary if the damage is minimal missing and deteriorated pieces of metal prior to cleaning. If deteriorated condition is left unrepaired, perforation of the metal will occur and as a result structural failure.
Structural iron maintenance may require the services of a structural engineer when severe erosion or distortion occurs, to assist in the development of repair techniques when material loss is involved. For these repairs use only a professional iron worker. Before installation of new material verify the metal type and thickness. Prior to installation, remove all oil, dirt, and other debris from the surface. All surfaces shall be dry and free from frost.