1.What are the reasons for the pre-processing stage?
Residual Surface Contamination
Oil: Rolling oil, rust-proof oil, and lubricating oil have not been thoroughly removed. Oil can block contact between the steel substrate and the zinc solution.
Iron Powder/Dust: This originates from the rolling process or storage environment and can affect the adhesion of the coating.
Graphite: Lubricating graphite remains on the surface of certain special steels.
Incomplete Removal of Iron Oxide Scale (Rust)
Oxide scale forms on the steel surface during hot rolling and storage. If pickling is not thorough, this hard scale residue prevents the zinc from reacting with the fresh steel substrate to form an alloy layer, resulting in flaking and delamination after plating.
Pickling Issues
Over-pickling: Prolonged pickling time or excessively high acid concentrations can cause excessive corrosion on the steel substrate, resulting in numerous micropores and hydrogen embrittlement, impairing coating adhesion.
Under-pickling: Insufficient pickling time prevents complete removal of iron oxide scale.
Acid Contamination: Excessive iron ion content in the acid can impair cleaning effectiveness.
2.What are the reasons for the galvanizing process stage?
Flux Issues
Flux Failure: Improper flux temperature and concentration, or excessive iron ion content, prevent the flux (usually a zinc chloride-ammonium chloride solution) from forming a uniform protective film on the steel substrate, leading to secondary oxidation before the steel enters the zinc bath.
Flux Residue: If workpieces are not fully dried after removal from the flux bath, carrying them into the zinc bath can cause zinc splashing and create missed plating spots.
Zinc Bath Composition and Temperature
Improper Aluminum Content:
Excessive aluminum content (common in alloy coatings like Galvalume) causes aluminum to preferentially react with steel, forming a brittle Fe2Al5 barrier layer. While this can inhibit alloy growth, an excessively thick or uneven Fe2Al5 layer is inherently brittle and prone to "powdering" or "flaking" during deformation.
Too Low Aluminum Content: In pure zinc coatings, an appropriate amount of aluminum can enhance brightness and improve adhesion. Too low a content can have a poor effect.
Improper Zinc Bath Temperature:
Too high a temperature accelerates the iron-zinc reaction, forming an excessively thick and brittle iron-zinc alloy layer (primarily Γ and δ1 phases). This brittle layer can detach extensively with slight bending or impact.
Too low a temperature results in an incomplete iron-zinc reaction, resulting in a thin alloy layer, weak adhesion, and a rough coating appearance.
3.What impact does unreasonable machining and deformation have on the peeling of the galvanized layer?
Severe bending or stamping: When the bending radius is too small or the deformation exceeds the plastic deformation limit of the coating, the brittle iron-zinc alloy layer will first crack and flake off. This is the most common cause of post-processing spalling.
Severe impacts and scrapes: Strong external forces can directly cause the zinc layer to separate from the substrate.
4.What effect does the corrosion of the operating environment have on the shedding of the galvanized layer?
Unsuitable for use in strong acid and alkaline environments: Zinc is an amphoteric metal and corrodes rapidly in both strong acid and alkaline environments, causing the coating to dissolve rather than peel.
Extended exposure to humid, enclosed environments may result in "white rust" (basic zinc carbonate), a corrosion product rather than peeling caused by adhesion issues.
5.What are the conclusions and preventive measures?
Pretreatment: Strengthen control and inspection of the degreasing and pickling processes to ensure a clean, corrosion-free surface.
Galvanizing: Strictly control the zinc bath temperature, immersion time, aluminum content, and flux specifications.
Post-processing: When designing the processing technology, consider the deformation capacity of the zinc coating to avoid excessive bending radius and excessive deformation.