Atmospheric Corrosion
153
Water repellents. Water repellents are water-repellent preservatives with
the fungicide, mildewcide, and preservatives omitted. They are not effective
natural finishes by themselves but are used as a stabilizing treatment prior
to priming and painting.
Solid color stains. Solid color stains provide an opaque finish and are available in a wide range of colors. They contain a much higher concentration of
pigment than the semitransparent stains. Solid color stains totally obscure
the natural color and grains of the wood. Oil-based and latex-based solid
color stains form a film similar to a paint film and as such can peel loose
from the substrate. Both of these stains are similar to thinned paint and
can usually be applied over old paint or stains, providing the old finish is
securely bonded.
Semitransparent penetrating stains. Semitransparent penetrating stains are
moderately pigmented and do not hide the wood grain. They do not form a
surface film, are porous to water vapor, and penetrate the surface. Because
they do not form a surface film, they do not blister or peel. Penetrating stains
are alkyd or oil based, and may contain a fungicide or mildewcide as well as
a water repellent. Latex-based stains are also available but do not penetrate
the wood surface as do the oil-based stains.
These stains are not effective when applied over a solid color stain or over
old paint. They are not recommended for use on hardwood but provide an
excellent finish on weathered wood.
Transparent coatings. Conventional spar or marine varnishes produce a
film-forming finish and are not generally recommended for exterior use on
wood. Shellac or lacquers should never be used outdoors because they are
brittle and very sensitive to water. Exposure to sunlight causes varnish coatings to become brittle and to develop severe cracking and peeling, often in
less than 2 years.
4.8.6 indoor atmospheric Corrosion
Atmospheric corrosion poses a problem indoors as well as outdoors. As
can be expected, there are obvious differences between outdoor and indoor
exposure conditions that lead to a difference between outdoor and indoor
corrosion behaviors.
Under outdoor exposure conditions, the aqueous layer is influenced by
seasonal and daily changes in humidity and by precipitation (dew, fog, or
snow), whereas indoors the aqueous layer is apt to be governed by relatively
constant humidity conditions. In this situation, for all practical purposes,
there will be an absence of wet-dry cycles, and therefore the effect of indoor
humidity will introduce a time-of-wetness factor.
In general, most gaseous pollutants found outdoors have considerably
lower levels of concentration indoors, with the exception of NH 3 and HCHO.
Pollutants such as HCHO and HCOOH are important indoor corrosion
153
Water repellents. Water repellents are water-repellent preservatives with
the fungicide, mildewcide, and preservatives omitted. They are not effective
natural finishes by themselves but are used as a stabilizing treatment prior
to priming and painting.
Solid color stains. Solid color stains provide an opaque finish and are available in a wide range of colors. They contain a much higher concentration of
pigment than the semitransparent stains. Solid color stains totally obscure
the natural color and grains of the wood. Oil-based and latex-based solid
color stains form a film similar to a paint film and as such can peel loose
from the substrate. Both of these stains are similar to thinned paint and
can usually be applied over old paint or stains, providing the old finish is
securely bonded.
Semitransparent penetrating stains. Semitransparent penetrating stains are
moderately pigmented and do not hide the wood grain. They do not form a
surface film, are porous to water vapor, and penetrate the surface. Because
they do not form a surface film, they do not blister or peel. Penetrating stains
are alkyd or oil based, and may contain a fungicide or mildewcide as well as
a water repellent. Latex-based stains are also available but do not penetrate
the wood surface as do the oil-based stains.
These stains are not effective when applied over a solid color stain or over
old paint. They are not recommended for use on hardwood but provide an
excellent finish on weathered wood.
Transparent coatings. Conventional spar or marine varnishes produce a
film-forming finish and are not generally recommended for exterior use on
wood. Shellac or lacquers should never be used outdoors because they are
brittle and very sensitive to water. Exposure to sunlight causes varnish coatings to become brittle and to develop severe cracking and peeling, often in
less than 2 years.
4.8.6 indoor atmospheric Corrosion
Atmospheric corrosion poses a problem indoors as well as outdoors. As
can be expected, there are obvious differences between outdoor and indoor
exposure conditions that lead to a difference between outdoor and indoor
corrosion behaviors.
Under outdoor exposure conditions, the aqueous layer is influenced by
seasonal and daily changes in humidity and by precipitation (dew, fog, or
snow), whereas indoors the aqueous layer is apt to be governed by relatively
constant humidity conditions. In this situation, for all practical purposes,
there will be an absence of wet-dry cycles, and therefore the effect of indoor
humidity will introduce a time-of-wetness factor.
In general, most gaseous pollutants found outdoors have considerably
lower levels of concentration indoors, with the exception of NH 3 and HCHO.
Pollutants such as HCHO and HCOOH are important indoor corrosion