Materials Performance

DEC 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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96 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 CORROSION BASICS Understanding the basic principles and causes of corrosion C ertainly very few vehicles are exposed to as wide a variety of environments in so short a time as the sleigh used by Santa Claus. It is rumored that many, even among the sophisticated NACE International membership, do not believe in Santa Claus for one reason or another. While that issue may be open to debate, there can be no doubt that corrosion mechanisms are real and must affect even the most hypotheti- cal among us. This is especially critical because the designers of the sleigh gave little thought to corrosion issues, as many designers are wont to do. Elf participation in NACE technical committees has been disappointingly low for many years. Most of the sleigh's components were selected for decorative purposes—a bit and simple stress corrosion associated with the incredible amount of cargo, not to mention Santa himself. Cyclic thermal stresses from changes in altitude and the numerous stops in the tropical, late- December temperatures of the southern hemisphere combine with the carbon dioxide from chimneys to produce sus- ceptibility to near-neutral stress corrosion cracking. The elves who help load the presents they've fashioned are always spilling hot chocolate on everything, and that's a sur- prisingly aggressive environment. Another situation that is unique to Santa's sleigh is related to its unconventional mode of locomotion: reindeer "exhaust" is loaded with microbiological grinches just waiting for the chance to spoil a good time. There is some advantage in the completion of the deliveries in a single night: most corrosion mechanisms are time-dependent and the acceleration of atmospheric corrosion by exposure to so many atmospheres in rapid succession is surely a double-edged sword (by the way, the double-edged sword is the third- most requested gift among fans of Conan the Barbarian). Still, Santa looks hope- fully under his tree each year for a neatly wrapped education in corrosion control (if only it were that simple). Even the auto- motive cathodic protection system Mrs. Claus gave Santa last year proved to be ineffective. Perhaps if he had parked the sleigh in a swimming pool the other 364 days of the year, there might have been a path to distribute protective current; but liquid fresh water isn't very plentiful at the North Pole. Season's Greetings (or don't you believe in seasons either?). strange considering the fact that avoiding observation appears to be a critical part of the operation—that includes a shiny red paint that provides little in the way of cor- rosion protection. The moisture content of the many clouds along the flight paths provides a source of condensate that can support all manner of corrosion reactions. Chief among these are the dissimilar metal cells that result from the connection of the rugged stainless steel runners to the lightweight aluminum superstructure (and between that superstructure and whatever the heck tinsel is made of ). Fortunately, deicing salts have limited impact on the sleigh's runners because most landings are restricted to rooftops; however, fly-ash from chimneys presents a formidable problem. The nut-and-bolt fasteners are subject to crevice corrosion Vehicular Corrosion This month, MP Technical Editor Norm Moriber provides a holiday-themed "Corrosion Basics"

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