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Boiler Corrosion & Fatigue in Steam and Hot Water Systems

corrosion mechanisms that may be evident in water tube and shell boiler operation

Circumferential Cracking	Form of fatigue stress corrosion, mostly at fire-side bends
Caustic Embrittlement	Occurs under high static stress in concentrations of caustic soda
Necking and Grooving	Waisting of fire-side stay bolts & rods at entry to boiler end plates
Boiler Stress Cracking	A fatigue corrosion in areas of repeated or alternating stresses, usually of low frequency and relatively high magnitude
Air Bubble Pitting	Associated with small bubbles of oxygen adhering to metal surfaces
Fatigue Corrosion	Caused by alternating stress fatiguing of a component, associated with crazy and circumferential cracking
Stress Corrosion	A consequence of high mechanical stress in a chemically active local environment
Galvanic Attack	Physically weakening electrochemical transformations between dissimilar metals in the presence of an electrolyte
Crazy Cracking	Most commonly occuring in the bore of a fire tube, particularly at bends. Surface has a fissured, wrinkly appearance
Crevice Attack	Occuring along seams between adjacent components, caused by electrochemical and/or chemical stress in the presence of mechanical straining
Scab Pitting	A deep-acting electrochemical cell penetratrating fure-side tube surfaces, although similar structures can develop under less specific circumstances
Priming	Consequent upon foaming caused by dissolved salts. Small bubbles aggregate from rough surfaces to form a stable foam, which is carried-over into steam spaces - causing explosive displacement of large quantities of water into (particularly) superheater drums

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