CORROSION MITIGATION

With the temperatures rising so does the risk of fire increase. This series will be dedicated to Emergency Corrosion Mitigation and the next several posts will enlighten you on this restoration category.

 CHEMISTRY OF FIRE

The chemistry of a fire is a combination of complex reactions, particularly where synthetic materials have burned. There are well over 100 chemical elements, which have been identified. Many of them are involved in the process of burning and the cleaning techniques used in restoration. Three conditions are required for a fire to occur:
(1) a fuel is needed;

(2) the fuel must have enough surface to be raised to its kindling point (e.g. a match will light wood shavings but not a log a foot in diameter); and
(3) sufficient oxygen to support combustion must exist. With these conditions and a source of ignition, fire results.

Fires may be classified into two groups -- simple and complex. A simple fire results in complete combustion and produces no soot, no free carbon, and no appreciable amounts of corrosive gases, fumes or smoke. A relatively pure fuel, such as natural gas, gasoline or a high quality fuel oil consists of many compounds of carbon, hydrogen and oxygen. However, if any of those relatively pure fuels are burned efficiently and completely, the products of combustion would be essentially carbon dioxide and water. Only trace impurities would be present. In disaster restoration, virtually no fires are simple.

Most fires are classified as complex. These fires are the result of incomplete combustion and are fueled by synthetic materials. Incomplete combustion occurs when there is insufficient oxygen present to react with the carbon and hydrogen in the fuels. The products of incomplete reactions could include carbon monoxide, unburned or free carbon and a variety of complicated hydrocarbon products. Materials acting as synthetic fuels form acid gases and corrosives such as hydrochloric, hydrofluoric, sulfuric and nitric acids. Some of the synthetic fuels are foams, films, polyethylene, polypropylene, melamine, acrilan, Saran, synthetic rubbers, Teflon, polyurethane, polyvinyl chlorides and fluorides. Objects made from these materials include: toys, carpets, flooring tiles, and sheet goods, furniture, clothing, shoes, appliances, plumbing, dishes and bathroom equipment. Even wood fires have been analyzed and found to produce over a dozen different organic acids.

In a fire, acid gases combine with heat and water vapor and penetrate cracks and crevices. When the surfaces cool, the gases condense, forming highly corrosive solutions. Even small amounts of PVC (polyvinyl chloride) pipe can produce enough hydrochloric acid to cause damage. Rubber containing sulfur produces sulfuric acid when burned. Burning Teflon or other fluorinated hydrocarbons produce hydrofluoric acid which etches glass. Some surfaces are especially sensitive to these gases and residues. Machinery and tools, electrical equipment, precision or sensitive metal apparatus, metallic building construction parts, household appliances, limestone, marble and terrazzo surfaces, aluminum and glass can be permanently pitted, etched, and stained from acid residues.


When acids attack metals, salts are produced which also continue to damage the metal surface. An example of salt action is the corrosion on cars from snow and ice treatments or salt-water spray in ocean areas. Anodized aluminum is extremely susceptible to permanent damage from both acids and alkalines. Anodized aluminum is actually aluminum with an extremely thin veneer of aluminum oxide. Once the veneer has been damaged, it cannot be restored.

Soot is comprised of carbon and other materials, which are incompletely burned or oxidized. Although some of the carbon particles have electrical charges causing them to stick together, more often greases and oils are the adhesives. Some soot particles are dry and can be vacuumed effectively. Other soot particles require alkalinity and/or solvent action to dissolve the greases or oils. The alkalinity of a cleaning solution plus the lifting-wetting action of surfactants chemically reacts with oil or grease to form a new product, a water-soluble soap, which is washed away. These reactions then release the insoluble carbon and permit it to be removed physically, often with water. The type of soot residues and the surfaces to be cleaned determine what process, chemicals and concentrations are necessary for the most effective cleaning. An alkaline wash will react with residual acids and some of the greases and oils in soot deposits. Other soot deposits may require a solvent additive. There are some deposits that may even need a putty knife or other physical means of removal. For example, the combustion products from certain plastics vaporize and condense into a solid form.

The most important action to take in fire restoration is drying out the area as soon as possible. Smoke, water vapor and other gases should be evacuated. Portable metal objects should be moved, cleaned and dried with warm air as soon as practical. Normal cleaning should proceed as usual, starting the salvage procedures in the wettest and heaviest areas. If a time lag is necessary, contaminated metal surfaces should be coated with a light coat of vegetable oil to stop further attack. If there is too great a time lag between the initial survey of the job and when work begins, a cost factor should be considered. Delays in cleanup may change the probability that soil will be removed from a particular substrate, requiring more time and more products than would have been necessary when the first test patch was made. Delay may also allow further chemical attack and decrease the level of success in cleaning. Some surfaces may even become so severely attacked that they are no longer salvageable.