natural water used in carbon steel passivationReading time:
In most cases, pure water will corrode carbon steel (see cathodic reactions and corrosivity in water treatment ). However, water extracted from natural sources contains dissolved gases and minerals (see table 1 - chapter what water should we treat ? and why ? ). The quantity and type of these dissolved substances will have a major impact on metal corrosion, especially corrosion affecting carbon steel.
This phenomenon had been quantified at the beginning of the 19th century by researchers such as Tillmans, Bayless, Langelier, Larson/Skold and Ryznar, who developed simple indices for use in the field. These methods, which continue to be used to prevent corrosion in drinking water networks, involve classifying water as "agressive" or "scale-forming"; this classification is mainly based on the carbonate balance. A detailed description of the methods used to establish and to modify these properties can be found in the section neutralisation – remineralisation.
The inhibition mechanism involves the two following passivation reactions :
cathodic inhibition using CaCO3
Cathodic reduction of oxygen causes the release of OH- ions at the cathode. In water that has sufficient calcium hardness (Ca2+) and alkalinity (HCO3-), the insoluble CaCO3 will precipitate at the active cathodes.
The CaCO3 that is formed, acts as a barrier both against the continued diffusion of oxygen at the surface and also against electron transfer since the CaCO3 layer is not a conducting layer.
anodic inhibition using alkalinity
The ferrous ion (Fe2+) is the initial product created by the anodic corrosion reaction. Most solid phases created by Fe2+ precipitation are largely soluble and deposits incorporating these phases are fragile and do not act as inhibitors. Fe2+ oxidation to produce Fe3+ is required for the formation of a stable anodic passivating layer.
The dissolved oxygen is thermodynamically capable of completing this oxidation; however, the kinetics of the reaction is depending of a lot of factors (see Iron removal). Slow oxidation of Fe2+ allows Fe2+ to migrate towards the corroding surface where it will oxidise as colloidal iron, producing "red water" or forming non-protective, porous layers. However, if Fe2+ oxidation occurs rapidly, it will produce a stable Fe3+ with layers, in the immediate vicinity of the active corrosion, that provide efficient inhibition.
The first researchers, Tillmans, Bayless and Ryznar found that, in drinking water circuits, alkalinity and pH had a markedly greater effect than CaCO3 precipitation alone. This effect was subsequently ratified by Pr Sontheimer in the «siderite model». This model explains the relationship between the pH and Fe2+ oxidization kinetics through the formation of insoluble Fe2+ → FeCO3 (siderite) compounds that progress to form the inhibiting lepidocrocite (Ɣ-FeOOH).
Lepidocrocite is a stable, oxidised form of iron capable of producing anodic polarisation.
Thus, both pH and alkalinity (M-alk) contribute to the powerful polarisation of the anodic and cathodic corrosion mechanisms.