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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11 NACE INTERNATIONAL: VOL. 53, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2014 Continued on page 12 Dark scale on Al-brass tubes Q: We fabricate heat exchangers with A l-brass tubes. Recently, we unpacked some A l-brass tube bundles, which had been stored outside without extra protection, and found ~40% of the tubes were discolored. The color ranged from dark greenish to dark brown at the tube surface, and were mostly dark brown on the inside. Most of the tubes were dripping with water. During the three months of outside storage, we had heav y rains that f looded the tube bundles. The tubing was shipped with a waterproof package but it seems it was not good enough. A: Dark brown flms on A l-brass are not unusual, and most probably are caused by cupric oxide (CuO). I suspect that if the tubes had been placed in service they would have lightened up if the velocities were in the 5 to 10 ft/s (1.5 to 3 m/s) range. The greenish f ilms probably contain some basic copper carbonates (malachite or azurite, depending on the chemistry of the water they were exposed to) or basic copper chloride. Rusting of stainless steel welds in seawater Q: I am working in a shipyard as a mechanical engineer. Is there a specif ic welding method to prevent stain- less steel (SS) welds from rusting in seawater? The previous boat went out only once, and some of the welded parts already rusted. Can anyone give me some professional advice on corrosion resis- tance? I think the two base metals being joined are Ty pe 316L SS (UNS S31603), but is this material good enough in harsh environments? I found that most of the rusting is occurring at the welded joint. Is it because of the f iller material or the polishing method? I have heard that pick ling and passiv- ation can prevent the welded joints from rusting, but I am not familiar with these methods. A: Ty pe 316L SS is extremely marginal for use in seawater. You can have general corrosion, weld corro- sion, and crevice corrosion, as well as other ty pes. A higher-alloy SS will perform better. While Ty pe 316L SS is frequently referred to as a "marine" grade SS, it is the very minimum cost grade that has any hope of performing in seawater. In previous times, the alloy contents were well within the respective allowable ranges, but with modern steelmaking, the alloy content is aimed toward the bottom end of the various ranges for reduced cost. What may have worked once may not work now. However, Ty pe 316L SS is so marginal that every thing has to be in your favor, including design detailing, for it to work.

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