Improved Corrosion-Resistant Steel for Highway Bridge Construction

CHAPTER 8. CONCLUSIONS

• Of the high-Cr alloy steels studied in this investigation, only ASTM A1010 had the strength and toughness required to meet modern bridge steel design specifications.

• The corrosion behavior of the experimental steels in CCT is not a function of the steel YS.

• The laboratory CCTs demonstrated that reducing the Cr content from 11 percent typical of ASTM A1010 caused a significant reduction in corrosion resistance.

• As the Cr content of the experimental steels decreased from 11 to 5 percent, the corrosion rate (thickness loss) steadily increased.

• The effect of adding 2 percent Si to the 9 and 7 percent Cr steels was significantly detrimental to corrosion resistance.

• Substituting 2 percent Al for 2 percent Si in the 7 percent Cr steel had a positive effect on the corrosion rate.

• Under the conditions of the 5 and 3 percent NaCl CCTs in the laboratory, the corrosion rate of the ASTM A1010 steel was one-tenth to one-fifteenth that of ASTM A588.

• All the steels exhibited a relatively linear rate of thickness loss in the CCT, indicating that the corrosion products formed were not significantly protecting against continued corrosion.

• The corrosion products on the 11 percent Cr steels consisted of significantly less maghemite than the steels with lower Cr content. This is the apparent reason for the significantly better corrosion rates observed for the 11 percent Cr steels.

• The overall amount of goethite and lepidocrocite was equal for all the steels, so these corrosion products were not responsible for the significant difference in the CCT corrosion rates among the various steels.

• Atmospheric corrosion tests conducted on the Moore Drive Bridge with lower-Cr experimental steels containing 9 and 7 percent Cr exhibited between one-third and one-half the corrosion loss of ASTM A588 weathering steel. ASTM A1010 steel exposed on the Moore Drive Bridge had one-fourth the corrosion loss of ASTM A588, confirming that it was the most corrosion-resistant steel studied.

• After field exposure, the rust composition was similar for all three experimental steels. Akaganeite was the most abundant oxide and comprised about 70 percent of the rust. Goethite made up about 24 percent of the rust, and lepidocrocite made up about 6 percent. Notably absent was maghemite.

• The CCT protocol based on the SAE J2334 test is fundamentally different that actual field environments for bridges exposed to deicing salts because the high time-of-wetness of the CCT promotes the formation of maghemite.

• A 125-year LCC study of a model bridge girder made from ASTM A1010 steel versus a girder made from painted carbon steel showed ASTM A1010 to be the lower LCC material under all assumed conditions. After 15 years in service, the probability exceeds 50 percent that the ASTM A1010 steel is the lower cost material. After 40 years in service, it is certain that the ASTM A1010 steel is the lower cost material.