Abstract
A study has been made of the influence of small additions of sodium metaphosphate glass (a sodium polyphosphate) on the corrosion of mild steel in a solution of 25 ppm sodium chloride at flow velocities of 1·2, 20 and 200 cm/sec. The inhibitor efficiency is found to depend on the concentration, the rate of flow and the presence of small concentrations of calcium ion. Small additions of metaphosphate glass can actually stimulate corrosion of mild steel at low flow velocities in the absence of calcium although in the presence of cqlcium they have a small but beneficial effect. The efficiency of inhibition increases with increase in flow velocity.
The inhibition is produced by a visible film largely of a colloidal nature the amount of product becoming smaller when the corrosion rate is lowered. Potential measurements have shown that polyphosphate acts mainly as an anodic inhibitor in the absence of calcium and as a cathodic inhibitor in its presence. Stimulation can occur owing to the complexing power of metaphosphate glass and its depolarising action at the anode.
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