Correlation of outgassing of stainless steel and aluminum with various surface treatments

A number of important applications of ultrahigh vacuum technology involve the use of vacuum systems which cannot be baked in situ, e.g., materials processing equipment and various designs of accelerators and fusion devices. A choice of vessel material and surface treatment which minimizes outgassing in unbaked systems has obvious advantages. We report on a systematic comparison of outgassing measurements from type 304 stainless steels and type 6061/63 aluminum alloys after various standard and advanced surface treatments. Correlations were made with high-resolution secondary ion mass spectroscopy and x-ray photoelectron spectroscopy profiles of the passivation oxide layer and profilometry measurements of the surface roughness. The observed outgassing rates Q obeyed a power law Q=Q0t−n where n varied from 0.94 to 1.27. Over the standard observation period (50 h), the outgassing was dominated (≳85%) by H2O. The absolute outgassing rates varied by only a factor of 2.8 for the stainless steels and by a factor of 1.4 for the aluminum alloys, even though the surface roughness differed by more than two orders of magnitude. The high-value-added surface treatments (i.e., electropolishing) did not have significantly lower outgassing in comparison to simple detergent washes.