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Discussion Forums - The Hendrix Group
HomeHomeDiscussionsDiscussionsGeneral Corrosi...General Corrosi...Should I choose Stainless or hot dip galv. steel rods?Should I choose Stainless or hot dip galv. steel rods?
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8/2/2007 9:00 AM
 
[updated:LAST EDITED ON Aug-02-07 AT 06:28 PM (CDT)]I have a construction contractor who wants to substiture hot dip galvanized A36 steel plates and 1-1/2" A572 rods for SS plates and A572 stainless steel Grade 50 rods underground. These would be embedded in concrete (50' long) and used to tie back a 25' high section of currently slightly overturning concrete wall. The rods would connect to the concrete wall using a steel tie bracket connection (designed as stainless steel 1/2" bent plate which is anchored into the concrete wall using 1-1/4" SS adhesive anchor studs and proposed to be substituted with hot dip galv'd A36). This bracket will be exposed to soil 5' below grade in Washington, D.C. - lots of rain, damp soil conditions). The rods will run embedded in concrete for 50' horizontally until reaching the deadman (anchor). The very end of the rods (about 6") will be exposed to earth and be secured with plate washers. I would like to know how much faster would the hot dip galvanized rods corrode and become unreliable, as opposed to the stainless steel rods. I'm also looking at how long the galv'd plates would last versus the SS. I'm looking for ballpark ranges such as , "Corrosion and unreliability may occur after 10, 20, 50, 100 years, etc". Are there any differences in strength betw. A36 and SS?
 
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8/3/2007 9:00 AM
 
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.
 
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8/6/2007 9:00 AM
 
Dear Debasis Mitra, Thank you very much for your thorough and timely reply. I appreciate your recommendation to understand the soil chemistry. We have a geotechnical report, but it doesn't appear to address acidity or corrosion effects of soil on metals. We are taking this topic back to our structural engineers to clarify their intent on the SS vs. CS ( galv'd.) and the amount of corrosion resistance envisioned. It appears that what we may be looking for the superior grade SS with good PREN, as long as it comes with the 50,000 psi yield point. We do not see a specific SS specified in our drawings or specifications, so we are pushing our engineers to define that. Per your discussion, it appears very important to pick the right SS. I am unfamiliar with yield points of SS and whether or not galv'd CS elemnents would have different dimensions ( because of strength characteristics of the two types of metals). If we were to choose the hot-dip galv'd CS A572 rods and assoc'd. accessories all dipped after fabrication, would we risk a problem of the galv. coating being breached where the 50 foot rods (actually 2 - 25' rods with a coupler in the middle) thread through the steel plate at the one end or at the soil-exposed end with the plate washers? We will definitely use only one kind of metal or the other and not use dissimilar materials to avoid the galvanic action. I am not familiar with the "bacterial test" for the coal tar wrap / coat. You mentioned that should be "as per standard spec". Is it possible for you to cite a reference, please, if that wouldn't be too time consuming? Thank you, Bonita Mueller
 
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HomeHomeDiscussionsDiscussionsGeneral Corrosi...General Corrosi...Should I choose Stainless or hot dip galv. steel rods?Should I choose Stainless or hot dip galv. steel rods?


  

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