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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12 NOVEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 11 Continued f rom page 11 THE BLOG A: Crevice corrosion occurs when SS is exposed to a narrow crevice rather than bulk seawater or the atmosphere. Te crevice creates condi- tions that destroy the corrosion resis- tance. Te higher the chromium and molybdenum contents, the better, but relying upon alloy content alone can be an expensive way to avoid the problem. It is better to design out the crevice if possible, or to consider a sealant in the crevice. SS welds form a porous oxide layer during the welding process, which prevents the stainless from passivating. This passivation process is what gives SS their corrosion resistance—it is an extremely thin layer of chromium and iron oxides. The porous oxide layer should be removed. Small items might be immersed in a pick ling solution, and large items might have a pick ling paste applied. You are not alone in being confused about pick ling and passivation. My understanding is that pick ling removes the porous oxide from the weld. When subsequently exposed to the atmosphere, it will passivate by itself as the chromium reacts with oxygen in the atmosphere. The passivation treatments encourage this process using an oxidizing solution of chemicals for a more reliable result. I would try pick ling alone f irst. There is an enormous amount of liter- ature available for free on the Internet from various national nickel and SS devel- opment associations, primarily the Nickel Institute in the United States (w w w.nickelinstitute.org). Test for stress corrosion cracking Q: Can someone please tell me how magnetic particle inspection works? I recently worked with another natural gas pipeline company on a line crossing where they had low potentials. They informed me that if they had found any disbonded coating, they would do such a test for stress corrosion cracking. A: A simple explanation of magnetic particle inspection is as follows: the technician sets up a magnetic feld in the part, then sprink les the surface with ferro-magnetic particles. At every discon- tinuity on the surface, the magnetic feld will result in north and south poles that will collect the particles, highlighting the discontinuities for further evaluation. There are numerous variations on this, but the basic theory is the same. Some people set up in the f ield with a "yoke" while others use "prods" or "f ield coils." Some tests are done with dry powder while others use wet f luorescent particles. Wet f luorescent magnetic particle inspection uses an ultraviolet light to illuminate the particles and is usually considered superior to dry powders in its ability to easily f ind f laws.

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