148
Fundamentals of Corrosion
of the lower permeability and presence of toxic substances. Table 4.22 shows
the relative resistance to decay of the heartwood of various tree species.
Table 4.23 provides a more comprehensive listing. Those not shown have
little or no resistance without being treated.
Wood-destroying fungi are primitive forms of plant life. They make use
of lignin (white rots) or the cellulose (brown rots) of the wood for food.
The metabolic process can be halted by controlling moisture, air, or temperature. Optimum conditions for growth are 10 to 25% moisture content,
20% of wood volume as air, and a temperature range of 75 to 95°F (23 to
35°C). Freezing of the wood does not kill the fungus; it only causes dormancy. Fungus growth can be inhibited by impregnation of the wood.
Wood is also subject to attack by insects, the most serious of which in the
United States are termites of the subterranean type. These insects remove
the wood internally, leaving few signs of their activity. The cellulose in the
wood is acted on by protozoa in the insect’s intestine and converted to sugar
for food. Control can be achieved by following construction techniques that
prevent the termites from gaining entry into the structure. Because termites
live underground, their entry can be prevented by the installation of metal
strips between the wood and the foundation material. The U.S. Department
of Agriculture has several publications available describing construction features to prevent termite entry. Termites can also be controlled by poisoning
the soil or pressure-treating the lumber with toxic chemicals.
Another insect that can cause extensive damage is the wood-boring beetle. Small surface holes and piles of sawdust are indications of the presence
of this insect. Thorough surface coatings of paint or varnish are the best
control measures.
TabLE 4.22
Relative Resistance to Decay (Heartwood)
Trees Having:
High Natural Decay Resistance Moderate Decay Resistance
Northern white cedar
Douglas fir
Southern cypress
Eastern white pine
Western red cedar
Southern yellow pine
Redwood
Western larch
Chestnut
White oak
Black locust
Red gum
Black walnut
Black cherry
Source: From Reference 23.
Fundamentals of Corrosion
of the lower permeability and presence of toxic substances. Table 4.22 shows
the relative resistance to decay of the heartwood of various tree species.
Table 4.23 provides a more comprehensive listing. Those not shown have
little or no resistance without being treated.
Wood-destroying fungi are primitive forms of plant life. They make use
of lignin (white rots) or the cellulose (brown rots) of the wood for food.
The metabolic process can be halted by controlling moisture, air, or temperature. Optimum conditions for growth are 10 to 25% moisture content,
20% of wood volume as air, and a temperature range of 75 to 95°F (23 to
35°C). Freezing of the wood does not kill the fungus; it only causes dormancy. Fungus growth can be inhibited by impregnation of the wood.
Wood is also subject to attack by insects, the most serious of which in the
United States are termites of the subterranean type. These insects remove
the wood internally, leaving few signs of their activity. The cellulose in the
wood is acted on by protozoa in the insect’s intestine and converted to sugar
for food. Control can be achieved by following construction techniques that
prevent the termites from gaining entry into the structure. Because termites
live underground, their entry can be prevented by the installation of metal
strips between the wood and the foundation material. The U.S. Department
of Agriculture has several publications available describing construction features to prevent termite entry. Termites can also be controlled by poisoning
the soil or pressure-treating the lumber with toxic chemicals.
Another insect that can cause extensive damage is the wood-boring beetle. Small surface holes and piles of sawdust are indications of the presence
of this insect. Thorough surface coatings of paint or varnish are the best
control measures.
TabLE 4.22
Relative Resistance to Decay (Heartwood)
Trees Having:
High Natural Decay Resistance Moderate Decay Resistance
Northern white cedar
Douglas fir
Southern cypress
Eastern white pine
Western red cedar
Southern yellow pine
Redwood
Western larch
Chestnut
White oak
Black locust
Red gum
Black walnut
Black cherry
Source: From Reference 23.