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Historic Preservation - Technical Procedures

Spectitle:

Galvanized Iron And Steel: Characteristics, Uses And Problems

Procedure code:

0501009S

Source:

Developed For Hspg (Nps - Sero)

Division:

Metals

Section:

Metal Materials

Last Modified:

01/07/2014

Details:

Galvanized Iron And Steel: Characteristics, Uses And Problems



GALVANIZED IRON AND STEEL:  CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of galvanized iron and steel and identifies typical
problems associated with these materials along with common causes
of its deterioration.


INTRODUCTION

Galvanizing is a process of coating iron or steel with zinc in
order to provide greater protection against corrosion for the iron
or steel base.  The process of galvanizing sheet iron was developed
simultaneously in France and England in 1837.  Both of these
methods employed a "hot dipping" process to coat sheet iron with
zinc.  Like tinplate, early galvanized metals were hand dipped.
Today almost all galvanized iron and steel is electroplated.

The following are the most common methods for applying protective
coatings of zinc to iron and steel:

1.   Hot-dip Galvanizing:  The immersion of iron or steel in molten
    zinc, after the surface of the base metal has been properly
    cleaned.  

    a.   This process gives a relatively thick coating of zinc
         that freezes into a crystalline surface pattern known as
         spangles.  

    b.   During the process, a multiple layered structure of iron-
         or steel-zinc alloys is formed between the inner surface
         of the zinc coating and the iron or steel.  These middle
         layers tend to be hard and brittle and may peel or flake
         if the iron or steel element is bent.

2.   Electrogalvanizing:  The immersion of iron or steel in an
    electrolyte, a solution of zinc sulfate or cyanide.
    Electrolytic action deposits a coating of pure zinc on the
    surface of the iron or steel.  

    Advantages:

    a.   The thickness of the coating can be accurately controlled
         using this process.  

    Limitations:

    a.   The thick coatings provided by the hot-dip galvanizing
         process are not usually possible with this method.

3.   Sherardizing:  The placing of a thoroughly cleaned iron or
    steel element in an air-free enclosure where it is surrounded
    by metallic zinc dust.  The architectural element is then
    heated and a thin, zinc alloy coating is produced.

    Advantages:

    a.   The coating will conform to the configurations of the
         element.  

    Limitations:  

    a.   This process is usually limited to relatively small
         objects.

4.   Metallic Spraying:  The application of a fine spray of molten
    zinc to a clean iron or steel element.  The coating can then
    be heated and fused with the surface of the iron or steel to
    produce an alloy.  

    Advantages:

    a.   Coating is less brittle than those produced by some of
         the other processes.

    b.   Coating will not peel or flake on bending.  

    Limitations:

    a.   The coating is more porous and becomes impermeable with
         time as products of corrosion fill in the pores.

5.   Painting:  Paint containing zinc dust pigments may be applied
    as a protective coating to galvanized iron and steel.

    Advantages:  

    a.   The paint may be applied in situ.

    Limitations:

    a.   This is a less effective method of zinc coating than the
         others listed above.

    b.   Paint does not adhere well to pure zinc, nor to
         galvanized iron or steel.

    c.   When paint peels from galvanized iron and steel, it
         usually comes off completely along with the primer,
         exposing a clean metal surface.

    d.   If sheetmetal features are well-painted, it is difficult
         to identify whether they are zinc or galvanized iron or
         steel.

         1)   If the metal is galvanized, it will have a spangled
              appearance and may show some rust or rust stains
              from the iron or steel base metal.  Both galvanized
              iron and steel are magnetic

         2)   If the metal is cast or pressed zinc, it will have
              a grayish-white appearance.  Pure zinc is not
              magnetic so a magnet will not stick.

         3)   A magnet test will also reveal whether a painted
              sheetmetal feature is zinc or galvanized iron or
              steel.  Both galvanized iron and steel are
              magnetic, pure zinc is not.


TYPICAL USES

Typical historical uses for galvanized iron and steel included:

-    Cornices and other wall ornaments

-    Door and window hoods

-    Decorative formed shingles and pantiles designed to imitate
    other materials

-    Roof ornaments such as crestings and finials

Typical uses today include:

-    Sheetmetal for flashing, and gutters and downspouts.

-    Hot-dipped galvanized steel nails.


PROBLEMS AND DETERIORATION

Problems may be classified into two broad categories:  1) Natural
or inherent problems based on the characteristics of the material
and the conditions of the exposure, and 2) Vandalism and human-induced problems.    

Although there is some overlap between the two categories, the
inherent material deterioration problems generally occur gradually
over long periods of time, at predictable rates and require
appropriate routine or preventive  maintenance to control.
Conversely, many human induced problems, (especially vandalism),
are random in occurrence; can produce catastrophic results; are
difficult to prevent, and require emergency action to mitigate.
Some human induced problems, however, are predictable and occur
routinely.  


NATURAL OR INHERENT PROBLEMS

CORROSION:

Galvanized iron and steel's resistance to corrosion depends largely
on the type and thickness of the protective zinc coating and the
type of corrosive environment.

The zinc coating on galvanized iron and steel may be corroded by:
Acids, strong alkalis, and is particularly vulnerable to corrosion
by sulfur acids produced by hydrogen sulfide and sulfur dioxide
pollution in urban atmospheres.  

1.   Natural Corrosion:

    a.   The zinc coating on galvanized iron and steel develops a
         natural carbonate on its surface by exposure to the
         atmosphere and by the action of rainwater.  This coating,
         however, is usually not thick enough to protect the metal
         from further corrosion.

    b.   The carbonate can become brittle and crusty and
         eventually split, exposing fresh zinc for corrosion.
         Since the zinc coating on the iron or steel is very thin,
         it can corrode up to the base metal exposing the base to
         the atmosphere as well.

    c.   In industrial atmospheres, the zinc carbonate coating can
         be broken down by the same acids that attack zinc.  These
         acids convert the carbonate to zinc sulfate, which is
         water soluble and washes away with rainwater, often
         staining the adjacent building elements.

2.   Chemical Corrosion:

    a.   Galvanized iron and steel have good corrosion resistance
         to:  Concrete, mortar, lead, tin, zinc and aluminum.

    b.   Galvanized iron and steel have poor corrosion resistance
         to:  Plasters and cements (especially Portland cements)
         containing chlorides and sulfates, acidic rainwater run-
        off from roofs with wood shingles (redwood, cedar, oak,
         and sweet chestnut), moss, or lichen, condensation on the
         underside of zinc plates and ponded water on the exterior
         surfaces of the zinc features

3.   Galvanic (Electrochemical) Corrosion:  This type of corrosion
    is an electrolytic reaction between the zinc coating and
    dissimilar metals when in the presence of an electrolyte such
    as rain, dew, fog or condensation.

    a.   To prevent the corrosion of the zinc coating due to
         galvanic action, contact between galvanized items and
         copper or pure iron or steel should be avoided.

    b.   Galvanized iron and steel are corrosive to all metals
         except lead, tin, zinc and aluminum.

    c.   Applying a protective coating such as paint to galvanized
         iron and steel will alleviate the problems caused by
         corrosion of the protective zinc coating.  


VANDALISM OR HUMAN-INDUCED PROBLEMS

Mechanical or Physical Deterioration:

1.   Abrasion:  Causes removal of the protective metal surface.
    The soft zinc coating on galvanized iron and steel make it
    vulnerable to abrasion damage, especially at roof valleys and
    gutters where the coating can be worn paper-thin by the
    scouring of rainwater.

2.   Fatigue:  A type of deterioration caused by cyclical expansion
    and contraction of sheet metal features, especially roofs,
    without adequate provisions for this movement.  

    a.   Zinc is very vulnerable to fatigue failure because it has
         a relatively high coefficient of thermal expansion.  

    b.   Fatigue failure may also occur when the metal sheets are
         too thin to resist buckling and sagging.  It results in
         the bulging and tearing of the zinc coating and resembles
         a cut or a crack.

3.   Creep:  The permanent distortion of a soft metal which has
    been stretched due to its own weight.  Thin areas of the metal
    are especially prone to failure.  Creep may be prevented by
    the use of properly sized individual sheets and bays, properly
    designed joints, and an adequate number of fasteners.

4.   Distortion:  Permanent deformation or failure may occur when
    a metal is overloaded beyond its yield point because of
    increased live or dead loads, thermal stresses, or structural
    modifications altering a stress regime.  

Connection Failure:

1.   Wind and thermal stress can damage a roof by pulling joints
    apart and loosening fasteners.

                         END OF SECTION
 


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