Runway deicing fluid is environmentally friendly

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2009

Biobased products have been developed with improved compatibility with cadmium-coated metallic parts. 

KEY CONCEPTS
• The currently used runway deicing fluid based on potassium acetate can cause corrosion of cadmium-coated metallic parts.
• A biobased runway deicing fluid has been developed based on a byproduct stream that exhibits comparable performance with reduced corrosion.
• The biobased runway deicing fluid also displays reduced oxidation of carbon-carbon brakes.

For those of us who need to fly, airplane delays have become a fact of life. Weather delays during winter can be particularly difficult to deal with because the airport runways need to be deiced. This process is very important obviously in order to keep air traffic moving.

Runway deicing fluids have been developed, which airport personnel spray on to break the bond holding ice and snow to the pavement, as shown in Figure 2. These products also are sprayed on the runway prior to the onset of snow to minimize ice adhesion in a technique known as anti-icing. Besides performing well in these critical functions, there are other concerns that need to be addressed with using the correct fluid.


Figure 2. A more environmentally friendly runway deicing fluid has been developed that also is less corrosive toward aircraft equipment. (Courtesy of Battelle Memorial Institute)

Dr. Satya Chauhan, senior program manager at the Battelle Memorial Institute in Columbus, Ohio, says, “Other factors that need to be considered include minimizing the potential corrosion of aircraft and airfield equipment and potential environmental concerns.” The primary runway deicing fluid used in the U.S. is a 50% solution of potassium acetate in water.

The problem with using potassium acetate is that this salt can cause corrosion of aircraft equipment, particularly those that are cadmium-coated. Chauhan indicated that runway deicing fluids also contain a small percentage of additives included to impart corrosion protection to the fluid to help protect aircraft when exposed. But corrosion inhibitors such as those based on triazole chemistry have been found to be toxic to the environment.

Runway deicing fluids need to pass over 30 tests to be certified for use, according to Chauhan (SAE AMS 1435). Testing includes an evaluation of the fluid’s physical, deicing, environmental and materials-compatibility properties. The original runway deicing materials were propylene glycol (PG) and urea. Chauhan says, “Urea and PG are not environmentally friendly because they exhibit high levels of biological oxygen demand (BOD) and chemical oxygen demand (COD). In addition, urea’s decomposition product is ammonia, which is toxic.”

The problems with urea and PG prompted the switch to potassium acetate. At the same time aircraft brakes were upgraded from steel to carbon. Chauhan adds, “Potassium acetate is quite active catalytically when in contact with carbon-carbon brakes. This aspect, combined with the heat generated when the aircraft lands, accelerates the oxidation of the brakes.”

An environmentally friendly runway deicing fluid that is just as effective as potassium acetate but does not degrade cadmium-coated metallic parts and carbon aircraft brakes has not been available until now.


The primary runway deicing fluid used in the U.S. is a 50% solution of potassium acetate in water.

BYPRODUCT STREAM
The objective that Chauhan and his co-workers at Battelle were looking for was to develop a biobased product that was sustainable, recyclable and generated a small carbon footprint. A key component was to find a suitable freezing point depressant that was also environmentally friendly.

The researchers looked to work with biobased C3-C5 polyols, which are a much more sustainable component than potassium acetate. An example of an appealing mixture containing such polyols is the byproduct from transesterification of a fatty oil with methanol or ethanol.

Included in this particular mixture are water, alcohol and a basic component derived from the catalyst used in the process. The researchers found that neutralizing the base with an organic acid such as acetic acid produces a salt that enhances the overall freezing point depression properties of the fluid.

Six biobased products were evaluated by the researchers. They display very low five-day BOD values between 0.1 and 0.3 and very low toxicity values toward the aquatic organisms Daphnia and fathead minnows.

A substantial improvement was seen in compatibility with cadmium-coated metallic parts. Chauhan says, “The cadmium-coated coupons are exposed to deicing fluids, and the weight change due to corrosion is recorded.” The biobased products exhibited weight changes, which are significantly less than potassium acetate.

The reduced conductivity of the biobased fluids, as compared to potassium acetate, also leads to a benefit in reducing corrosion. The biobased fluids have values less than 35 microsiemens. In contrast, the potassium acetate fluid has a conductivity in excess of 100 microsiemens.

Catalytic oxidation of carbon-carbon brakes also was assessed. Chauhan says, “This procedure is an aggressive test that is carried out for 24 hours at 650 C using a test developed by Honeywell. The weight loss from standard coupons is measured, and thermal oxidation by itself typically leads to a 2%-3% loss.”

The potassium acetate-based fluid exhibits a 50%-70% weight loss. The biobased fluids display a 15%-20% weight loss. A standardized test for carbon-brake oxidation is currently being developed by the Society of Automotive Engineers (SAE).

Performance testing also showed that the biobased products are comparable in performance in the deicer penetration and deicer undercutting tests. Both of these procedures are designed to determine how the deicing fluid debonds ice from a runway surface.

Friction testing also was conducted because the deicing fluid must ensure that the aircraft can properly grip the runway and not skid off into an embankment. Chauhan says, “A SAAB automobile is fitted with a device to measure slippage.” The biobased products displayed comparable results with potassium acetate at concentrations suitable for deicing (two gallons/1,000 feet²) and anti-icing (0.5 gallon/1,000 feet²). 

Chauhan indicates that the U.S. Federal Aviation Agency recently evaluated the runway fluids and found the friction values to be comparable to currently used acetate deicers. Further details on the biobased product technology can be found in a U.S. Patent (1) and in a presentation made at a 2007 SAE conference (2).

REFERENCES
1. Chauhan, S., Samuels, W., Kuczek, S., and Conkle, H. (2006), “Process for Producing a Deicing/Anti-Icing Fluid,” U.S. Patent 7,048,871.
2. Chauhan, S. (2007), “Development of Environmentally Benign and Reduced Corrosion Runway Deicing Fluid,” SAE Aircraft & Engine Icing International Conference, Seville, Spain, Paper # 07ICE-54.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.