Abstract
On-load corrosion affects the mild or low-alloy steel surfaces of boiler tubes where steam is being generated at temperatures up to 360°c, and pressures up to 180 bar. The corrosion rate under these conditions is low in pure water or dilute solutions, but is much higher in more concentrated solutions af either high or low pH. On-load corrosion results from the local formaiion of either concentrated alkaline or acidic solutions. These are generated by the concentration of dilute solutions by boiling when the heated surface is obstructed by a steam blanket, a crevice or a porous deposit.
Corrosion due to concentrated alkaline solutions was recognised first in the thirties. The alkalinity of boiler water consequently tended to be reduced and corrosion due to acidic solutions increased during the forties and fifties. The effect of acid-forming contaminants was demonstrated in the sixties and acidic corrosion due to oxygenated chloride solutions was established even more recently. Understanding the mechanism of on-load corrosion has led to the present satisfactorily low incidence of corrosion to boilers.
Prospects of changes in water treatment, design or alloy composition are briefly discussed.
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