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07453 A NEW UNDERSTANDING FOR THE MECHANISM OF CAUSTIC GOUGING OF BOILER TUBES

Picture for 07453 A NEW UNDERSTANDING FOR THE MECHANISM OF CAUSTIC GOUGING OF BOILER TUBES
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Product Number: 51300-07453-SG
ISBN: 07453 2007 CP
Author: H. M. Shalaby
Publication Date: 2007
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Gouging of boiler tubes has been commonly attributed to the dissolution of a protective magnetite film due to caustic attack, followed by precipitation of a non-protective magnetite scale. Furthermore, copper and nickel compounds were assumed to preferentially precipitate in the gouged area and to enhance the corrosion process. The present paper challenges some of the accepted assumptions and presents a new understanding for this corrosion phenomenon. It is argued that the magnetite film loses its protective nature as a result of localized high-heat input caused by flame impingement. This step is followed by formation of a non-protective porous magnetite scale as a result of a reaction between the metal and oxygen-bearing species. The growth of the scale is consistent with a process whereby the magnetite layer is constantly pushed up from below by oxide forming at the metal surface. The oxidation process becomes self-sustaining due to the increase in heat transfer resistance across the tube wall. Copper and nickel were found in all tube surfaces. However, the amounts were more in the gouged area due to build up of local concentrated solutions through wick-type boiling. The gouging process was judged to be autocatalytic, as the build up of concentrated solutions requires the presence of the porous magnetite.
Gouging of boiler tubes has been commonly attributed to the dissolution of a protective magnetite film due to caustic attack, followed by precipitation of a non-protective magnetite scale. Furthermore, copper and nickel compounds were assumed to preferentially precipitate in the gouged area and to enhance the corrosion process. The present paper challenges some of the accepted assumptions and presents a new understanding for this corrosion phenomenon. It is argued that the magnetite film loses its protective nature as a result of localized high-heat input caused by flame impingement. This step is followed by formation of a non-protective porous magnetite scale as a result of a reaction between the metal and oxygen-bearing species. The growth of the scale is consistent with a process whereby the magnetite layer is constantly pushed up from below by oxide forming at the metal surface. The oxidation process becomes self-sustaining due to the increase in heat transfer resistance across the tube wall. Copper and nickel were found in all tube surfaces. However, the amounts were more in the gouged area due to build up of local concentrated solutions through wick-type boiling. The gouging process was judged to be autocatalytic, as the build up of concentrated solutions requires the presence of the porous magnetite.
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Under-deposit corrosion is a common mode of failure in boiler tubes. Mechanisms include phosphate attack, caustic gouging, hydrogen damage, and wall thinning due to metal loss. Many aspects of these mechanisms and their mitigation are well-understood, while certain specific details remain uncertain or subject to controversy.

 
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