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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60 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIALS SELECTION & DESIGN above threshold levels. As for hydrogen, Johnson stated that Troiano "delineated sy st ematical ly, th e ph enom enolog y of hydrogen-induced delayed failure, thus bringing order into w hat had been an uncorrelated and somewhat chaotic series of practical and laboratory observations." 7 Environmental HE (EHE) is indicated if the hydrogen responsible for the cracking ent ered th e st eel during ser vice. Thi s hydrogen is usually generated as a corro- sion byproduct of the steel, zinc coating, or both. Internal HE (IHE) is indicated if the hydrogen that entered the steel during manufacturing, such as steelmaking and acid pickling, was responsible for the crack- ing. In the case of the S1 and S2 anchor rod failures on Pier E2, EHE is the only failure mechanism that can explain why all 32 fail- ures occurred exclusively in the bottom threads and not in the top threads. 8 Caltrans conducted a field hardness FIGURE 4 Effects of zinc coating on stress intensity factor KI EHE . and laboratory test program on over 1,300 ASTM A354 Grade BD rods that had been installed on the SAS span. This program included full-size (up to 4-in diameter) anchor rod EHE testing in a 3.5% sodium chloride (NaCl) solution, which was still in progress as of September 2014. The cost of the anchor rod test program was estimated at $20 million, including $800,000 to modify four EHE test rigs to test "four 2013 rods" (i.e., four new Grade BD rods produced in 2013). EHE traditionally has been categorized as stress corrosion cracking (SCC) because the role of hydrogen in the crack initiation and growth was not understood. Even now, the term SCC is used for IHE, EHE, or both in some literature. Since IHE occurs with- out corrosion of any kind, it is inappropri- ate to characterize or treat it as SCC. Hydrogen may or may not take part in SCC, whereas hydrogen must be present above a threshold concentration in both IHE and EHE. Unlike most SCC, both IHE and EHE do not involve active path corro- sion for crack initiation and growth. Also, several phenomenological differences exist between HE cracking and SCC. For exam- ple, HE cracking susceptibility peaks at room temperature, whereas SCC suscepti- bility increases with temperature. Further- more, while cathodic protection can sup- press SCC, it will promote HE cracking. Therefore, HE cracking should be differen- tiated from SCC. Accordingly, in Figure 4 by Townsend, 9 K IEHE represents KI SCC . It shows that a zinc coating increases the susceptibility of steel to EHE cracking. For example, at 36 Rock- well C Hardness (HRC), KI EHE of Alloy 4140 steel (UNS G41400) in a 3.5% NaCl aqueous solution decreased from 77 to 32 ksi√in (85 to 35 MPa√m) when zinc coated. Carbon and low-alloy steels without zinc coating can be susceptible to HE cracking at a sur- face hardness of 40 HRC or higher. When HDG or zinc electroplated, however, the same steel can be susceptible to EHE at 36 HRC. The 3-in diameter HDG ASTM A354 Grade BD anchor rods for S2 that failed on Pier E2 had 36 HRC near the surface. The 39 HRC maximum requirement at mid-radius might be inappropriate for Grade BD rods with a diameter of >2.5 in (64 mm). This is because the surface hardness could be 40 HRC or higher and could fail due to EHE during service, especially if the rods are HDG. On the other hand, ASTM A325 bolts (34 HRC max for 0.5 in [13 mm] ≤ diameter ≤ 1.0 in [25 mm] and 30 HRC max for 1.125 in [28 mm] ≤ diameter ≤ 1.5 in [38 mm]) are safe from EHE failures even when HDG. Grade BD Rods with Cold-Rolled Threads and M-shaped Hardness Traverse Profiles ASTM A354 requires that both Grades BC and BD be hardened and tempered at 800 °F (426 °C) minimum, with no require- ments for holding time at temperature. Nor- mally, low-alloy steels, like Alloy 4140, would show a hardness traverse cur ve across the diameter that is U- or V-shaped when hardened and tempered. Many 3-in,

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