Secondary gettering

Addition of Cr to Fe-Al alloys aids the formation of Cr sulphides and AI2S3 which together markedly reduce the sulphidation rate". Between 2% and 5% Cr then, more than 3% Al is required to obtain protection. At 9% Cr, however, only 1 % Al is needed to give protection since the Cr is sufficiently active to lower the S potential seen by Al (secondary gettering). Thus... 

Milton and Hutton [21] evaluated aluminum, copper, silver, indium, lead, and tantalum as possible secondary cathode materials. The first three candidates (Al, Cu, and Ag) sputtered at rates too high to allow production of ions characteristic of the glass sample (i.e., tended to produce too thick a metallic layer). Indium and lead are soft materials that lead to the overcompression of the insulator-cathode-sample sandwich, consistently resulting in electrical short-circuiting between the anode and cathode. Finally, tantalum does indeed exhibit the desirable characteristics for application as secondary cathode materials. Although not explicitly required, the fact that Ta is a getter element is likely to provide some added benefits as well. 

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