1.Why does passivation have little effect on cold-rolled carbon steel?
Material Differences:
Effective passivation requires the metal itself to form a dense, stable, and self-healing oxide film (such as chromium, aluminum, titanium, and their alloys). The "passivation" treatment of stainless steel strengthens this chromium-rich oxide film.
The naturally formed oxide film (iron oxide) on the surface of cold-rolled carbon steel (iron-based) is very loose and unstable, unable to prevent the intrusion of corrosive media. Acid pickling for "passivation" cannot form a protective layer similar to that of stainless steel.
Different Process Purposes:
In the steel industry, similar "passivation" treatments performed on cold-rolled carbon steel are usually called chemical passivation or oiling. Their fundamental purpose is not to form an oxide film, but rather:
Short-term rust prevention: Preventing fingerprint rust and water rust during transportation and short-term storage.
Preparation for subsequent coating: Some phosphating or passivation treatments aim to improve paint adhesion.
The rust prevention period for this type of treatment is very short, typically ranging from a few hours to about 24 hours in salt spray tests, which cannot meet medium- to long-term salt spray resistance requirements.
2.Can galvanizing improve the salt spray resistance of cold-rolled coils?
Hot-dip galvanizing (GI): Thick coating, strong protective ability, good salt spray performance, but the surface has zinc flowers, and the appearance is generally not very attractive.
Electro-galvanizing (EG): Uniform and bright coating, aesthetically pleasing, but the coating is relatively thin, and the salt spray performance depends on the coating thickness (e.g., 10μm or more can achieve hundreds of hours).
3.Can zinc-based coatings improve the salt spray resistance of cold-rolled coils?
Aluminized zinc (GL): Offers superior corrosion resistance compared to ordinary zinc plating, especially with better resistance to edge corrosion.
Zinc-iron alloys and zinc-nickel alloys: Provide excellent corrosion resistance and are commonly used in automotive and high-demand applications.
4.Can passivation post-treatment improve the salt spray resistance of cold-rolled coils?
Here, "passivation" refers to the treatment of the zinc plating layer (such as chromium-free/chromium-containing passivation, fingerprint-resistant treatment), the purpose of which is to seal the porous structure of the zinc plating layer and delay the appearance of "white rust". It must be based on the zinc plating layer and cannot be used alone for cold-rolled substrates.
5.Can applying rust-preventive oil/wax improve the salt spray resistance of cold-rolled coils?
It is low in cost and easy to operate, but its rust prevention period is limited, and it is prone to dust accumulation, which may affect subsequent welding or painting. It is suitable for short-term protection or inter-process rust prevention.