Wood Tanks for Corrosive Applications

Wood tanks offer superior performance in corrosive environments. Without the use of expensive and exotic liners or coatings, wood tanks serve satisfactorily and economically, holding corrosive solutions and gases including organic solvents, petroleum hydrocarbons, organic acids, inorganic acids, alkaline chemicals and salts.

Specify wood tanks with confidence

Wood's inherent durability and high corrosion resistance are readily understood by those who have been a party to its demonstrated performance. This research report will help the new or unacquainted engineer, specifier and user to more confidently choose tankage after due consideration for wood's applications and advantages.

The cost factor

Here are how costs compare for some typical tanks constructed of various materials. The performance of wood tanks under highly corrosive conditions In addition to low initial cost, maintenance costs can be remarkably less in many industrial uses where non-corrodible tanks are required. In fact, most wood tanks are placed in service and then given only token maintenance until they are replaced after as long a service life as 50 years or more. These are usually the tanks which the new generation of plant engineers want to replace with another material because they seep and/or drip and look terrible. Try that maintenance program out on tanks made of other materials and compare their service life to wood.

Cell wall resistance

The chemical resistance of wood depends primarily upon the cell wall resistance to a particular chemical action-and upon the degree of chemical penetration into the wood.

All wood cells contain the three major chemical components: cellulose, hemicellulose and lignin. Mineral matter and a variety of substances extractible with water or neutral organic solvents are also present in the cell walls and voids.

Wood tanks are durable for any solution which is not actively destructive to the wood fiber. The deterioration of the wood will depend upon the action of the solution upon either the lignin or the cellulose fiber, or, in some cases, upon both. Some solutions attack only one or the other, while a very few will attack both.

Organic acids and alkaties

Generally, organic acids aren't as likely to produce as great a disintegrating effect on wood as some of the inorganic group. Some of the organic acids, even in concentrated form, show no particularly harmful effects on wood. Acetic acid is a notable example of this. Wood is most severely attacked by strong acidic and alkaline solutions representing the extremes in pH values, and by strong oxidizing agents and acids, such as nitric and chromic acids and sodium and calcium hypochlorite.

Strong alkalies cause increased swelling and increased accessibility as well as acting to dissolve the lignin and hemicellulose components of wood.

Generally, degradation of wood occurs more rapidly with highly alkaline solutions than it does with acid solutions. Excellent performance of wood tanks and pipe have been obtained over a range of pH values from 2 to 11. Best performance can be expected over a range of 2 through 9.

Temperature and deterioration

High temperatures, depending on the type of chemical handled and the duration that the high temperature is maintained, will increase the rate of deterioration.

Deterioration will approximately double for each 10"C. rise in temperature. This rate is similar for both acid and alkaline solutions although the nature of the deterioration is different in each environment.

Wood is superior to rubber and most plastics for handling hot solutions and corrosive gases. Operating temperatures up to 95"C. and higher, at infrequent intervals, are common practice with wood construction.

Performance and long life are related to the abrasive nature of the contained solution as well as to the solution's chemical characteristics and temperature.

Service Life

Wood water tanks have been reported to have served for more than 90 years. Service lives of 40 to 50 years are common for wood water tanks.

Many factors affect the service. life of industrial tankage, such as the particular use environment, the operating conditions and care, as well as the design, materials and construction quality of the tankage itself. Service lives of wood tanks and pipe in industrial applications have equaled those in use for water storage. However, for most industrial uses, a satisfactory service life is defined as a minimum of 20 years.

Wood tailings pipe lines have served 20 years and more carrying abrasive slurries of 43% to 60% solids at pH's ranging from below 2to 11.

Examples of abrasion resistance

Examples of where wood's abrasion resistance can be put to effective use may be found in agitated ore leaching tanks and wear plates in pickling tanks. Wood is especially useful in those industries where rough treatment is common.

Different woods, varying resistances

Different species and types of wood vary in their resistance to chemical degradation due to differences in basic composition and permeability.

In nearly all species, sapwood is more pervious than heartwood. This difference in accessibility together with the nature and greater amount of extractive materials contribute to heartwood's greater resistance to chemical attack.

It is the small differences in the chemical composition between sapwood and heartwood, and between species, which determine relative corrosion resistance and durability.

Hardwoods contain more hemicellulose which is more susceptible to attack by acids and alkalies than is cellulose. In addition, the hemicellulose of hardwoods is more easily attacked than is the hemicellulose of softwoods. Softwoods have more lignin than do hardwoods. Lignin is resistant to acids, but is attacked, although to a lesser degree than hemicellulose, by alkalies. Lignin is susceptible to attack by strong oxidizing acids and agents.

Tropical hardwoods include a wide variety of species, many of which compare more closely to softwoods in their resistance to chemical attack.

The effect of impregnants

lmpregnants, such as paraffin and coke-oven coal tar to reduce permeability, phenolic resin to increase acid resistance and furfuryl alcohol to increase alkaline resistance, have been used to improve wood's performance without adversely affecting structural strengths or wood's other natural advantages.

Glue lamination

Glue-laminated construction of wood tank staves and bottom pieces can eliminate permeability problems with porous woods. The glue line between each laminate acts as a plastic liner which slows down or prevents the attack of the outer laminates from the contents of the tank.

Treatments, such as phenol-resin impregnation; and material construction techniques, such as glue-laminating and finger-jointing, can be utilized to improve the basic wood properties for tank and pipe use and double or triple the service life.

Summary

Long-life and satisfactory performance of wood tanks are determined not only by the wood's resistance to corrosion, but also by the quality of materials, fabrication and tank construction, and operational care and maintenance. Service problems can be avoided through proper design and specification of wood and metal or other non-wood components. Working with knowledgeable and experienced wood tank and pipe manufacturers is important.

You are urged to consult with a NWTI Member Manufacturer for answers to your specific questions regarding use and/or environment