Last year I cleaned the innards of my W126 (1982 300SD) fuel sender unit. It worked fine for a while, then started sticking at 1/4 tank again. Lo and behold, there's enough new crud on the center rod to block the float, even though the rest of the device is still clean.

The center rod is stainless steel (I think). When I scraped the crud last year and polished the rod with emery cloth, there were some discolored and pitted areas. It appears those areas have generated new crud consisting mainly of rust (soaked with diesel, of course). And I thought stainless isn't supposed to rust.

A little online research yielded some information about "passivation" of stainless steel. Apparently, if stainless steel is physically abraded, the protective surface layer of chromium or nickel oxides is removed, exposing iron which can then rust. The problem can be alleviated by "passivating" the metal with an acid such as citric acid.

Any metallurgists on this board who can corroborate this and provide further guidance? I don't want to clean the rod again, only to have it rust and block the float again in a few months.

Thanks.

pjc