Inhibition of Lead Corrosion With Sodium Hexametaphosphate

Abstract

application of sodium hexametaphosphate,* in concentrations of 0.5 to 10.0 p.p.m., to municipal as well as industrial water supplies for the control of the corrosion of ferrous metals has been discussed previously by a number of writers (1, 2, 3, 4, 5). It has been suggested that this inhibitive action is due to an adsorbed film of metaphosphate, or some complex thereof, upon the metal or metal oxide surface (2). In view of the evidence of a similar adsorption on numerous other metals and their compounds (6), it appeared quite probable that the inhibitive action of metaphosphate upon corrosion would not be limited to iron and steel and, as a result, tests for the quantitative evaluation of the effect of these low concentrations of metaphosphate upon the corrosion of a number of non-ferrous metals were contemplated. Of these proposed investigations, that on lead has been completed and is to be described here. It was felt by the author that, because of the serious physiological aspect, i.e., the lead carried into the water, of the corrosion of this metal, its investigation was of primary importance. Since the factor of prime interest in the corrosive attack upon lead is the total amount of the metal carried into the water, rather than the pitting, tuberculation or destruction of the metal itself, it was this quantity which was measured in the investigation described. The effect of metaphosphate upon the lead pick-up was determined at pH values ranging from 5.2 to 8.9; and measurements of the pick-up which resulted upon continuous passage of water through lead wool and through lead tubing were made. In the latter case, the increase in the lead content of water after standing for 24 hr. in the tubing was also determined.