Schweitzer P.A. Fundamentals of corrosion. Mechanisms, causes, and preventative methods
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Atmospheric Corrosion 149
4.8.5.1 Plywood
Wood layers or plies of veneer, or veneer and lumber in which alternate
plies are laid with the grain at right angles, are given the name plywood. By
alternating the grain direction of each ply, in adjacent plies, the two face
directions are equalized in strength, stiffness, and dimensional changes.
Plywood is produced as either interior or exterior type. Exterior-type ply-
wood is designed to retain its shape and strength when repeatedly wetted
and dried under adverse conditions and be suitable for permanent outdoor
exposure. It is sometimes referred to as marine plywood. Exterior plywood is
usually bonded with hot-pressed phenol resins.
TabLE 4.23
Resistance of Wood to Decay
Softwoods Hardwoods
Bald cypress
Ε
Ash
Ρ
Douglas r
F–G
Aspen
VP
Hemlock, western
Ρ
Balsa
Ρ
Larch, western
F
Beech, American
Ρ
Pine, ponderosa
F
Birch, yellow
G
Redwood, virgin
Ε
Cherry, black
Ε
Spruce, Sitka
F
Chestnut, American
Ε
White cedar
Ε
Cottonwood, eastern
VP
Elm, American
F
Elm, rock
F
Hickory, shayback
G
Magnolia, southern
P
Mahogany
G
Maple, sugar
Ρ
Maple, silver
Ρ
Oak, red
G
Oak, white
G
Sycamore
Ρ
Sweetgum
F
Walnut, black
Ε
Poplar, yellow P
Note: Ε = excellent; G = good; F = fair; P = poor; and VP = very
poor.
Source: From Reference 23.
150 Fundamentals of Corrosion
4.8.5.2 Reconstituted Wood Products
Reconstituted wood products are produced by the formation of small pieces
of wood into large sheets. The nished product can be classied as berboard
or particle board, depending on the nature of the basic wood component.
Fiberboards are produced from mechanical pulps. Hardwood is a rela-
tively heavy type of berboard that is designed for exterior exposure.
Some reconstituted wood products can be factory primed with paint and
some may have a factory-applied topcoat to protect the wood from fungi and
insects; treatment with toxic chemicals provides the necessary protection.
A variety of materials can be used but oil and oil-borne preservatives pre-
dominate. The most widely used is coal tar creosote, which is a by-product
distilled from the coal tar produced by the high-temperature carbonization
of bituminous coal. It is a heterogeneous mixture of liquid and solid hydro-
carbons. To increase penetration and reduce the coat, coal tar solutions and
creosote-petroleum solutions are used extensively. Solutions as high as 50%
have been used. A disadvantage to this type of treatment is the inability to
apply paint.
When cleanliness and paintability are required, pentachlorophenol in vol-
atile petrochemical carriers is used. Concentrations of not less than 5% are
used. These solutions are in the same price range as coal tar emulsions and
provide the same degree of protection.
Water-borne solutions of inorganic salts are also used. These have the
advantages over the oils of greater ease of penetration and freedom from re
hazards and odor. The disadvantage is that they cause swelling and some
react with metal. The primary preservative used is chromated zinc chloride.
Other typical salts used include:
1. Acid cupric chromate
2. Ammonial copper arsenate
3. Chromated copper arsenate
4. Chromated zinc arsenate
5. Copperized chromated zinc chloride
Wood can be treated with preservatives in several ways, including:
1. Under pressure in closed vessels
2. Dipping
3. Hot and cold soak
4. Diffusion
Of the four methods listed, treatment under pressure in closed vessels is the
predominant method employed for lumber used in engineering structures.
Atmospheric Corrosion 151
4.8.5.3 Applied Exterior Wood Finishes
There are a variety of nishes that can be applied to wood that is to be
exposed to the weather and atmospheric conditions. The nish selected will
depend on the appearance and degree of protection desired and on the spe-
cies of wood to be protected. In addition, different nishes provide varying
degrees of protection; therefore, the type, quality, quantity, and application
method of the nish must be considered when selecting and planning the
nishing of wood and wood products. Table 4.24 shows the relative painting
and weathering properties of various woods. The classication of paintabil-
ity results from the nature of the specic wood. The higher the rating, the
greater the care that must be taken in applying the nishes.
Paints. Paint coatings on wood provide the most protection because they block
the damaging ultraviolet light rays from the sun. They may be either oil based
or latex based. Either type is available in a wide range of colors. Oil or alkyd
paints are borne by an organic solvent, whereas latex-based paints are water-
borne. The three primary reasons for using paints are to protect the wood sur-
face from weathering, to conceal certain defects, and for aesthetic purposes.
Paints do not penetrate the wood surface too deeply. A surface lm is
formed while obscuring the wood grain. Paints perform best on smooth,
edge-grained lumber of lightweight species. If the wood becomes wet, the
paint lm blisters or peels.
A nonporous paint lm provides the most protection for wood against
surface erosion and the largest selection of colors of any of the wood n-
ishes. Paint accomplishes this by retarding the penetration of moisture, and
reducing the problem of discoloration by wood extractives, paint peeling,
and warping of the wood. However, paint is not a preservative. It does not
prevent decay if conditions are favorable for fungi growth. Wood preserva-
tives must be used for this purpose.
Water-repellent preservatives. Water-repellent preservatives contain a fun-
gicide or mildewcide (the preservative), a small amount of wax for water
repellence, a resin or drying oil, and a solvent such as mineral spirits or tur-
pentine. A water-repellent preservative can be used as a natural nish. These
preservatives do not usually contain coloring pigments. The type of wood
determines the color of the resulting nish. The mildewcide prevents wood
from darkening.
During the rst few years of application, the water-repellent preservative
has a short life. Additional applications are usually required each year. Once
the wood has weathered to a uniform color, the treatments are more durable
and renishing is required only when the surface starts to become unevenly
colored by fungi.
Special color effects can be achieved by adding inorganic pigments to the
water-repellent preservatives. The addition of pigment to the nish helps
stabilize the color and increase the durability of the nish. Colors that match
the natural color of the wood and extractives are usually preferred.
152 Fundamentals of Corrosion
Water-repellent preservatives can also be used on bare wood prior to prim-
ing and painting or in areas where old paint has peeled, exposing bare wood.
This treatment prevents rain or dew from penetrating into the wood, partic-
ularly at joints and end-grain, thereby reducing the swelling and shrinking
of wood. This reduces the stress placed on the paint lm, thus extending its
service life.
TabLE 4.24
Painting and Weathering Characteristics of Various Woods
Woods
Ease of Keeping
Painted
Resistance to
Weathering
Softwoods
Cedar 1
A
Cypress 1
A
Redwood 1
A
Pine, ponderosa 3
Β
Fir 3
Β
Hemlock 3
Β
Spruce 3
Β
Douglas r 4
Β
Larch 4
Β
Hardwoods
Aspen 3
Β–Α
Basswood 3
Β
Cottonwood 3
D–B
Magnolia 3
Β
Yellow poplar 3
Β–Α
Beech 4
D–B
Birch 4 D–B
Gum 4
D–B
Lauan (plywood) 4
Β
Maple 4
Β
Chestnut 5–3
C–B
Walnut 5–3
C–B
Elm 5–4
D–B
Hickory 5–4
D–B
Oak, white 5–4
D–B
Oak, red 5–4
D–B
Note: 1 = easiest to keep well painted; 5 = most difcult. A = most
resistant to weathering; D = least resistant to weathering.
Source: From Reference 24.
Atmospheric Corrosion 153
Water repellents. Water repellents are water-repellent preservatives with
the fungicide, mildewcide, and preservatives omitted. They are not effective
natural nishes by themselves but are used as a stabilizing treatment prior
to priming and painting.
Solid color stains. Solid color stains provide an opaque nish and are avail-
able 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 lm similar to a paint lm 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 nish 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 lm, are porous to water vapor, and penetrate the surface. Because
they do not form a surface lm, 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 nish on weathered wood.
Transparent coatings. Conventional spar or marine varnishes produce a
lm-forming nish 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 coat-
ings 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 inuenced 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
154 Fundamentals of Corrosion
stimulants. They originate from adhesives, tobacco smoke, combustion of
biomass, and plastics.
Another factor contributing to a decreased indoor corrosion rate is the
decreased levels of indoor atmospheric oxidants, many of which are photo-
chemically produced.
As discussed previously, not only the concentration of pollutants but also
the air velocity determines the dry deposition velocity of corrosion stimu-
lants. Because the air velocity is decreased indoors, signicantly lower dry
deposition velocities will take place.
Based on the differences between the indoor and outdoor factors affecting
atmospheric corrosion rates, it follows that the corrosion rate of many metals
is lower indoors than outdoors. This has been veried by examining the cor-
rosion rates of copper, nickel, cobalt, and iron in eight indoor locations. In all
cases, they exhibited a lower corrosion rate indoors than outdoors.
These factors do not eliminate the possibility of indoor atmospheric cor-
rosion of materials. Designs must take into account the possibility of indoor
atmospheric corrosion.
In an uncontaminated atmosphere at constant temperature, and with the
relative humidity below 100%, corrosion of metals would not be expected.
However, this is never the case because there are always normal temperature
uctuations: as the temperature decreases, the relative humidity increases;
and because of hygroscopic impurities in the atmosphere or in the metal
itself, the relative humidity must be reduced to a much lower value than
100% in order to ensure that no water condenses on the surface. For all met-
als there is a critical relative humidity below which corrosion is negligible.
These critical relative humidities fall between 50 and 70% for steel, copper,
nickel, and zinc.
In design, areas where dust particles can accumulate should be eliminated,
as well as any crevices or pockets. Even in indoor atmospheres, carbon steel
should be protected from corrosion by means of rust preventatives, painting,
galvanizing, or other protective coatings, depending on the conditions of
exposure. The use of low-alloy steel also helps to reduce or eliminate corro-
sion. Atmospheric corrosion is reduced when steel is alloyed in small con-
centrations with copper, potassium, nickel, and chromium.
References
1. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 20.
2. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 17.
Atmospheric Corrosion 155
3. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 22, 23.
4. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 42.
5. Kucera, V. and Mattsen, E., 1989, Atmospheric Corrosion, in Manseld, F., Ed.,
Corrosion Mechanisms, New York: Marcel Dekker, p. 258.
6. Kucera, V. and Mattsen, E., 1989, Atmospheric Corrosion, in Manseld, F., Ed.,
Corrosion Mechanisms, New York: Marcel Dekker, p. 266.
7. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 117.
8. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 119.
9. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 132.
10. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 133.
11. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 137.
12. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 136.
13. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 150.
14. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 145.
15. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 144.
16. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 146.
17. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 147.
18. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 172.
19. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 176.
20. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 179.
21. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 193.
22. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 193.
23. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 198.
24. Schweitzer, P.A., 1999, Atmospheric Degradation and Corrosion Control, New York:
Marcel Dekker, p. 201.
25. Leygraf, Atmospheric Corrosion in P. Marcus, J. Oudar, Eds., Corrosion
Mechanisms in Theory and Practice, New York: Marcel Dekker, p. 423.
157
5
Corrosion of Polymer (Plastic) Materials
As discussed previously, metallic materials undergo a specic corrosion rate
as a result of an electrochemical reaction. Because of this, it is possible to
predict the life of a metal when in contact with a corrodent under a given set
of conditions. This is not the case with polymeric materials. Plastic materi-
als do not experience a specic corrosion rate. They are usually completely
resistant to chemical attack or they deteriorate rapidly. They are attacked
either by chemical reaction or by solvation. Solvation is the penetration of
the plastic by a corrodent, which causes softening, swelling, and ultimate
failure. Corrosion of plastics can be classied in the following ways as to the
attack mechanism:
1. Disintegration or degradation of a physical nature due to absorption,
permeation, solvent action, or other factors
2. Oxidation, where chemical bonds are attacked
3. Hydrolysis, where ester linkages are attacked
4. Radiation
5. Thermal degradation involving depolymerization and possibly
repolymerization
6. Dehydration (rather uncommon)
7. Any combination of the above
Results of such attacks will appear in the form of softening, charring, craz-
ing, delamination, discoloration, dissolving, or swelling.
The corrosion of polymer matrix composites is also affected by two other
factors: the nature of the laminate and, in the case of the thermoset resins,
the cure. Improper or insufcient cure time will adversely affect the cor-
rosion resistance, whereas proper cure time and procedures will generally
improve the corrosion resistance.
All polymers are compounded. The nal product is produced to certain
specic properties for a specic application. When the corrosion resistance of
a polymer is discussed, the data referred to are that of the pure polymer. In
many instances, other ingredients are blended with the polymer to enhance
certain properties, which in many cases reduce the ability of the polymer to
resist attack of some media. Therefore it is essential to know the makeup of
any polymer prior to its use.
158 Fundamentals of Corrosion
5.1 Radiation
Polymeric materials in outdoor applications are exposed to weather
extremes that can be extremely deleterious to the material, the most harm-
ful of which is exposure to ultraviolet (UV) radiation, which can cause
embrittlement, fading, surface cracking, and chalking. Most plastics, after
being exposed to direct sunlight for a period of years, exhibit reduced
impact resistance, lower overall mechanical performance, and a change
in appearance.
The electromagnetic energy from sunlight is normally divided into UV
light, visible light, and infrared energy. Infrared energy consists of wave-
lengths longer than the visible red wavelengths, and starts above 760 nm.
Visible light is dened as radiation between 400 and 760 nm. UV light con-
sists of radiation below 400 nm. The UV portion of the spectrum is further
subdivided into UV-A, UB-B, and UV-C. The effects of the various wave-
length regions are shown below:
Ultraviolet Wavelength Regions
Region
Wavelength
(nm) Characteristics
UV-A 400–315 Causes polymer damage
UV-B 315–280 Includes the shortest wavelengths at the Earth’s surface
Causes severe polymer damage
Absorbed by window glass
UV-C 280–100 Filtered by the Earth’s atmosphere
Found only in outer space
Because UV is easily ltered by air masses, cloud cover, pollution, and other
factors, the amount and spectrum of natural UV exposure is extremely vari-
able. Because the sun is lower in the sky during the winter months, it is
ltered through a greater air mass. This creates two important differences
between summer and winter sunlight: changes in the intensity of the light
and in the spectrum. During the winter months, much of the damaging
shortwavelength UV light is ltered out. For example, the intensity of UV at
320 nm changes about 8 to 1 from summer to winter. In addition, the short-
wavelength solar cut-off shifts from about 295 nm in summer to about 310
nm in winter. As a result, materials sensitive to UV below 320 nm would
degrade only slightly, if at all, during the winter months.
Photochemical degradation is caused by photons of light breaking chemical
bonds. For each type of chemical bond there is a critical threshold wavelength
of light with enough energy to cause a reaction. Light of any wavelength