The objective for this exploratory study on the Resistance to Iron Chloride Test specified in AMS-C-27725 (Coating, Corrosion Preventative, for Aircraft Integral Fuel Tanks for Use to 250 °F (121 °C)) was to get a better understanding on a possible rationale behind this corrosion test required for the aircraft integral fuel tank coating systems. This test employs a high-voltage direct current (DC) resistance measurement technique to measure the coatings’ electrical insulating resistance after being immersed in a 0.5% (by weight) ferric chloride (FeCl3) solution for 10 days at 60 °C. After immersion, the coatings are first examined visually for blisters or loss of adhesion, and then the electrical resistance is measured and is required to be at least 5.0 megohms per mil coating thickness. In recent years, this test has been in question for its relevancy as a valid measure of corrosion resistance performance. The concern is that there is no clear understanding of the test method itself and the specified requirement. The test was first implemented in early 1960s and has remained in the specification ever since with only minor changes over the years.
The study was conducted under the assumption that the test in its current version represents an accelerated corrosion resistance test for assessing barrier protection properties of integral fuel tank coatings. It was confirmed that the ferric chloride solution undergoes a fast hydrolysis reaction that makes the solution highly acidic; this rapid change in acidity is believed to be a predominant factor in accelerated coating degradation. Various factors affecting the iron chloride resistance test were investigated, including the determination of drying rate for the coatings after immersion in the test solution, and the solution chemistry of ferric chloride. A chemically stable alternative solution was proposed and tested for use in the accelerated corrosion test and as a replacement for the inherently unstable colloidal ferric chloride solution. In all these steps, the electrochemical impedance spectroscopy (EIS) method was investigated as an alternative analytical technique for the direct current resistance method (Table 1).