Rust Formation on Alloyed Galvanized Steel Sheets (Electrochemical Corrosion)

1. Production Process:

The main production process of alloyed galvanized steel sheets (GA sheets) involves the strip steel, after exiting the zinc bath but before the coating solidifies, entering a zinc alloying furnace. Within 5 seconds, the strip steel is heated from approximately 450°C at the zinc bath to 550°C, completely converting the pure zinc layer into a zinc-iron alloy layer. This alloy layer has an iron content of 8-15%.

2. Alloying Properties and Characteristics:

After alloying treatment, the surface of the hot-dip galvanized steel sheet loses its beautiful zinc bloom and luster, exhibiting a silvery-white or dark gray appearance. The treated coating can undergo complex bending and forming processes and possesses good weldability, coatability, heat resistance, and corrosion resistance.

3. Mechanism of Condensation Rust (Electrochemical Corrosion)

a. Causes of Condensation Formation

Condensation occurs when warm, humid air comes into contact with a cooler medium surface and is rapidly cooled. When the air cools below its dew point temperature, water droplets form on the medium surface, producing condensation, commonly known as condensation.

Two basic conditions for condensation formation: 1. Temperature difference: If there is no temperature difference between the humid air and the medium surface, the air saturation will not change, and condensation will not occur. 2. Humid air: If the air is relatively dry (low relative humidity), even with a temperature difference, condensation will not occur if the air does not reach its dew point after cooling.

Steel plates have high thermal conductivity and poor insulation. In winter, the temperature difference between indoors and outdoors is large, and when humidity is high, condensation easily occurs. The condensed water remains on the steel plate surface, accelerating corrosion.

b. Analysis of Rust Causes in Alloyed Zinc Plating

The principle of rust prevention for galvanized products (including alloyed galvanized products) is sacrificial anode protection, which sacrifices the coating to protect the integrity of the steel substrate, extending the service life of the steel substrate. If galvanized products are not properly protected before processing, they are prone to rust, especially in environments with high humidity and large temperature differences, where condensation corrosion is most likely to occur.

4. Post-galvanizing Rust Prevention Treatment for Alloyed Galvanized Products

The principle of rust prevention for galvanized products (including zinc-iron alloy products) is sacrificial anode protection, which sacrifices the coating to protect the integrity of the steel substrate. This method extends the service life of the steel substrate, but does not mean that the product will be completely rust-free. Therefore, even zinc-iron alloy products require surface protection treatment after production. Currently, the most typical post-galvanizing rust prevention treatments for galvanized products are passivation or oiling.

Passivation: The passivation process involves a chemical reaction between the metal and the medium, generating a very thin protective film. This protective film is usually composed of compounds of oxidants and metals, which inhibits the anodic reaction, thereby protecting the zinc layer.

Oiling: Surface oiling is currently the simplest and most effective rust prevention method for steel products, and it plays an irreplaceable role in subsequent transportation or stamping processes.

5. Protection after Alloyed Galvanizing

To isolate the steel plate from the air and prevent oxidation:

a. Spray rust-preventive oil onto the cross-section of the steel plate to form a protective oil film, isolating it from the air.

b. Seal the finished steel plate completely with rust-preventive paper to prevent air convection.

c. Store in a dry, well-ventilated warehouse.

d. Manage inventory reasonably and use as soon as possible.