Atmospheric Corrosion
147
The weathering process is a slow-acting surface phenomenon. Most soft
woods erode at an average rate of approximately 1/4 in. per century, whereas
dense hardwoods erode at a rate of only 1/8 in. per century. In cold northern
climates, erosion rates of as low as 1/32 in. per century have been reported.
Western red cedar, which is a typical very-low-density softwood, may erode
at a rate as high as 1/2 in. per century.
Several factors affect the physical loss of wood substance from the wood
surface during weathering. These factors include not only wood species and
density, but also
1. Growth rate
2. Ring orientation
3. Wind
4. Degree of exposure
5. Amount of irradiation
6. Rain action
Swelling and shrinkage stresses, caused by fluctuations in moisture content, accompany the loss of wood from the surface. Grain raising, surface
roughening, differential swelling of earlywood and latewood bands, and the
formation of small parallel cracks and checks are the result. In addition, warpage might occur, and larger and deeper cracks might develop.
The growth of dark-colored spores and micelia of fungi or mildew on the
surface usually accompanies the weathering process. These impart a dark
gray, uneven, and unsightly appearance to the wood. In such species as redwood and Western red cedar, highly colored wood extractives add to the
color of the weathered wood. In areas where the wood is not exposed to the
sun and where the extractives are not washed away by rainwater, the dark
brown color of the extractives may persist for a long time.
Wood deteriorates due to two principal causes: chemical and biological
attack. The chemical resistance of the wood depends on the ability of the
wood cells to resist chemical action and the extent to which the chemical
penetrates into the wood.
Wood is most resistant to chemical attack in the pH range of 2 to 9. It is
resistant to weak acids but concentrated mineral acids tend to hydrolyze the
cellulose and hemicellulose constituents. Oxidizing agents such as ozone
attack the grain in the wood, which tends to bind the fibers together. The
cellulose can also be oxidized by strong oxidizing agents such as nitric
acid. Aqueous solutions of nitric acid, sulfuric acid, sodium hydroxide, and
hydrochloric acid can cause the most damage because swelling and degradation are simultaneous.
Decay resistance of timbers varies greatly, depending on the type of tree
and the type of wood. Hardwood is more resistant than softwood because
147
The weathering process is a slow-acting surface phenomenon. Most soft
woods erode at an average rate of approximately 1/4 in. per century, whereas
dense hardwoods erode at a rate of only 1/8 in. per century. In cold northern
climates, erosion rates of as low as 1/32 in. per century have been reported.
Western red cedar, which is a typical very-low-density softwood, may erode
at a rate as high as 1/2 in. per century.
Several factors affect the physical loss of wood substance from the wood
surface during weathering. These factors include not only wood species and
density, but also
1. Growth rate
2. Ring orientation
3. Wind
4. Degree of exposure
5. Amount of irradiation
6. Rain action
Swelling and shrinkage stresses, caused by fluctuations in moisture content, accompany the loss of wood from the surface. Grain raising, surface
roughening, differential swelling of earlywood and latewood bands, and the
formation of small parallel cracks and checks are the result. In addition, warpage might occur, and larger and deeper cracks might develop.
The growth of dark-colored spores and micelia of fungi or mildew on the
surface usually accompanies the weathering process. These impart a dark
gray, uneven, and unsightly appearance to the wood. In such species as redwood and Western red cedar, highly colored wood extractives add to the
color of the weathered wood. In areas where the wood is not exposed to the
sun and where the extractives are not washed away by rainwater, the dark
brown color of the extractives may persist for a long time.
Wood deteriorates due to two principal causes: chemical and biological
attack. The chemical resistance of the wood depends on the ability of the
wood cells to resist chemical action and the extent to which the chemical
penetrates into the wood.
Wood is most resistant to chemical attack in the pH range of 2 to 9. It is
resistant to weak acids but concentrated mineral acids tend to hydrolyze the
cellulose and hemicellulose constituents. Oxidizing agents such as ozone
attack the grain in the wood, which tends to bind the fibers together. The
cellulose can also be oxidized by strong oxidizing agents such as nitric
acid. Aqueous solutions of nitric acid, sulfuric acid, sodium hydroxide, and
hydrochloric acid can cause the most damage because swelling and degradation are simultaneous.
Decay resistance of timbers varies greatly, depending on the type of tree
and the type of wood. Hardwood is more resistant than softwood because