Investigation of the Corrosion Propagation Characteristics of New Metallic Reinforcing Bars

About the report:

The threshold chloride concentrations for solid 316LN stainless steel, 316L stainless steel clad, 2101 LDX duplex stainless steel, MMFX-2 (Fe-9%Cr), and carbon steel (ASTM A615) rebars were investigated through laboratory tests in saturated Ca(OH)2 + NaCl solutions. The chloride threshold for carbon steel was found to be less than a Cl-/OH- molar ratio of 1, which was consistent with the literature. Solid 316LN stainless steel rebar in a pickled condition was found to have a much higher chloride threshold (i.e., threshold Cl-/OH- ratio > 20) than carbon steel (0.25 < Cl-/OH- < 0.34). Pickled 2101 LDX (UNS S32101) had a chloride threshold Cl-/OH- ratio of 9.7 and un-aged pickled MMFX-2 (Fe-9.3% Cr) had a chloride threshold Cl-/OH- ratio of 4.9. 316L stainless steel-clad rebar possessed a chloride threshold Cl-/OH- ratio of 4.9 with intact cladding. Surface preparation, duration of period exposed to a passivating condition in Ca(OH)2 solution prior to introduction of chloride, and presence of cladding defects all affected the threshold chloride concentration obtained. For instance, the presence of mill scale on any of the corrosion-resistant materials reduced the chloride threshold to approximately that of carbon steel. Therefore, pickling is highly recommended in any reinforcement substitute. The chloride threshold for 316L clad rebar was also highly dependent on any defects that exposed the carbon steel core. It was similar to solid stainless steel when intact and when defective, it was similar to that of carbon steel rebar. The model-predicted extension of time until corrosion initiation in concrete could extend to 100 years or more in a pickled condition by using rebar materials such as 316L or 316LN stainless steel with a higher corrosion resistance. Corrosion propagation studies indicated that while radial propagation might be similar on all materials once local corrosion was initiated, lateral spread of corrosion would be limited on clad and solid stainless steels. This finding has significant engineering ramifications as the depth of penetration of corrosion of stainless-steel reinforcement would in this case have to be far more extensive in order to damage overlying concrete by oxide wedging. Lastly, corrosion products were found to be either similar on all materials or, if different, to possess similar molar volumes alleviating concerns that oxide wedging could be worse on new candidate rebar materials. These findings indicate the total corrosion lifetime, given by the time until initiation and the time of propagation until concrete damage, can be improved to well over 100 years by using pickled stainless steel. The VTRC should investigate use of highly alloyed stainless steel in metropolitan applications where access for repair and maintenance is limited.

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