SPALLING
The major problem with sprayed metal coatings in BLRB's has been their tendency to spall.The spalling problem was extensively described in a previous paper by the author (Unger,Thorpe,and Easterly,1985).
Spalling is an unacceptable,premature separation of the coating from the substrate.Ith can occur either as adhesive failure,at the interface between the coating and the virgin tube,or cohesive separation,within the coating itself.Spalling is caused by either chemical or mechanical means,or a combination thereof.In boiler applications,this can lead to entire loss of portions of the coating,and producing isolated,unprotected areas on the boiler tubes which are directly exposed to the corrosive medium.Obviously,when such spalling occurs it greatly decreased coting effectiveness and produces uneven tube wastage and an unpredictable maintenance schedule.
Chemical Spalling
The primary cause of chemical spalling in BLRB's is diffusion of the corrosive media through the coating to the substrate.
Allsprayed coatings have an inherent degree of porosity.However,this does not necessarily allow penetration of the corrosive media to the substrate.Through porosity can lead to diffusion of the corrosive medium and resultant corrosion at the coating/substrate interface.Consequently,voluminous corrosion products form at the substrate interface,weaken the coating bond,and eventually cause affected areas of the coating to lift or pop off(spalling).
A properly designed and properly applied coating should not contain through porosity.This problem only occurs when the coating is too thin,contains excessive porosity as a result of misapplication,or is cracked or lifted by overheating during application.Any of these deficiencies can result in "through porosity"--a direct path to the substrate and subsequent chemical spalling.
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