Research on the Influence of pH on Scaling Ion Forms and Deposition Mechanism in Cooling Water Dielectric of HVDC Converter Valves

The internal water cooling system of converter valves, as a critical auxiliary system in HVDC converter stations, faces significant reliability threats due to scaling deposition on grading electrodes. This study investigates the scaling mechanism in the internal cooling water system by constructing a simulated experimental platform to systematically analyze scaling characteristics under different water quality conditions. The scaling phenomena and morphological characteristics were comparatively analyzed, supplemented by theoretical calculations of scaling ion speciation, to evaluate the influence of different ionic forms on scale formation behavior. Experimental results demonstrate that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{p H}$</tex> critically determines scaling processes by altering the existing states of scaling ions. Under weakly acidic and neutral conditions, positively charged aluminum species such as <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Al}_{7}(\text{OH})_{17}{ }^{4+}, \text{Al}^{3+}$</tex>, and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Al}(\text{OH})^{2+}$</tex> migrate and deposit on cathodes under electric field effects. Conversely, in weakly alkaline environments, negatively charged <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Al}(\text{OH})^{-}$</tex> ions accumulate at the anode to form scaling layers. This study elucidates the decisive role of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathbf{~ p H}$</tex> regulation in electrode scaling behavior, providing theoretical foundations for water quality optimization in converter station cooling systems.