TIN:  CHARACTERISTICS, USES AND PROBLEMS


This standard includes general information on the characteristics
and common uses of tin and identifies typical problems associated
with this material along with common causes of its deterioration.


INTRODUCTION

Characteristics of Tin:

-    Silvery-white metal

-    Non-magnetic

-    Fairly resistant to corrosion

-    Non-combustible

-    Lightweight

-    Durable

-    Soft

-    Ductile

-    Malleable

-    Expensive, but can last long time when properly maintained

-    Low maintenance material consisting primarily of routine
     inspection and periodic painting.

Tin is typically used in alloying with other metals (i.e. alloying
tin with copper to form bronze).  It is also used to coat harder
metals such as iron and steel.  Before the 20th century, sheets of
iron and steel were hand-dipped in molten tin or a combination of
tin and lead to make tin- and terneplate.  In the 20th century,
electroplating, or the process of coating a base metal with tin
using an electric current, became popular.

-    Tinplate:  Sheet iron or steel which has been coated with pure
     tin.  The tin offers a light weight, corrosion resistant
     finish highly suitable for a roofing (and walling) material.

-    Terneplate:  Sheet iron or steel which has been coated with a
     mixture of lead (75-90%) and tin (10-25%).  The addition of
     the lead provides more durability.

-    These materials must be painted.  For roofing, both the
     surface and the underside of the material should be painted. 
     They are typically painted a red or reddish-brown color or
     green to simulate copper.  When properly maintained, tin- and
     terneplate roofing can last 50-100 years.


TYPICAL USES

Typical historical uses for pure tin included:

-    Lighting devices such as perforated lanterns, candle shields,
     wall sconces, and mirror frames.

Typical historical uses for tinplate and terneplate included:

-    Roofing material:  Sheets of terne- and tinplate were soldered
     and/or mechanically fastened together to form a continuous
     waterproof covering.  

-    Decorative machine-pressed shingles:  These began to be
     manufactured in the late 19th century to simulate tile roofs.

-    Sheetmetal wall covering formed to imitate masonry or other
     building materials.  

-    Flashing, gutters and downspouts

-    Dormers

-    Fire protection on wood doors and shutters

-    Ornamental elements such as door and window heads, balusters
     and urns, or roof ornaments


NATURAL OR INHERENT PROBLEMS

1.   Chemical Corrosion:

     a.   Tinplated coatings generally have good corrosion
          resistance to:  Oxygen, moisture, sulfur dioxide and
          hydrogen sulfide.

     b.   Tin- and terneplated coatings generally have poor
          corrosion resistance to:  Asphaltic and bituminous
          roofing materials such as building paper and roofing
          cement, and paints containing either asphalt or bitumen,
          acids, graphite or aluminum.

2.   Galvanic (Electrochemical) Corrosion:

     a.   Galvanic corrosion will occur, causing tin- or terneplate
          to corrode, if these metals come in contact with copper.

     b.   Tin- and terneplate will cause aluminum and bare iron or
          steel to corrode.

     c.   Mixing metals used for flashing, gutters and downspouts,
          decorative elements, windows or roof covering will also
          cause galvanic corrosion.


VANDALISM OR HUMAN-INDUCED PROBLEMS

Mechanical or physical deterioration:

1.   Tin- and terneplate are resistant to corrosion as long as the
     tin or terne coating is not damaged.  If, however, the coating
     becomes damaged by falling objects, such as tree limbs or
     heavy roofing materials for instance, the base metal may
     become exposed and begin to corrode.  Galvanic action between
     the tin and the iron or steel will also accelerate the
     deterioration.

Tin- or terneplate roofing may be suitably replaced with lead-coated copper or terne-coated
stainless steel.  The initial cost
for either of these materials is higher, but more durable and
easier to maintain.

                          END OF SECTION