Materials Performance

MAY 2015

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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Page 48 of 100

46 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5 COATINGS & LININGS ESSENTIALS Continued on page 49 Innovative Slurries Developed for Steel Aluminization Steam pipes and superheater tubes in power generation boilers are generally fabricated of iron-based alloy components. A common technique to reduce oxidation and corrosion of Fe, Ni, and Co-based alloy components is aluminum surface enrich- ment by aluminizing. Aluminum slurry coatings are widely used for this due to their low cost and straightforward manu- facture. In slurry processing, the coatings are produced by a solid-liquid reaction and subsequent interdifusion between the substrate and the metallic powder contained in the slurry. Slurry coatings, however, can have several disadvantages. Phosphate, chromate, or halide salts used as binders or to activate the difusion species are toxic and harmful to the environment. Also, conventional slurry aluminizing processes, because of their high aluminum activity, have a tendency to form Al-rich intermetallic phases in iron-based alloys that can lead to crack formation. Another limitation is the thermal expansion mismatch between the substrate and the coating, which also can lead to coating cracks. Researchers with DECHEMA- For schungs institut in Frankfurt am Main, Germany have developed nonpolluting, nontoxic aluminum slurry coatings for repairing and protecting boiler components from high-temperature oxidation, high gas velocity oxidation, thermal fatigue, and corrosion such as sulfdation, halogen attack, and hot corrosion. Chemical compo- sitions and mechanical properties of the coatings can be customized by tailoring the slurry composition; and cracking of the resulting aluminide coatings is avoided by the technique used to mix the metallic powder into the slurry. Te main advantage of the newly developed slurry treatment is the possibility of controlling the activity of aluminum so the produced coating will not form a brittle phase, which has the tendency to form cracks. Details are discussed in CORROSION 2014 paper no 4105, "Innovative Heat Treatment and Novel Non-Harmful Slurries for 'In Situ' Aluminization or Co-Deposition

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