The mechanism of the inhibition of zinc corrosion in 1N HCl solution by tetraphenylphosphonium bromide

Abstract

A number of organic corrosion inhibitors of zinc were tested in a 1N HCl medium by the weight loss method. The tafel cathodic slopes relating to solutions containing conventional compounds are 120 mV per current density decade, whereas the solution containing tetraphenylphosphonium bromide gives a 240 mV slope. The inhibition mechanism for the latter compound was studied electrochemically as well as by analytical methods, showing its own reduction to be involved with two different processes: (i) phosphorus-carbon bond fracture with subsequent formation of triphenylphosphine oxide, and (ii) aromatic ring reduction. These two types of reduction have opposite effects on the inhibition process: triphenylphosphine and its oxide formed during the inhibition process are strong inhibitors, but the reduction of aromatic rings at the interface favours the desorption of adsorbed compounds and limits their inhibiting efficiency. An insoluble compound such as (Ph 4P) 2ZnCl 4 may increase the inhibition efficiency by precipitation at the interface, or decrease it by precipitation in the solution.