What causes stripes on the workpiece coating during the production process using acidic zinc plating brightener?

In the production process of using acidic zinc plating brightener, sometimes there may be stripe defects in the coating of the workpiece. What is the reason for this?

When acid galvanizing is carried out, we found that when linear bubbles are generated on the surface of the workpiece, stripes will appear on the corresponding positions of the workpiece surface, indicating that the stripes were originally caused by hydrogen evolution.

When the NH₄Cl content increases, the [K⁺] decreases. According to NH₄++H₂O ⇌ NH₃ · H₂O+H₂, the [H⁺] in the solution increases and the pH value decreases. At this time, the area of Zn²⁺adsorbed on the cathode surface is relatively large, so hydrogen evolution will only occur when the current density is high enough. Moreover, due to the relatively large [H⁺], when hydrogen evolution occurs, a large amount of H₂ is produced, making it difficult for H₂ to adsorb on the cathode surface. Instead, it rises to the liquid surface through buoyancy, which hinders the deposition of Zn²⁺at that location during the upward movement, resulting in stripes.

When the total salt concentration in the plating solution decreases, the [K⁺] also decreases, and the adsorption sites of Zn²⁺on the cathode surface account for the majority. Therefore, hydrogen evolution only occurs when the current density is high enough. H² is not easily adsorbed on the surface of the galvanized layer, so it quickly rises to the liquid surface under buoyancy. Due to the upward movement of H₂, it hinders the further deposition of Zn²⁺at that position, resulting in stripes.

So, in the production process of acidic zinc plating brightener, there may be stripe defects in the coating of the workpiece, which is how it comes about. For other technical questions about acid zinc plating brightener, please consult Bigley customer service, and professional engineers will answer your questions.