Passivation of carbon steel alloy in de-oxygenated alkaline pH media. The effect of various additives

The ability of certain additives to influence the oxide film formation characteristics on carbon steel alloy under de-oxygenated alkaline pH conditions at 473 K has been investigated. The additives used in the study are: boric acid, glucose, hydrazine, morpholine and cyclohexylamine. Their relative impacts on the passivation processes are evaluated by using the data on base metal loss, soluble and insoluble iron released to the medium and scanning electron microscopic studies of the topography of the coating and electrochemical investigations on the protectivity of the oxide coating. These results have been compared with the passivation behaviour of this alloy obtained under simple alkaline pH treatment with LiOH without any additive. The effectiveness of the passivation treatment is found to be greatly enhanced in the presence of boric acid and hydrazine. Based on the effectiveness in inducing passivity to a carbon steel surface, the additives can be arranged in the following order: boric acid ≈ hydrazine > cyclohexylamine > morpholine > LiOH(blank) > glucose.