Assessment of Microbiologically Influenced Corrosion Potential in the International Space Station Internal Active Thermal Control System Heat Exchanger Materials: A 6-Month Study

The fluid in the Internal Active Thermal Control System (IATCS) of the International Space Station (ISS) is water based. The fluid in the ISS Laboratory Module and Node 1 initially contained a mix of water, phosphate (corrosion control), borate (pH buffer), and silver sulfate (Ag₂SO₄) (microbial control) at a pH of 9.5±0.5. Over time, the chemistry of the fluid changed. Fluid changes included a pH drop from 9.5 to 8.3 due to diffusion of carbon dioxide (CO₂) through Teflon^® (DuPont) hoses, increases in dissolved nickel (Ni) levels, deposition of silver (Ag) to metal surfaces, and precipitation of the phosphate (PO₄) as nickel phosphate (NiPO₄). The drop in pH and unavailability of a antimicrobial has provided an environment conducive to microbial growth. Microbial levels in the fluid have increased from <10 colony-forming units (CFUs)/100 mL to 10⁶ CFUs/100 mL.

The heat exchangers in the IATCS loops are considered the weakest point in the loop because of the material thickness (≈7 mil). It is made of a Ni-based braze filler/CRES 347. Results of a preliminary test performed at Hamilton Sundstrand indicated the possibility of pitting on this material at locations where Ag deposits were found. Later tests confirmed that corrosion of the heat exchanger material is a concern for this system. Accumulation of microorganisms on surfaces (biofilm) can also result in material degradation. This is known as microbiologically influenced corrosion (MIC). MIC can amplify the material damage caused by the exposure to the fluid.

This paper will discuss the results of a 6-month test performed to characterize and quantify the damage from microbial accumulation on the surface of the ISS IATCS heat exchanger materials. The test was designed to quantify the damage to the materials under worst-case conditions with and without microorganisms present at pH 8.3 and 9.5.