377. Corrosion by commercial sodium hypochlorites and its inhibition

Abstract

Commercial sodium hypochlorites, whether containing KMnO4 or not, are shown to be corrosive at 150 p.p.m. available chlorine and 40° C. to metals such as aluminium, tinned copper, nickel silver and cast stainless steel (18 Cr, 8 Cu, 3 Mo) which are used in dairy equipment. Hypochlorites containing KMnO4 when aged are potentially dangerous to wrought 18/8 stainless steel. The attack is by pitting and therefore especially dangerous to all the metals investigated, and, in general, increases with increase of time of exposure and temperature.Decay of sodium hypochlorite solutions results in conversion of NaOCl to NaCl and NaC103, which reaction is shown to follow a simple equation fairly closely in various storage conditions.Attack on metals by sodium hypochlorite can be efficiently inhibited by addition of sodium silicate, which has a specific action in addition to the effect exerted by increase of alkalinity. NaOH and Na2CO3 additions to the same pH are not so effective and increase attack on aluminium.Increase of pH from 9 to 10·5 by addition of Na2CO3 or sodium silicate reduces bactericidal efficiency of hypochlorites to the same extent.