1.What are the effects of insufficient or excessive flux concentration zinc chloride?
Insufficient concentration:
The oxide film on the workpiece surface cannot be completely dissolved, resulting in residual oxide film. This residual oxide film hinders the bonding of the zinc solution to the workpiece substrate, leading to skipped plating (partially missing coating), poor coating adhesion, and even the appearance of "black edges" and "bubbling."
Excessive concentration:
The flux viscosity increases, and excess zinc chloride solution easily remains on the workpiece surface. This residual solution, when entering the zinc pot, reacts violently with the zinc solution, producing a large amount of zinc slag. This zinc slag adheres to the coating surface, causing roughness, nodular bumps, and spots, while increasing zinc consumption and subsequent cleaning costs. Furthermore, high zinc chloride concentrations can easily crystallize, resulting in uneven flux coverage on the workpiece surface and a "mottled" coating.
2.What are the effects of insufficient or excessive ammonium chloride concentration?
Insufficient concentration:
The melting point of the flux increases, making zinc chloride crystallization more likely at low temperatures. This results in uneven flux coating on the workpiece surface (without a protective film in some areas). Furthermore, it fails to effectively buffer the pH value, causing zinc chloride to hydrolyze and form zinc hydroxide precipitates (ZnCl₂ + 2H₂O ⇌ Zn(OH)₂↓ + 2HCl). This precipitate adheres to the workpiece surface, forming a "white frost" and causing pinholes and pores in the zinc coating (the precipitate reacts with the zinc bath to produce gas). Furthermore, the workpiece is susceptible to secondary oxidation after fluxing and before entering the zinc bath (due to the lack of a complete protective film), causing the coating to appear black or gray.
Excessive concentration:
The flux increases volatility (NH₄Cl decomposes easily upon heating into NH₃ and HCl), resulting in high levels of fumes in the fluxing tank, polluting the environment and causing fluctuations in composition. At the same time, excessive NH₄Cl will form a loose "salt film" (rather than a dense protective film) on the surface of the workpiece. This film is prone to rapid decomposition and gas production when in contact with zinc liquid, resulting in dense pores and pitting in the coating. The film may also be too thick, causing the zinc liquid to be unable to evenly infiltrate the workpiece, resulting in "uneven coating thickness."
3.What is the effect of activators (such as stannous chloride SnCl₂, antimony chloride SbCl₃, cupric chloride CuCl₂, etc.)?
Proper addition can significantly reduce the rate of missed plating, resulting in a more uniform coating and improved bonding strength.
Excessive addition can lead to over-activity of the flux, potentially corroding the substrate (especially thin steel sheets), causing the coating to become thinner and whiten. It can also cause intermetallic nodules (such as Sn-Zn alloy nodules) to form on the surface of the coating, resulting in a rough coating.
4.What is the effect of stabilizers (such as sodium fluoride NaF, potassium fluoride KF, etc.)?
Proper addition can resolve plating skipping issues on high-silicon and high-manganese workpieces and reduce coating spotting.
Excessive addition can corrode the metal flux tank and hangers. It can also react with Zn²⁺ to form ZnF₂ precipitates, causing turbidity in the flux and residual precipitates on the workpiece surface, leading to "inclusions" in the coating.
5.What are the effects of corrosion inhibitors (such as hexamethylenetetramine, polyethylene glycol and other organic compounds)?
Proper addition maintains the workpiece's surface finish and creates a smoother coating.
Excessive addition can form an "organic film" on the workpiece surface, hindering the reaction between the flux and the oxide film, reducing the fluxing effect and weakening the coating's adhesion.