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18 APRIL 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIAL MATTERS Continued f rom page 17 cause of structural failure that puts air- craft and other equipment out of service. Because so many disparate materials are used, galvanic corrosion is the most seri- ous corrosion mechanism in aircraft (Naval aircraft in particular). Cracks commonly initiate from galvanically driven corrosion pits around fastener holes, and a U.S. Air Force study 1 con- cluded that 80% of structural failures originated from corrosion pits. Conse- quently, there is a strong drive to improve protection from galvanic corrosion. The proliferation of new materials, however, creates more opportunities for serious galvanic corrosion and stress cor- rosion cracking (SCC) issues, say NACE International members A lan Rose, CEO, and Keith Legg, chief technolog y off icer, with Corrdesa. For example, they note, the push to reduce the weight of aircraft components is leading to more use of car- bon f iber composites with aluminum air- frames, which are not galvanically com- patible with each other. At the same time, environmental regulations are requiring that long-standing protective coatings containing cadmium and hexavalent chromium be replaced with alternative materials, which may not perform as well as their predecessors. According to Rose and Legg, the pri- mary military specif ications governing galvanic corrosion prevention in defense systems—MIL-STD-889 2 and MIL- DTL-14072 3 —as well as other galvanic design guides, are based on the galvanic potential difference (ΔE) between two materials in the galvanic series, and call for designs based on galvanic potential tables that draw from data that are half a century old. 4 Modern corrosion analysis, Rose says, has demonstrated that corrosion risk is determined by the corrosion current between two objects—which is a function of surface chemistry and electrochemical reactions that vary with surface chemis- try, treatment, and service environment— and not solely the difference between their galvanic potentials on the galvanic series table. The corrosion current can be calcu- lated using modern electrochemical data, and the corrosion rate—and therefore corrosion risk—can be determined from the corrosion current. "When we showed this approach in a meeting of aircraft maintainers at one of our Nav y depots, they immediately recognized it as a para- digm shift that could greatly reduce the time and money spent on corrosion repair," Rose comments. Currently, he says, U.S. Department of Defense acquisition requirements perpet- uate the galvanic corrosion problem because they require corrosion analysis based on the existing military specif ica- tions that are founded on galvanic poten- tial rather than galvanic current. Rose and Legg cite an example of a galvanic corrosion decision based on the materials' ΔE and how it affected the wings of F/A-18 Hornet f ighter jets that have seen a few years of service. When the aircraft come back for depot overhaul, almost every fastener hole showed corro- sion because of galvanic interaction between the fastener, the carbon f iber composite skin, and the aluminum air- frame. A depot repair, based on MIL- STD-889, consists of grinding away the corroded aluminum, drilling out the fas- tener hole, and inserting a stainless steel bushing. The next time the aircraft returns to the depot, however, corrosion around the bushing is even worse. So, again following MIL-STD-889 guidelines for repairs, the bushing is removed, the corrosion is ground out, and a new, larger bushing is inserted. Wherever a cathodic bushing is inserted into an anodic aluminum com- ponent, pitting corrosion occurs in the aluminum around the periphery of the bushing, no matter how well-protected it is. A computer-aided engineering (CA E) analysis of the wing area around the small bushing showed that the galvanic corro- sion rate of the aluminum is proportional to the area of the bushing. Since the bush- Whether Onshore or Offshore…. We're working to help you Protect Your Assets and Our Environment! Canadian Office: 780-447-1114 Email: sales@irtrectifier.com U.S. Office: 918-805-7587 Website: www.irtrectifier.com