Electrochemical studies on zinc in boric acid containing electrolytes


Electrochemical studies on zinc in boric acid containing electrolytes

Harm, U.; Kryk, H.; Hampel, U.

During the sump recirculation phase after a leak in the primary cooling circuit of a pressurized water reactor (PWR), corrosion of hot-dip galvanized containment internals (e.g. grating treads, supporting grids of sump strainers) in the boric acid containing coolant may occur, which could later cause problems due to the possible precipitation of formed zinc borates (fouling) at hot regions of the reactor core [1, 2].
Beside other safety related investigations, generic zinc corrosion studies in boric acid electrolytes were conducted to investigate the dependency of the zinc corrosion rates in PWR coolants on different boundary conditions (fluid temperatures, pH, boric acid concentration, flow conditions nearby the zinc surface).
Corrosion experiments with dipped zinc sheets in stirred boric acid solutions already had shown that moderate variations of the fluid temperatures or the boric acid content only caused small changes in resulting zinc corrosion rates, but an increase of flow rates or turbulences often led to significantly increased corrosion rates [2].
For a better understanding of these results, additionally, some series of electrochemical measurements were carried out using a rotating disc electrode (zinc) as working electrode, a platinum counter electrode and tempered aqueous boric acid solutions (plus 0.1 M Na2SO4 as conducting salt) as electrolyte. Linear anodic and cathodic polarization was realized (up to potentials of 200 mV different from the free corrosion potential) under variation of fluid temperature (20 to 60 °C), boric acid content (1000 to 3000 ppm boron), pH (4.7 to 7) and the rotation speed.
First results of these experiments (e.g. comparing tafel plots) showed similar dependencies of the zinc corrosion rates than described above for the zinc dissolution experiments. The cathodic polarization curves mostly showed a plateau of the current densities with increasing cathodic polarization (overvoltage) indicating a strong control of the cathodic reaction (and thus the corrosion process as a whole) by the transport limitation of the dissolved oxygen to the zinc surface. Comparison of the tafel plots resulting from measurements at different rotation speeds (similar otherwise conditions) also demonstrated a strong increase of the zinc corrosion rates with increasing flow rates. For example, calculated zinc corrosion rates for infinite high rotation speeds (Levich extrapolation) usually are more than ten times higher compared to those of similar experiments without rotation. Therefore, also these results of the electrochemical investigations confirm the earlier results of a transport controlled corrosion process (see above) and may help to quantify the possible ranges of resulting corrosion rates at different boundary conditions.
This work is funded by the German Federal Ministry of Economic Affairs and Energy (BMWi) with the grant number 1501496 on the basis of a decision by the German Bundestag.

[1] Seeliger, A.; Alt, S.; Kästner, W.; Renger, S.; Kryk, H.; Harm, U.: Zinc corro¬sion after loss-of-coolant accidents in pressurized water reactors - thermo- and fluid-dynamic effects. Nuclear Engine-ering and Design, 2016, 305, 489-502
[2] Harm, U.; Kryk, H.; Hampel, U.: Generic Zinc Corrosion Studies at PWR LOCA Conditions. Annual Meeting on Nuclear Technology (AMNT 2017), 2017

Keywords: Nuclear energy; corrosion; zinc release; electrochemistry; experiments

  • Poster
    Electrochemistry 2018, 24.-26.09.2018, Ulm (Universität Ulm), Deutschland

Permalink: https://www.hzdr.de/publications/Publ-28112
Publ.-Id: 28112