Chloride pitting and water chemistry control in cooling or boiler circuits

The effect of different anions (bicarbonate, phosphate, sulphate, hydroxyl) on the pitting corrosion by chloride ions has been investigated for a number of ferritic or austenitic steels and nickel alloys at different temperatures up to 175°C. For most of the alloys the normal order of inhibition is PO₄³ > SO₄^2- HCO₃, but some anomalies have been encountered for stainless steels at lower temperatures (<100°C), where the inhibitive action of SO₄^2- maybe more pronounced that the inhibition which can be obtained by adding equal amounts of phosphate. In the absence of strongly passivating alloying elements, such as chromium, high SO₄² concentrations may have a deteriorative effect. The same result may also be found under cathodic polarization conditions, where SO₄^2- and HCO₃ seem to stimulate reductive dissolution of the protective surface layer and may even cause some kind of pitting-type attack.