Pitting corrosion with anodic inhibitors

Hatch, G. B., Maximum Self-generated Anodic Current Density as an Inhibitor Pitting Index , III. State Water Surv., Circ. No. 91, 24 (1966) C.A., 66, 8l8l4f Herbsleb, G., Pitting Corrosion on Metals with Elearon-conductive Passive Layers , tVerksl. Korros., 17, 649 (1966) C.A., 66, 5337m ... 

Aluminium pipes Aluminium might become an important material for carrying water if its liability to pitting corrosion could be overcome. Very soft waters are difficult to accommodate when normal pipe materials are used, and it is for these that aluminium offers most promise ". The possibility of using it for domestic water pipes, however, appears at present to depend upon finding a cheap and effective inhibitor that could be added to the water, or upon the use of internally clad tube, e.g. Al-1 25 Mn alloy clad with a more anodic alloy, such as Al-lZn. Such pipes are at present mainly used for irrigation purposes. ... 

In water with a pH near 7.0, a low concentration of chlorides, silicates, and phosphates cause passivation of steel when oxygen is present hence, they behave as anodic inhibitors. Another anodic characteristic is that corrosion is localized in the form of pitting when insufficient amounts of phosphate or silicate are added to saline water. However, both sUicates and phosphates from deposits on steel increase cathodic polarization. Thus, their action appears to be mixed, i.e., a combination of both anodic and cathodic effects. 

The relationship between additive concentration and corrosion rate raises the question of just what can be accomplished in reducing corrosion. Should complete stifling of corrosion be the goal If some small amount of corrosion is acceptable, is this then in the form of increased pitting, compared to the untreated system, thus making the situation worse than without the inhibitor This consideration is particularly important when working with anodic-type inhibitors such as chromate. 

Detection is obviously specific to a particular active inhibiting chemical. Some tests are expensive and difficult. Much work has gone on to develop a test for aminoalcohols, with mixed results so far. As stated earlier, it is far easier to carry out test on admixed inhibitors than for those applied to hardened concrete. There can be dangers with under dosing inhibitors, in that corrosion may occur very aggressively in small anodic pits. It is therefore important that the risk of pitting and under dosing is fully understood. 

Recently, the idea of applying inhibitors to stop corrosion in existing structures has received some attention from researchers and the producers of corro,sion inhibitors. The principle of most inhibitors is to develop a very thin chemical layer usually one or two molecules thick, on the steel surface, that inhibits the corrosion attack, Inhibitors can prevent the cathodic reaction, the anodic reaction or both (cathodic, anodic and ambiodic inhibitors). They are consumed and will only vork up to a given level of attack i.e, chloride content). The concern with inhibitors is that they will suppress the generalized corrosion but if the amount available is inadequate due to low dosage or consumption, there could be localized, pitting attack. This phenomenon has not been clearly demonstrated in concrete but can occur in theory and has been observed in other corrosion environ men t,s. 

Given some basic information about the corrosion inhibition mechanisms of chromates, many studies have been conducted for chromate replacements. For effective replacement of hexavalent Cr, however, an inhibitor has to inhibit the oxygen reduction reaction as well as anodic dissolution/pitting, and several studies indicate that hybrid formulations seem to be the best way to do just that. Typically, in these hybrid formulations an organic oxygen reduction reaction inhibitor is included with environmentally benign anodic inhibiting anions. 

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