Secondary circuit | Water chemistry
Amine-based chemistry1 January 2013
Analysis of carbon steel samples in three amine solutions have found two amines that maintain favourable pH and inhibit corrosion as well, or better, than cyclohexylamine. By Maria Radulescu, Doina Stefanescu and Maria Mihalache
Most of the deposits found on NPP secondary circuit components, especially pipes, are composed of iron oxides and oxyhydroxides, along with other metallic oxides depending on the materials used in the construction of the system. Starting from much exhaustive research to reduce the formation and transport of corrosion products in the secondary circuit, it has been decided to add some amines in the feedwater. In this context, it is very important to find those amines that, simultaneously with the action of pH regulation, should have an inhibiting effect on the corrosion behaviour of carbon steels.
The aqueous media used in autoclaving carbon steel SA106 samples were of three types: classical AVT solution (Amine Volatile Treatment) containing morpholine and cyclohexylamine dissolved in demineralized water, and two other types of solutions in which triethanolamine (TEA) and triethylamine (TREA), respectively, were used as a substitute for cyclohexilamine. The role of all these amines dissolved in demineralized water was to assure a final pH of about 9.7.
After the autoclaving of the samples in the three types of solutions for several periods, the weighing of the samples prior and after these periods was executed and using a mathematical processing program of these data, the kinetic equations and curves as a function of autoclaving periods have been obtained. The curves corresponding to corrosion rates/autoclaving period variations (Fig. 1 ) are decreasing power functions: y = kx–n.
Correlating the corrosion rates and the quantity of deposits of corrosion products formed on autoclaved carbon steels samples, it can conclude that the samples in the AVT solution suffered a greater corrosion than in [MF + TEA] and [MF + TREA] solutions, a fact that was also metallographically confirmed.
There is supplementary evidence of a lower porosity of films formed during the tests executed in solutions containing TEA and TREA compared to films formed in the AVT solution. After maintaining all these samples in electrolyte (0.05M boric acid + 0.001M Na-tetraborate) 24 hours prior to EIS determination, it was observed that the characteristics of films formed in solutions containing TEA and TREA remained unchanged, while the impedance of films formed in AVT solution increased (Fig. 2 ).
As aromatic amines (such as CHA) are much weaker bases than aliphatic amines (such as TEA and TREA), they dissociate themselves more readily in aqueous solutions and thus they react much less than the aliphatics. Therefore, aromatic amines such as CHA help form a less-protective film than the two aliphatic amines.
All experimental results showed that the passivation efficiency was greater in the case of solutions containing TEA compared to those containing TREA and AVT, respectively. All these amines have two functions, as a pH regulator and as a corrosion inhibitor. The efficiency difference is due to the particular structure of these amines; as TEA has in addition three hydrophilic groups with polar character, it is more easily adsorbed on the samples surfaces covered with iron oxides and oxyhydroxides. For this reason, absorbed TEA molecules move more easily than TREA ones, the water molecules from the outer superficial film forming several complex compounds with superficial iron oxides.
The absorption process of these amines will lead, finally, to a diminution of corrosive attack of ions from solution due to formation of some hydrogen bonds between these amines and the intermediate corrosion products, such as the adsorbed compound [Fe(OH)ads]. While this absorption due to some hydrogen bonds with [Fe(OH)ads] will give a small contribution at the corrosion inhibition, nevertheless will result in a noticeable adherence of these two amines on superficial films compared with that on uncovered metal. In these zones, the superposed amine layers, tied to superficial films by hydrogen bonds, will be more compacted, forming an efficient protective barrier-layer.
Maria Radulescu, Doina Stefanescu and Maria Mihalache, Institute for Nuclear Research 115400 Pitesti – Mioveni PO Box 78 Romania
A longer version of this paper was first published in the Journal of Nuclear Research and Development, no 1, May 2011, as ‘Management of CANDU 6 Secondary Circuit Water Chemistry using Several Amines’.
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