Laboratory methodologies for propellant corrosion research

Storable liquid propellants are stored for extended periods of time in metal tankage prior to usage in rocket engines. Knowing the chemical interaction of the propellant and the tankage material is essential to evaluating the structural integrity of the tankage in service and determining if the propellant remains within specifications at the time of use. Some of this information has been obtained through long duration storage studies for periods of over 20 years in some cases. It is desirable to establish valid methods to obtain quantitative data to project long-term corrosion rates in lieu of real-time storage experimentation. Experimental methods and techniques currently used in obtaining such corrosion data and their theoretical basis are described in this article. These include 1) electrochemical: dc polarization and ac impedance measurements; 2) weight loss; and 3) surface analytical: x-ray photoelectron spectroscopy, auger electron spectroscopy, and optical microscopy. This article presents a description of the fundamental methods used by two research organizations and a comparison of these methods and equipment. These techniques are valid for evaluation of corrosion rates on various fuel and oxidizer propellants. The results of specific research with nitric acid based oxidizers with various aluminum alloys are presented in a companion article.