Identification of commercially available alloys for corrosion-resistant metallic reinforcement and test methods for evaluating corrosion-resistant reinforcement

A literature review was conducted with the goal of identifying alternative low-cost corrosion-resistant steel reinforcement materials. The most promising alternate reinforcing materials seen to date that are less expensive than 300 series stainless steels include low-nickel austenitic stainless steels and a variety of ferritic or martensitic 12-15 weight percent chromium steels. Steels with 2.5-10 weight percent chromium may also be of interest because they offer a marginal gain in corrosion performance at a very low cost. Several steel types that should undergo further evaluation are 201LN, 216, Duracorr, Enduramet 32 and Enduramet 33, HSS2, Lapealloy, S41425, S41426, and S42300. Corrosion-resistant steels are alloyed to ensure the steel itself has sufficient corrosion protection qualities; therefore, it is sensitive to cost fluctuations in raw materials. Based on the last 7 years, bars with higher nickel and molybdenum contents are sensitive to the cost of these alloying elements, whereas bars with higher chromium contents have been only slightly sensitive to the raw material cost. The cost of alloying materials also reflects the cost of different types of stainless steels. Both martensitic and ferritic stainless steels demonstrated slight increases in the average surcharge over a 7-year period, whereas austenitic, duplex, and precipitation hardening stainless steels increased dramatically. The most promising test for determining chloride threshold (initiation) in the laboratory is the +100 mV vs. SCE (or +200 mV vs. SCE) potentiostatic hold. The Cl- threshold can be established for the new rebar materials by conducting potentiostatic holds at +100 mV vs. SCE at various fixed Cl- levels. This method can also be extended to mortar-covered bars immersed in a simulated pore water solution with a thin mortar layer thickness. Propagation tests can also be conducted by conducting either potentiostatic holds at selected potentials or galvanic coupling in a split cell. A propagation law and repassivation potential (i.e., a "no propagation threshold" threshold potential) can be established. Concerning field testing, the ASTM G109 method is recommended primarily for comparison to existing research data. This test can be used to assess Cl- thresholds either by varying Cl- levels in the mortar mix or core drilling/sampling. Initial recording of galvanic current indicates initiation, whereas spalling provides an engineering indication of propagation. The Florida Department of Transportation's tombstone method should also be considered as a variation of the ASTM G109 method in high-permeable/low-permeable concrete mixes in order to test candidate rebar in concrete. ASTM G109 and Florida Department of Transportation tombstone concrete specimens can be artificially cracked to accelerate the onset of corrosion. Finally, the mechanical properties for each steel will need to be determined. Data will need to be gathered on specimens that have been rolled to the final reinforcing steel dimensions, although some of the bars identified could potentially function in the same capacity as the MMFX-2. However, additional research is required for the higher strength steels for structurally critical areas.