Federal agency urges Minnesota to re-examine bridges for deadly design flaw.

2. Federal agency urges Minnesota to re-examine bridges for deadly design flaw. In continuing coverage from yesterday's briefing, NBC Nightly News (1/15, story 5, Williams) reported that the National Transportation Safety Board (NTSB) "today said a design mistake involving 16 separate plates from the center span was what they called 'the critical factor' in" the deadly Minneapolis, Minnesota bridge collapse last year. CBS Evening News (1/15, story 7, Couric) pointed out that federal investigators described steel connectors as "simply too thin." The New York Times (1/16, A17, Wald, Davey) adds, "The Federal Highway Administration moved quickly Tuesday to tell state authorities to re-examine bridges to make sure they do not have the newly discovered design flaw that doomed the Interstate 35W bridge." Revealing "its findings in the bridge collapse," the NTSB also "recommended an analysis of the flawed parts, known as gusset plates, every time major work was done on a bridge. There are about 12,600 bridges nationwide that could be vulnerable to the kind of design error found in Minneapolis." The AP (1/16, Frommer) explains that, although the NTSB is saying that "a design error led to the collapse" of the Minneapolis bridge and that "undersized gusset plates are used in the interstate bridge," the safety board "is stopping short of saying the gusset plates caused the collapse." NTSB chairman Mark Rosenker said that "the plates, which connected steel beams, were roughly half the thickness they should have been" and that "investigators found 16 fractured gusset plates from the bridge's center span." However, Rosenker "says the agency's investigation found no evidence that cracking, corrosion or other wear 'played any role in the collapse of the bridge.'" Investigators "also found no flaws in the steel and concrete material used in the bridge."

3. Evaluation of Corrosion Chapter 1

6. Corrosion of Zinc Plated Screws in Sulfuric Acid

7. Definitions Anode Cathode Point where reduction occurs Atoms gain electrons and become reduced Reduced atoms may “plate” out on the cathode • Point where oxidation occurs • Atoms lose electrons and become oxidized • Oxidized atoms can become soluble in the electrolyte

9. Reactions at Anode • Metal is oxdizedM Mn+ + ne- • Fe Fe2+ + 2e- • Al Al3+ + 3e- • Cu Cu2+ + 2e-

10. Reactions at Cathode • Deaerated Water (No Oxygen)2H+ + 2e-H2 • Aerated Water (Oxygen Present)O2 + 4H+ + 4e- 2H2O (Acidic)O2 + 2H2O + 4e-  4OH- (Neutral/Basic) • Dissolved IronFe3+ + e-  Fe2+

12. Corrosion Rate Measurement

13. Corrosion Rate Experiment • If Samples 1 and 4 have the same mass loss, then the corrosiveness of of the solution is unchanged • If Sample 4 has a lower mass loss, then the corrosiveness decreased • If Sample 4 has a higher mass loss, then the corrosiveness increased • If Samples 3 and has the same mass loss as Samples 1 and 2 combined, then the corrodibility does not change • If Sample 3 has a lower mass loss, then the corrodibility decreased • If Sample 3 has a higher mass loss, then the corrodibility increased

14. In-Class ProblemCorrosion of Carbon Steel • 2” x 3” x 0.123” with 0.25” hole • Sample 1: 12 hours, 25 mg loss • Sample 2: 108 hours, 130 mg loss • Sample 3: 120 hours, 150 mg loss • Sample 4: 12 hours, 15 mg loss (inserted when Sample 2 was removed) Calculate the corrosion rates for each sample and determine if there are changes in corrodibility or corrosiveness. D = 7.9 g/cm3

15. Homework #1 Due 1/25 (Friday) • Read Chapter 1 of textbook • Answer Problems 1-5 and 1-7 at end of chapter • Use the internet to find how corrosion contributed to an accident. Prepare a brief (1 page) summary of the type of corrosion (crevice, stress corrosion cracking, etc) and how it precipitated the accident.