1.How well do galvanized coils and aluminum-zinc coated coils form?
Galvanized Coil (GI):
Advantages: The pure zinc layer is relatively soft, possessing good ductility and lubrication. During stamping, bending, and deep drawing processes, the zinc layer deforms along with the substrate without easily cracking or peeling, making it particularly suitable for complex parts requiring deep drawing.
Disadvantages: Under extreme deformation, "zinc shavings" or "powdering" may occur, contaminating the mold.
Aluminum-Zinc Coated Coils (GL):
Advantages: The aluminum-zinc alloy coating (55% Al, 43.4% Zn, 1.6% Si) has a higher hardness than pure zinc, giving it better scratch resistance and rigidity.
Disadvantages: Due to its harder and more brittle nature, the coating is less ductile than pure zinc. Microcracks are prone to form in the coating during sharp bending or deep drawing. While these microcracks usually do not immediately lead to substrate corrosion (because the coating itself is corrosion-resistant), they are not suitable for deep-drawn parts where coating integrity is extremely important.
2.What are the differences in cutting and cut protection between galvanized coils and aluminum-zinc coated coils?
Galvanized Coil (GI):
After cutting, the exposed steel substrate is revealed. Zinc acts as a sacrificial anode, preferentially corroding through electrochemical action to protect the steel substrate at the cut from rusting. This protection lasts for a long time until the surrounding zinc is depleted. The "zinc dross" produced at the cut is also relatively easy to clean.
Aluminum-zinc coated coils (GL):
After cutting, the problem becomes more complex. The aluminum and zinc in the coating form an electrical couple with the steel substrate. However, because aluminum has a more positive electrical potential than zinc, its willingness to "sacrifice" corrosion is not as strong as that of pure zinc. Therefore, the protection at the cut is relatively weak, and red rust may appear more quickly. Corrosion products (usually a mixture of white and red) are also more noticeable at the cut edges.
3.What are the differences in weldability between galvanized coils and aluminum-zinc coated coils?
Galvanized coil (GI):
Its weldability is relatively good. When spot welding, a slightly higher current is required than when welding ordinary cold-rolled steel sheets because zinc has a low melting point and will melt first, contaminating the electrode tip and shortening electrode life. However, by adjusting parameters and using specialized electrodes, stable weld quality can be obtained.
Aluminum-zinc coated coils (GL):
Poor weldability. There are two main reasons:
High resistance: Aluminum has good electrical conductivity, resulting in low surface resistance of the coating, requiring a larger current to generate sufficient heat.
Alumina problem: Aluminum readily forms a hard, high-melting-point alumina film on its surface. This film is non-conductive and infusible, severely hindering current flow, leading to weak welds, spatter, and drastically accelerating electrode wear and corrosion. This typically requires very expensive special electrodes (such as dispersion-strengthened copper) and strict process control, and the weld quality is inconsistent. Therefore, aluminum-zinc coated sheets are generally not recommended for structural components requiring multi-point welding.
4.How well do the coatings adhere to galvanized coils and aluminum-zinc coated coils?
Galvanized coil (GI):
It is one of the most ideal substrates for color coating (color steel sheet). Its surface is uniform and reacts well with chemical pretreatment agents (such as phosphating solutions and passivation solutions), forming a dense conversion film that provides excellent adhesion for subsequent paint coatings.
Aluminum-zinc coated coil (GL):
The surface contains aluminum-rich areas. These areas have poor chemical reactivity with the pretreatment solution, which may lead to an uneven conversion film, thus affecting coating adhesion. More stringent or specialized pretreatment processes are usually required to ensure adhesion quality. Although modern technology can solve this problem, higher requirements remain for process control.
5.How to choose according to different scenarios?
Choose galvanized coil (GI) when: Your processing involves a large amount of stamping, deep drawing, and welding, and you have extremely high requirements for coating adhesion.
Choose aluminum-zinc coated coil (GL) when: Your products primarily require extremely high corrosion resistance and a certain degree of heat resistance, and the processing methods mainly involve simple shearing and bending, with almost no welding involved.