1.What are the three conditions that are usually required for foam to form?
The coating is defective or its adhesion has decreased (as is often the case after oxidative aging).
There is moisture or corrosive substances beneath the coating.
External driving forces (such as temperature changes, osmotic pressure).
2.What are the processes and causes of osmotic foaming?
Process: Moisture (rainwater, condensation) seeps into the coating through edges, cuts, and minor mechanical damage, reaching the space between the coating and the substrate. If the coating has become less dense due to oxidation and aging, moisture penetrates more easily.
Cause of blistering: Water forms an electrolyte between the steel plate (anode) and the zinc layer (cathode), initiating corrosion. Corrosion products (rust, zinc salts) expand in volume, generating osmotic pressure that lifts the aged, brittle, and less adhesive coating, forming blisters.
Relationship with oxidation: Oxidized (aged) coatings have poor adhesion and are easily peeled off, facilitating blistering.
3.What are the processes and causes of blistering during cathode stripping?
Process: When the substrate (especially at cuts or damage areas) begins to corrode, an electrochemical corrosion cell forms. During the corrosion reaction, alkaline substances (OH⁻) are generated in the cathode area (the intact zinc plating).
Cause of blistering: These alkaline substances break the chemical bonds between the coating and the substrate (saponification reaction), causing the coating to peel off from the substrate. The aging (oxidation) of the coating itself further reduces its alkali resistance, accelerating this process.
4.Can defects in the coating itself or improper application cause blistering?
Process: During production, air bubbles and impurities within the coating, or inadequate pretreatment of the substrate (oil, dust), can lead to inherently weak adhesion in certain areas.
Cause of blistering: In outdoor use, temperature changes cause air or trace amounts of moisture at these weak points to expand, directly lifting the coating. In this case, blistering will occur even if the coating has not yet shown significant oxidation.
5.What is coating oxidation/aging?
Under the long-term effects of ultraviolet radiation, heat, moisture, and chemicals, the resin in the coating degrades, manifesting as:
Loss of gloss and fading
Powdering (the coating surface becomes powdery)
Becoming brittle and losing elasticity