Materials Performance

NOV 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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61 NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2014 Corrosion on the New Eastern Span of the San Francisco-Oakland Bay Bridge 3.5-in (89-mm), and 4-in diameter Grade BD rods on the SAS span displayed M-shaped hardness traverse profiles (Figure 5[a]). Cal- trans data showed peak hardness as high as 43 HRC, typically at 0.50 to 0.75 in (13 to 19 mm) from the surface, whereas the sur- face hardness was 34 to 38 HRC. This could occur if the tempering temperature did not soak through the rod cross sections. Also, for ASTM A354 rods, the threads may be cut or rolled either before or after heat treatment. All of the 4-in diameter tower saddle tie rods, 80% of the 3.5-in diameter PWS anchor rods, and some other Grade BD rods on the SAS span were cold rolled to form the threads after hardening and tempering. The cold-work effect of thread rolling would increase the surface hardness, par- ticularly at the thread roots, as illustrated by the dashed lines in Figure 5(b), with little effect on the hardness at mid-radius. The surface hardness at the roots of roll-formed threads could have increased to 40 HRC or even to 43 HRC on some ASTM A354 Grade BD rods. The surface layer with a cold- worked structure and high hardness may have increased the EHE susceptibility of many Grade BD rods at critical locations shown in Figure 2. Thread rolling after heat treatment would leave residual compressive surface stresses. Kephart showed that this would increase the resistance of low-alloy steels against SCC in a boiling ammonium nitrate (NH 4 NO 3 ) (BAN) solution . 10 He warned , however, that the BAN solution is so aggres- siv e th at hi s S C C d at a mi g ht n o t b e extended directly to other environments such as EHE in atmospheric condensates. EHE fai lures of hig h-stren g th st eel in marine air are sensitive to surface hardness whereas SCC is sensitive to specific corro- dents and less so to material hardness. HDG at ~840 °F (449 °C) could decrease the beneficial residual stresses from thread rolling. Whether the increased resistance of rolled threads against SCC in BAN can equate to an increased resistance against EHE in atmospheric condensates is still unknown without verification test data. 11 The HDG ASTM 354 Grade BD rods with rolled threads on the SAS span might be susceptible to EHE failures until proven otherwise by laboratory data, field experi- ence, or both. 150-Year Bridge Life Steel bridges typically have a 75-year design life. 12-13 Caltrans and its contractors claim a 150-year life either as a "design or service life" for the new eastern span. 14 No other steel bridges over seawater have a design life greater than 100 years in the United States. To achieve a 150-year life for the SAS span, critical components must include design and fabrication to prevent any pre- mature failures due to fatigue cracking and corrosion deterioration. Materials selec- tion would need to prevent any metallurgi- cal conditions that could lead to premature failures such as EHE cracking, SCC, or both. The PWS main cable for the SAS span cannot be replaced without first building a temporary bridge to support the steel OBG road decks. At six locations, such as the splay chambers and deviation saddles, the main cable wires (only 175 to 525 ft [53 to 160 m] above seawater) have no protection from marine atmospheric corrosion other than the initial zinc coating that is only 1.7 mils thick and dehumidifiers. These steel wires cannot be repaired if damaged by corrosion pits or even EHE cracking. Simi- larly, the HDG ASTM 354 Grade BD anchor rods for the tower base are not repairable should th ey exp erience EHE cracking. These 3- and 4-in diameter anchor rods were installed in 2008, only a few feet from seawater. They have shown red rust since around 2012. As of May 2014, the dehumid- ifiers that had been planned for the tower bottom had not yet been installed. The PWS assemblies were produced in China, shipped across the Pacific Ocean, and installed on the SAS span in 2012; and all the while were exposed to diurnal con- densates of the marine atmosphere, which FIGURE 5 (a) 3-in diameter tower base anchor rods (ASTM A354 Grade BD, cut threads, HDG). (b) 4-in diameter tower saddle tie rods (Grade BD, rolled threads, HDG); 3- and 4-in diameter Grade BD rods with M-shaped hardness traverse profles. (a) (b)

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