Oxygen pitting

Oxygen pitting of boiler tubes by boiler feed water due to inadequate de-aeration is also a problem, but controllable by proper maintenance of de-aerators, coupled with regular boiler feed water analysis, or, preferably, continuous dissolved-oxygen monitoring. 

Apart from caustic cracking, the main forms of corrosion in low-pressure plant are oxygen pitting and scab formation. The residence of oxygen at sensitive locations on-load, possibly following off-load initiation, means that... 

Kitchen steam generators to provide live steam for steam tables are widely available. They tend to be very small, perhaps only of 150 to 250 lb/hour generating capacity. However, because they typically receive 100% cold, city supply as a FW source, and where the boiler vessels are constructed of carbon steel, oxygen pitting and subsequent boiler failure can occur rapidly. 

In many systems where oxygen pitting corrosion has taken place, it often is difficult to decide whether the ingress of causative oxygen stemmed primarily from DO in the MU water required to compensate for excessive water losses or simply overall system air-ingress, especially because these conditions tend to go hand-in-hand. 

Inadequate FW deaeration. Oxygen pitting corrosion to deaerator, economizer, FW tank, FW line Pitting in tubes and shell (FT) Pitting to top drum (WT)... 

The points of attack are primarily the waterline (where oxygen pitting may also occur), in areas where steam-water velocities are highest, and at points where stresses are inherent. These include ... 

These features may indeed provide cost-effectiveness and other tangible benefits however, unless the boiler plant operates constantly and FW quality remains consistent, these benefits are unlikely to be fully realized. Where operating conditions vary, the use of multiblend programs exposes the potential for constant readjustment, poor waterside treatment (scales, deposits, oxygen pitting, etc.), and water-level control problems. 

For the smallest low-pressure heating boilers, simple one-drum programs that are designed for greater than 99% condensate return are used. Where condensate is only 70 to 80%, boiler waterside surfaces get dirty quickly, under-deposit and oxygen pitting corrosion may develop, and tubes may fail. 

Oxygen pitting corrosion, in fuel systems Oxygen removal 686... 

The free molecule, in the gas phase, has a planar, triangular structure that may be considered to be a resonance hybrid involving pn—pn S—O bonding, as in (12-XXIV), with additional n bonding via overlap of filled oxygen pit orbitals with empty sulfur dn orbitals, to account for the very short S—O distance of 1.41 A. 

OXYGEN PITTING - Damage caused due to the presence of oxygen in the feedwater. Damage results are small pit-like holes in the metal. 

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