B+-+corrosion+in+bridges

Q and A Wednesday——Chengcheng Xu October 6th, 2010 How do engineers slow down corrosion in bridges? Problem & statistic: ﹒   The U.S. Federal Highway Administration (FHWA) has rated almost 200,000 bridges or one of every three bridges in the   U.S.  , as structurally deficient or functionally obsolete. ﹒ More than one-fourth of all bridges are over 50 years old, the average design-life of a bridge. ﹒   The Strategic Highway Research Program (SHRP) of the U.S. National Research Council recently estimated that the cost of damage to   America   's bridges currently stands at about $20 billion and is increasing at the rate of $500 million per year. ﹒   SHRP concluded that the structural deterioration found in these bridges is primarily the result of corrosion. Corrosion causes: ﹒  Pitting, which is usually related to passive alloy, the local breakdown of a passive film and the presence of halide ions in solution, is caused by extremely acidic local environment within a pit. ﹒   Crevice corrosion, which occurs in restricted or occluded regions whose dimensions are typically less than 1 mm. Because of the low concentration of oxygen in the crevice,  cathodic oxygen reduction from the region adjacent to the crevice takes over, allowing the anodic current on the interior metal surface to continue, the metal in the crevice will be oxidized. ﹒  Corrosion can also be accelerated in situations where two dissimilar alloys are in electrical contact with each other and with the same solution. This form of corrosion is known as galvanic corrosion. Because the two alloys have different corrosion potentials, a voltage difference arises which drives the anodic reaction on one alloy and the cathodic reaction on the other. Solutions: ﹒ Designing the structure with corrosion prevention in mind and selecting corrosion-resistant materials when constructing and refurbishing bridges; ﹒ The application of protective coatings, membranes, and protective sealers to the bridge which can serve as a barriers to corrosive environments; ﹒   The use of direct electrical current and sacrificial materials to mitigate corrosion on reinforced concrete and bridge decks, a process known as "cathodic protection". Cathodic protection is the application of a cathodic current to a structure that prevents it from sustaining an anodic reaction. This can be achieved either by coupling the metal to be protected with a metal having a more negative open-circuit potential (sacrificial anode, such as magnesium) or by applying a cathodic current through an auxiliary electrode (impressed current). ﹒   Electrochemical chloride extraction. ECE treatment of reinforced concrete structures general] y takes 4 to 10 weeks until sufficient chlorides are removed from the structure. After treatment, concrete structures are left in a noncorroding (passive) state. Reference: ﹒  Whitmore, D.(1996). Electrochemical Chloride Extraction From Concrete Bridge Elements: Some Case Studies. NACE International. Retrieved October 11, 2010 ([]) ﹒  David A. Shores, R. M. Latanision & Peter C. Searson (2008). "Corrosion," in AccessScience, ©McGraw-Hill Companies. Retrieved October 11, 2010 ([]) ﹒White papers: Bridge Corrosion. NACE International. (  http://www.nace.org/content.cfm?parentid=1046&currentID=1421)