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Caustic corrosion, a form of under deposit corrosion, occurs when the protective oxide layer breaks down. This process normally begins with a disruption in fluid flow or buildup of caustic. Caustic concentrates within the deposit by evaporation of water leaving behind the hydroxide. The wastage occurs between the thick deposit and the steel. A second form is caustic gouging. Under the “right” flow and pH conditions, caustic may concentrate at the edges of steam bubbles. Caustic removes the protective iron oxide, and bare steel reacts with sodium hydroxide (NaOH) to form sodium ferroate (NaFeO2) or sodium ferroite (Na2FeO2), as shown below. The affected surface appears to be smooth and undulating.
Fe3O4 + 4NaOH à 2NaFeO2 + Na2FeO2 + 2H2O
2NaOH + Fe à Na2FeO2 + H2
Overload failures, thin edged ruptures or pinhole leaks
Caustic corrosion is caused by concentration of sodium hydroxide from boiler water, high heat flux, and deposits. Deposits are formed from feedwater system corrosion products or from condenser in-leakage constituents. As porous deposits build up in high heat input areas, sodium hydroxide can concentrate within the deposit to a locally corrosive level. An increase in the tube metal temperature due to the heat transfer resistance of the deposit supports the concentrating mechanism.
Caustic corrosion also occurs during steam blanketing. Sodium hydroxide is insoluble in steam. Therefore, sodium hydroxide is concentrated on the edges of steams bubbles. This local high concentration of hydroxides initiates corrosive attack.
The root cause of caustic corrosion can be verified by an investigation into the water chemistry practices and the amount of feed water corrosion product deposition on the boiler tubes. Condenser leakage from fresh water cooling bodies can be monitored. Tube sampling can be performed to measure the relative thickness and amounts of deposit buildup on the heated side of the internal surface. Tube sampling practices and test methods are detailed in ASTM Standards D 887-82 and D 3483-78. Upsets in water chemistry from malfunctions and operating errors can be verified by review of the chemical control logs, online water chemistry records, and instrumentation alarms. Radiographic and ultrasonic (UT) techniques have been used to detect wall thinning that results from caustic corrosion. Since deposit buildups are likely to occur at tube joints, use the UT technique at the welds. The weld crown on the external surface needs to be ground flush with the surface to allow access for the UT transducer. The tube end preparation on the internal surface must be known to differentiate any reduction in wall thickness due to weld fit-up requirements from wall thinning that is caused by caustic corrosion. Tube metal temperature monitoring by thermocouples can be used to indicate when deposition of feed water corrosion products have reached a significant level; however, corrosion damage can occur without any measurable increase in tube metal temperature.
Corrective actions involve control of boiler water chemistry, minimization of ingress of feed water corrosion products and condenser leakage products into the boiler, and removal of corrosion product deposits by periodic chemical cleaning. Elimination of welds with backing rings or other welded joint surface contour irregularities can be beneficial. Low chromium ferritic steel (such as ASME SA 213 Grade T-11) and rifled or ribbed tubing are less susceptible but not immune to caustic corrosion damage.
Water cooled tubes can experience caustic corrosion at locations that have:
(1) Flow disruptions such as welded joints with backing rings or protrusions, bends, or deposits.
(2) Horizontal or inclined tubing.
(3) High heat flux or flame impingement.
Caustic corrosion produces thin-edged ruptures or pinhole leaks in tubing where the corrosion on the waterside has reduced the wall thickness. Caustic gouging and ductile gouging are other names for this mechanism, since the tube fails in a ductile manner after the wall thickness has been significantly corroded through. A thick deposit is usually present on the internal surface.
This condition can be minimized by keeping the boiler clean on the water side, preventing the ingress of boiler water contaminants, reducing flow disruptions, reducing burner impingement, and reducing steam blanketing. Rifled tubing can reduce the risk associated with under-deposit corrosion. Periodically clean the boilers to reduce the risk of under-deposit corrosion. Reduce flow disruptions in the tubes. For example, avoid using backing rings during butt welding. Keep the amount of particulate iron oxide as low as possible by blow-down to prevent build-up of thick scales.