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Historic Preservation - Technical Procedures

Spectitle:

Cast Iron: Characteristics, Uses And Problems

Procedure code:

0501004S

Source:

Outdoor Sculpture Manual - Center For Public Buildings

Division:

Metals

Section:

Metal Materials

Last Modified:

11/12/2013

Details:

Cast Iron: Characteristics, Uses And Problems



CAST IRON:  CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of cast iron and identifies typical problems
associated with the material.  See 05010-01-S for guidance on
inspecting cast iron failures.


INTRODUCTION

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 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.  

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,
castings may be given a variety of treatments to reduce rusting and
corrosion in the environment.  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.

 


REFERENCES

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.

TYPICAL USES

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
    -  columns, balusters
    -  stairs
    -  structural connectors in buildings and monuments
    -  decorative features
    -  fences
    -  tools and utensils
    -  ordnance
    -  stoves and firebacks
    -  piping.

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,
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-S.


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:

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:

1.   the degree of protection (usually a protective coating)
    provided to keep moisture from contact with the metal, and

2.   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:

1.   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

2.   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.

GRAPHITIZATION:

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.

COATINGS FAILURE:

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 FAILURE:

Mechanical failures of cast iron are typically of two types and are
relatively common problems.

1.   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.

2.   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 which 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.  Repairs which create openings
that allow water penetration, or "pockets" which collect and hold
water, are both potential 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 where the cast iron 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:

1.   High silicon
2.   High chromium
3.   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:

1.   Prevent rust and corrosion - paint and plug holes.

2.   Maintain structural soundness - keep it together with binding
    and bolts, welding, etc., and brace loose elements by
    resetting.

3.   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.

                         END OF SECTION
 


cast iron, iron, gray cast iron,rusting, graphitization, cast iron alloys, cast iron maintenance