Phosphine's Corrosive Effects on Metals: Phase II Study Completed

The use of phosphine as an alternative to methyl bromide for fumigation in food processing and stored-grain facilities has been controversial. Phosphine's potential corrosive effects on metal and its chemical volatility have caused many to disavow its use as a fumigant, but others have embraced it as a viable alternative to methyl bromide.

The corrosiveness of phosphine is relative, dependant on its concentration, type of exposed metal, temperature, and relative humidity, among other variables. In the United States, all types of corrosion across all industries cost approximately $300 billion per year, according to theNational Association of Corrosion Engineers.

Robert Brigham, consultant to the Environmental Bureau, Agriculture and Agri-Food Canada, recently completed a follow-up to his 1997–1998 study of the steady-state exposure of metals to phosphine. The study was partially funded by the USDA's Agricultural Research Service. By expanding the parameters of the first study—types of metals used, temperatures, phosphine concentrations, relative humidity, carbon dioxide levels, and exposure times—Brigham could more precisely predict the effect of phosphine on metals.

While corrosion of copper is directly related to the phosphine concentration and exposure time, the form of the surface deposits was quite a surprise. Brigham points out that, "sometimes the surface deposits on the metal will be wet and sometimes dry." The type, or morphology, of surface deposit, is related to the relative humidity in the storage facility. Logically, a wet morphology would occur in high humidity conditions.

"The actual wet and dry morphologies occur counterintuitively. A dry morphology occurs in high relative humidity and wet morphology occurs in low relative humidity," according to Brigham. Wet surface deposits occur on copper exposed to more than 100 ppm phosphine at ambient temperatures if the relative humidity is less than about 50 percent. Higher relative humidity results in dry surface deposits on copper. Generally, surface deposits, either wet or dry, increase with phosphine concentration. Higher temperatures and longer exposures result in more surface deposits and more corrosion on copper.

With the ability to minimize its corrosion of metals, phosphine's effectiveness as a fumigant can be explored. Brigham's research suggests that conditions in a food processing and grain-storage facility can be manipulated to safely and effectively use phosphine. "The lab data fit well with controlled field exposures," says Brigham.

Brigham's results provide useful data for proprietors when deciding whether fumigation with phosphine is a viable option. David Mueller of Fumigation Service and Supply, Inc., Westfield, Indiana, feels the use of phosphine as a replacement fumigant for methyl bromide is commercially viable. "If phosphine concentrations are kept at low levels of 85–100 ppm and the temperature at approximately 35 °C, then phosphine is a viable alternative to methyl bromide."

Brigham also examined the effect of phosphine on electrical component parts of various types of machinery. While failures did occur, the components were found to be much more resistant to failure than expected. Forced failures of electrical components were achieved due to high contact resistance from buildup of nonconducting surface deposits, electrical shorting from the formation of wet surface deposits, and disruption of circuits due to the corrosion of metals. These failures occurred after extremes in relative humidity, phosphine concentrations, and repeated exposures were inflicted.

Mueller says, "this study is part of the puzzle of how phosphine can have niche uses as a replacement for methyl bromide. It provides the data that phosphine users have needed for years."

Last Updated: February 24, 2000