What is the effect of corrosion on the interior surface of a waveguide on insertion loss?

The effect of corrosion on the interior surface of a waveguide increases the insertion loss by degrading the electrical conductivity of the inner walls where the RF surface currents flow. In a waveguide operating in the TE10 mode: the RF current flows on the inner surfaces within a skin depth of the metal. The skin depth at 10 GHz in copper is approximately 0.66 micrometers. Any surface degradation within this thin layer directly increases the RF resistance and loss. Corrosion effects: oxide formation (copper oxide (CuO) forms on bare copper surfaces; the oxide has much lower conductivity than copper (approximately 100-1000× lower); even a thin oxide layer (1-10 micrometers) significantly increases the surface resistance; the waveguide insertion loss can increase by 50-200% (from 0.01 dB/m to 0.02-0.03 dB/m) with moderate oxidation), pitting corrosion (localized corrosion creates rough, pitted surfaces; the increased surface roughness scatters the surface currents, increasing the effective path length and resistance; surface roughness comparable to or larger than the skin depth (approximately 1 micrometer at 10 GHz) causes significant additional loss), galvanic corrosion (occurs when dissimilar metals are in contact in the presence of moisture; common in waveguide systems where aluminum waveguide meets brass flanges; the corrosion products (aluminum oxide, hydroxide) have very low conductivity), and moisture-induced corrosion (water inside the waveguide (from condensation or leaks) causes: surface corrosion (chemical reaction between water and the metal), dielectric loss from the water film itself (water has very high loss tangent at microwave frequencies: tan_delta approximately 0.15 at 10 GHz), and increased reflection (the corrosion products change the waveguide's dimensions and impedance)).