HISTORY OF COATING SYSTEMS
It has been demonstrated over the past two decades that sprayed metal coatings can offer some protection against the corrosion reaction.For instance,approximately 20 years ago(Tallent and Plumley,1966,1969),Combustion Engineering developed a multicoat coating system for protection of tube walls in black liquor recovery boilers(BLRB's).This system,applied by the flame spray process,consists of a bond coat of nickel aluminum,an 18/8 stainless steel coat,followed by a coating of pure aluminum.Often the coatings are further protected by the addition of a final coat of a silicone aluminum sealer.
Virtually all recovery boilers that have utilized thermal spray protective coatings over the last ten years have used such a multicoat system.Areas as large as 5,000 square feet(465 m2)have been coated in the lower furnace zone,generator tube section,nose arch section and superheater tubes.
However,multicoat installations have frequently required maintenance recoating after one year of service.In general,five to fifty percent of the surface area of a multicoat will require at least touch-up recoating due to spalling.
When intact,sprayed metal coatings protect tubes by providing a barrier between the corrosive medium and the tube.The coatings should be considered sacrificial,that is,they slowly corrode while protecting the tubes.Although the service history of the multicoat system has shown that thermal spray coatings can be effective to some degree in combatting this boiler tube problem,it is also clear that there is a great need for a more corrosion resistant,non-spalling coating.
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