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Wood Preservative


The use of wood preservatives is not required. However, their use in wood under severe service conditions may pay for itself many times in decreased decay and borer attack and thus decreased repair and replacement costs. Their proper use should be encouraged since it increases the chance of the vessel remaining sound until her next inspection and thus contributes to maintaining a reasonable standard of safety.

Wood preservatives used for protection against decay fungi and marine borers either kill the organism or prevent it from growing. For marine use the preservative must offer no toxic hazard to the crew, must be free from objectionable odors and must be able to remain in the wood and do its work in the presence of moisture. No known wood preservative is ideal for marine use but certain ones have proved effective for specific applications.

There are two general classes of wood preservatives, oil soluble and water soluble. Both have been used in the marine industry.

  1. Oil Soluble Preservatives
    1. Coal Tar Creosote. One of the most effective of the oil soluble preservatives is coal tar creosote. This preservative is highly toxic to wood attacking organisms, is relatively insoluble in water and is easy to apply. It has a distinctive unpleasant odor, is somewhat of a fire hazard when freshly applied and causes skin irritation in some individuals. Its main disadvantage is that it is a hazardous material to the environment and thus has become unavailable for boat building applications. However, some older vessels with deadwood, keel, stems and heavy timbers which were originally treated with creosote, are still in service.

    2. Copper Naphthanate Solutions. Copper naphthanate solutions form one of the most used groups of marine wood preservatives. A three percent solution, equivalent to one half of one percent copper by weight, provides good protection against decay when properly applied. The protection afforded against marine borers is slight. Wood treated with copper naphthanate is a distinctive green color. Much of the "treated wood" which can be purchased is preserved with copper naphthanate. The paintability, glue bonding ability, and structural stability of the wood is only slightly affected by the copper salts. These properties will vary, however, depending upon the oil used as a solvent. It is important to note that this substance poses a serious health hazard to humans. Full body protection should be worn during application.

    3. Pentachlorophenol Solutions. "Penta" solutions have proven satisfactory for marine use. Field tests have shown that a 5% solution offers adequate protection against decay when proper application techniques are used. Little if any protection against marine borers is provided.

      Pentachlorophenol does not give wood any distinctive color. In itself, it affects the characteristics of wood very little. The final effect of the preservation treatment on physical characteristics depends upon the petroleum solvent used. Pentachlorophenol solution remains effective for approximately 2-3 years before it begins to break down.

  2. Water Soluble Preservatives
    1. Water Soluble Preservatives. Copper naphthanate and "penta" are often combined with water repellents. These repellents aid in stabilizing the moisture content of the treated wood. This is a material aid in reducing the chance that decay growth conditions will occur. In order to be effective these solutions should contain no less than 5% pentachlorophenol or 2% copper in the form of copper naphthanate.

    2. Solvents. Almost any petroleum product from mineral spirit to used engine oil can be used as a vehicle for the preservative depending upon local conditions. In general, the heavier high viscosity residuum types offer the best retention. The choice of solvent is usually a compromise of effectiveness, paintability and initial cost.

    3. Water Preservatives. Waterborne preservatives include zinc chloride, tanalith, copper arsenite, chromated zinc arsenate and many others. Their major applications are those in which the leeching out of the preservative by moisture is not a problem. In general, these preservatives have not proven satisfactory for severe marine service. Some preserved wood obtained for repair use may have been pressure treated with one of these preservatives. It can give satisfactory service if care is taken to use it in a location where it is protected from the action of rain and sea water.

Methods Of Treatment

  1. Pressure Treatment. In the commercial treating of wood a method utilizing high pressure is often used. This method requires expensive equipment and is seldom seen in a boat yard. Nonpressure treatments available to the boat yard are brushing, cold soaking, and various types of "hot and cold" bath processes.

  2. Brush Treatment. The simplest way of applying a preservative solution is to brush it on. Every crack and check must be flooded with preservative if the treatment is to be effective. Small pieces such as butt blocks can be dipped into the preservative. Solutions of pentachlorophenol or copper naphthanate available commercially, have proved effective when used in this way.

    "Penta" stock solutions are available in what is know as 1:5 and 1:10 strengths, (i.e. the solution must be diluted one part of solution to five or ten parts of solvent to achieve a "normal" wood preserving solution). These stock solutions are used without dilution for applications such as preserving cracks, holes resulting from old fastenings, and coating joints and hard to get spots. Care must be exercised since wood preservatives are toxic. When using the brush-on method the entire surface must be thoroughly coated.

  3. Soaking. Cold soaking in copper naphthanate or "penta" solutions for periods of up to 48 hours provides much better retention of the preservative than does a brushing. An even better method consists of heating the wood in a hot preservative bath and then transferring it to a cold bath of preservative. The heating causes the air entrapped in the wood to expand. The sudden cooling sets up a vacuum which aids preservative penetration.

Preservative solutions or other chemicals which release copper ions into wood or into the bilgewater should be avoided in vessels containing ferrous fastenings. Copper ions are more stable than iron, and will spontaneously plate out on steel or on zinc coatings, replacing equal numbers of iron or zinc ions, which go into solution (replacement corrosion). While the amount of direct wastage of iron or zinc from this mechanism is likely to be minimal, the presence of copper-plated regions on the surface of the steel fittings cause them to become small, isolated galvanic cells. The further corrosion of the steel or galvanizing may be significantly increased by the presence of copper surface inclusions.

Copper naphthanate (Cuprinol), Chromated Copper Arsenate (CCA) and Ammoniacal Copper Arsenate (ACA) wood preservatives are one common source of copper ions in the wood or bilgewater. Another source is the addition of chemical treatments to bilgewater. A traditional solution to the problem of sour bilges due to generation of hydrogen sulfide gas by bacteria breaking down spilled diesel fuel is to dissolve copper chloride crystals in the bilgewater.

Excerpted from Wood Hull Inspection Guidance (NVIC 7-95)

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