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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68 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 BLOG Continued from The MP Blog, p. 13. The following items relate to materials selection & design. Please be advised that the items are not peer-reviewed, and opinions and suggestions are entirely those of the in- quirers and respondents. NACE Interna- tional does not guarantee the accuracy of the technical solutions discussed. MP welcomes additional responses to these items. They may be edited for clarity. Corrosion rates of wrought iron Q: Does anyone have any f igures or estimates for the corrosion rates of wrought iron in atmospheric exposure but in a sea salt-spray environment in the following conditions? • In a crevice (a riveted joint between two wrought iron plates) • Under a damaged paint coating • On a bare corroded surface • On a clean unpainted surface This is a structure build in the mid-19th century with "genuine" wrought iron. We are trying to build up a model of its future deterioration rate after restora- tion. There are very few data on corrosion rates of wrought iron that I could f ind in the literature. We have a number of wrought iron piers in the United K ingdom along with a huge number of wrought iron railway bridges. I am trying to f ind out what data there are from the many investigations, repairs, and conservation projects that have been carried out on them. According to Frank Speller in Corrosion Causes and Prevention, an Engineering Problem, wrought iron corrodes at a rate of 2.57 mils per two years in seawater. The rate of penetration in mild steel is 3.58 mpy in the same environment, more than two times the rate of wrought iron. I am not surprised, because wrought iron is very low in carbon, lacking pearlite, and almost all pure ferrite. This minimizes the effect of galvanic action of local cells. A: A paper by G.O. Lloyd entitled "Atmospheric Corrosion" gives the following information. Tese rates are for ingot iron at Lagos, where there is a tropical surf beach. Approximate Distance from Surf Salt Content of Air (mg NaCl/dm 2 ) Rate of Rusting (mm/y) 50 m 11.1 0.95 200 m 3.1 0.38 400 m 0.8 0.055 1,300 m 0.2 0.04 25 mi (40 km) — 0.048 Average rates in the United K ingdom vary between 0.048 mm/y at rural sites, 0.079 mm/y at marine sites, and up to 0.170 mm/y in some industrial atmospheres. In the driest atmosphere in K hartoum, the rate can be as low as 0.003 mm/y. A: I can't ofer an exact number for the corrosion rate, but do ofer the following case history. About 10 years back, I had the opportunity to conduct a cursory inspection of a wrought iron pier. It was a riveted construction and the harbor authority was investigating the requirements to extend its life by another 50 years. Te clue to the real age came from the original drawings retrieved from the archives. Tey were absolutely beauti- ful and incredibly neat. Te giveaway was the date, simply refected as '97—clearly this was 1897 and not 1997. Extensive use was made of riveted back-to-back angles. If I recall correctly, even the elements we would consider "I" or "H" beams today were fabricated from riveted plate and angles. The inspection was to enable us to prepare a budget for the investigation and we only had access to the shore edge of the jetty from the rock embankment. This was the tidal zone and the seawater temperature varied between 10 and 15 °C. From this limited access, we could deter- mine that all of the rivets appeared to be in place (or rather did not observe any that were missing). The structure was still functioning as a commercial f ishing harbor pier pretty much the same as my

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