Dear Bonita Mueller , Hot Dip Galv CS and SS has its independent and individual mode of soil based corrrosion. Your issue could have been more clear if a pictorial presentation / Line diagram of the system can be put forth to visualise. Soil chemistry and pH at varied depth plays significant role in corrosion. This issue needs to be addressed on a long term life basis. So mere substitution of "galv. item by SS item " or " SS item by galv. item" should not be the short cut . But w.r.t. to prevailing soil chemistry the items are better to subject to at least "lab simulation test" by some good chemist who has the idea that after which duration of test the result are safe to extrapolate to a comfortable extent. In Galv. items the protection barrier depends on type, purity & nature of Zn Bath and galv. procedure and the depth of subsurface penetration of the layer. Whereas in SS the protection depend on surface passive layer which has active/passive transition behaviour. Now if U use concrete embedded 50" galv. rod having 6" deadend exposure to soil, fastened with with 1/2" SS bracket , w.r.t. acidic pH ( probable with Washington rain/other leak paths ) can cause a Galvanic End Corrosion and the Zinc end ( anodic) will corrode at a much faster rate which can have a detachment. In harsh u/g environment it is normally suggested that if U use any disimilar metal combination the maximum potential difference should be limited to 0.15V. But as the Anodic index of austenitic SS is of the order of 0.50V and that of Galv. CS is 1.20V and thus the difference is 0.70V which is much greater than the recommended one. As per this line your Contractor is in right track. But at the same time U have to consider the mechanical strength issues also where SS Rod is at the low side compared to CS. You hv also mentioned that ur 25' section is slightly overturned where SS bar will be subjected to Stress. And as the bars are fastened at end by anchors and bracket all the locked-up stresses will try to concentrate at the deadends. Again if the soil condition is acidic and under seepage stagnancy it can cause pitting and also stress corrosion cracking in case of SS. So in one side is it not adviced to use dissimilar metal combination and on the other side use of simple commercial quality SS may have some short/long term reliability problems. So first it is advisable to assess soil chemistry and sample soil test at 5' - 6' depth at random location at metal exposed portion and depnding on that under advice of a chemist's consultancty either use high Quality galv. CS in all items or use superior SS with good PREN ( pitting resistance equivalent No.) Value looking into long term aspect. For galv items as an additional protection U can also use coal tar based wrap/coat on exposed portion but the wrap/coat coal tar material has to pass "a bacterial test " as per standard spec. I am just giving my own view. Other's pl. comment/ critic / educate me. regards.