The Effect of Runway De-icing Chemicals on the Corrosion of Cadmium Coating

Abstract

De-icing chemicals are spread on the runways to prevent the formation of ice and snow. Formerly used urea, which causes environmental concerns, has been replaced by mainly potassium formate and potassium acetate based de-icing chemicals. Also betaine based products, developed in Finland, are used as de-icers. The introduction of these new alkali metal based de-icers was followed by an increase in the occurrence of corrosion in different parts of aircrafts, such as cadmium-plated steel parts. The aim of this thesis was to study the corrosive effect of several new de-icing chemicals on cadmium coat-ings and to compare results obtained using various test methods. This thesis consists of theoretical and experimental parts. In the literature review, the corrosion mechanisms of cadmium are presented, the de-icing chemicals as a corrosive environment are evaluated, and different corrosion tests for examining the in-fluence of de-icing chemicals on cadmium coatings are described. In the experimental part, the results from different corrosion tests are compared and the corrosive effect of de-icers is researched. Also the effect of de-icing chemical concentration and the cad-mium coating quality on corrosion are examined. The chemicals used in the tests were commercial potassium formate and acetate, pure potassium formate and acetate as well as betaine based commercial de-icing chemical. In this thesis, four different test pro-grams were used, from which two are immersion tests and two cyclic exposure tests, in which the specimens are in turns immersed in the de-icing chemical and in turns in en-vironmental exposure in a heating chamber. The specimens contained four different types of cadmium coatings deposited in three different coating plants. The corrosive ef-fect of each de-icer on cadmium coatings were evaluated after the corrosion tests by weight change measurements and scanning electron microscope. According to the results, potassium formates were more corrosive than potassium acetates based on the higher measured weight losses. Betaine based Betafrost B did not cause significant weight changes, thus being the least corrosive of the studied chemicals. In the one month long cyclic exposure test, Boeing 2004, corrosion inhibitors included in commercial potassium formate and commercial potassium acetate decreased the corrosion rate of cadmium, whereas according to the two weeks long cyclic exposure test, AMS G-12 (2009), the corrosion inhibitors did not decrease the corrosion rate. Undiluted chemicals caused significantly more corrosion on the cadmium coatings than chemicals diluted to 50 % and 25 % by their weight. According to the one month long cyclic exposure test, dull finished specimens lost less weight than bright finished, but otherwise the quality of the cadmium coatings did not influence substantially the results. Immersion tests did not produce very reliable results because of the large devia-tions caused by the small weight changes. One day long standardized immersion test ASTM F1111 was insufficient for evaluating the corrosive effect of de-icers on cad-mium coatings, and there is a need to develop a new better standard test procedure. Test results were very sensitive to the variation of test parameters, for example temperature and humidity, and therefore the test parameters should be defined precisely when devel-oping a new test method. /Kir10