Abtragungsraten von Zink bei Gleich‐ und Wechselstrombelastung

Abstract

Die Untersuchungen ergaben, dal3 Zink sowohl als Reinzink als auch als Verzinkung von Bandeisenerdern durchaus zur Ableitung von Wechselstrom eingesetzt werden kann. Eine durch Wechselstrom bedingte Verschiebung des Potentials in positiver Richtung, wie sie bei Magnesium auftritt, ist ni&t gegeben. Bei Wechselstrombelastung wird die Abtragungsrate des Zinks nur unwesentlich erhoht.Allerdings ist beim Einsatz sowohl von verzinkten Bandeisenerdern als auch von Zinkanoden zur Ableitung von Wechselstromen darauf zu achten, da8 bei zusatzlichem kathodischem Schutz keine Potentialwerte negativer als -1,5 V eingestellt werden. Anderenfalls besteht die Moglichkeit, da es sich bei Zink urn ein amphoteres Metall handeft, daR durch die dann entstehende Natronlauge ein erhohter Materialabtrag stattfindet. Die Untersuchungen zeigen, daB bei Potentialwerten zwischen -1,l V bis -1,5 V der Zinkabtrag erheblich vermindert wird. Bei Einstellung des Potentials muB beriicksichtigt werden, dal3 durch die zusatzliche Wechselstrom-Uberlagerung das Zink-Potential bis etwa 0,l V negativer werden kann, d. h. dab der Potentialwert nur auf ungefahr -1,3 V bis -1,4 V eingestellt werden sollte.

Flow‐dependent zinc corrosion in boric acid‐containing electrolytes

“…The zinc corrosion process in neutral or weekly acid electrolytes (e.g., resting or slowly stirred solutions/pH 4–11) is mainly determined by the mass transfer limitation of the dissolved oxygen to the zinc surface whereas, at stronger acidic solutions (pH < 3), the cathodic reduction of protons (H 3 O + ions) to form hydrogen will become the main reaction. [ 1,6 ] In such situations, the subsequent cathodic reduction of oxygen (electron transfer) or the anodic zinc dissolution processes are much faster than the mass transfer of oxygen to the zinc surface by diffusion or convection.…”

“…[2][3][4] The courses of zinc concentrations were determined by analysis of electrolyte samples during the experiments. The main results of the investigations can be summarized as follows: Typical zinc corrosion rates ʋ observed in the experiments (slowly stirred boric acid-containing electrolyte/pH [5][6] were in the range of 1.5-2.5 g/m² h. [2,3] These values are much higher than typical zinc corrosion rates under atmospheric conditions (0.16-13.0 mg/m² h) or in unmoved neutral salt electrolytes (0.037-0.060 g/m² h). [1,6,7] In slightly acidic electrolytes (pH 5-6), typical corrosion rates often increase up to values in the range of 0.20-0.60 g/m² h. This effect can be explained by the removal of OH − ions nearby the zinc surface preventing the formation of passivating layers (which mainly consist of zinc hydroxides).…”

“…The main results of the investigations can be summarized as follows: Typical zinc corrosion rates ʋ observed in the experiments (slowly stirred boric acid-containing electrolyte/pH [5][6] were in the range of 1.5-2.5 g/m² h. [2,3] These values are much higher than typical zinc corrosion rates under atmospheric conditions (0.16-13.0 mg/m² h) or in unmoved neutral salt electrolytes (0.037-0.060 g/m² h). [1,6,7] In slightly acidic electrolytes (pH 5-6), typical corrosion rates often increase up to values in the range of 0.20-0.60 g/m² h. This effect can be explained by the removal of OH − ions nearby the zinc surface preventing the formation of passivating layers (which mainly consist of zinc hydroxides). [1,6,7] Boric acid electrolytes (pH range 4.5-6) showed similar or even better prevention of passive layer formation causing significantly enhanced corrosion rates (active metal surface) whereupon the boric acid acts as Lewis acid to widely (and fast) remove free hydroxide ions leading to the formation of borate ions (B(OH) 4…”

“…[ 1,6,7 ] In slightly acidic electrolytes (pH 5–6), typical corrosion rates often increase up to values in the range of 0.20–0.60 g/m² h. This effect can be explained by the removal of OH − ions nearby the zinc surface preventing the formation of passivating layers (which mainly consist of zinc hydroxides). [ 1,6,7 ] Boric acid electrolytes (pH range 4.5–6) showed similar or even better prevention of passive layer formation causing significantly enhanced corrosion rates (active metal surface) whereupon the boric acid acts as Lewis acid to widely (and fast) remove free hydroxide ions leading to the formation of borate ions (B(OH) 4 − ). [ 2–5 ]…”