Graduation Year

2011

Document Type

Thesis

Degree

M.S.M.E.

Degree Granting Department

Mechanical Engineering

Major Professor

Alberto A. Sagues

Keywords

2205, 316L, Magnesium Chloride, Relative Humidity, XM 29

Abstract

Prestressed concrete piles are commonly used to support over-water highway bridges in marine environments. The reinforcing steel within will ultimately be degraded via corrosion damage due to the penetration of chloride ions from sea water. The service life of these structures is, in part, dictated by the time required to diffuse chloride ions through the concrete cover and subsequently corrode the steel. Therefore, by slowing the rate of diffusion or increasing the chloride threshold of the steel (or both) an increased service life can be expected. This thesis focuses on the latter whereby stainless steel reinforcing alternatives were investigated to elevate the chloride threshold before corrosion begins.

The designation "stainless" steel implies corrosion resistance. However, corrosion resistance in itself is not a sufficient condition to make it a suitable alternative for prestressed concrete applications. In this study, the corrosion susceptibility of stainless steel alloys was scrutinized with the understanding that high strength stainless steels are vulnerable to stress corrosion cracking (SCC). This investigation screened three candidate alloys that span the norms of stainless steel compositions: a common austenitic stainless steel with high nickel content (316L), a less common austenitic stainless steel with low nickel but high manganese (XM 29), and a duplex stainless steel with high chromium and an additional constituent, molybdenum (2205). Each alloy was subjected to two stress conditions imposed by varied mechanical fixtures then subjected to various forms of high chloride concentrations. The pH of these conditions was also varied and in one case simulated the high pH common to concrete pore water solutions. Elevated temperatures were used to accelerate the effects of these exposures.

Results of Phase 1 showed that for exposure at 135oC (275oF) cracking of alloys 316 L and 2205 occurred after 1 hour while XM29 experience cracking after 24 hours. At 90oC (194oF) alloy 316L cracked after 4 hours; XM29 did not crack after 96 hours while 2205 did crack after 96 hours. The results were interpreted with an Arrhenius relationship between time to cracking and test temperature to extrapolate toward the anticipated service regime. Results of Phase 2 showed that SCC was less likely to initiate in high pH conditions than in low pH conditions at typical marine environment temperatures and chloride concentration. In these limited tests the SCC performance of XM29 was better relative to that of the other two alloys.

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