A brief discussion on the adhesion problem of galvanized sheets
1. The hot-dip galvanized layer of galvanized sheets has poor adhesion to itself;
2. Gas penetrates into the hot-dip galvanized layer and expands during baking, causing the polyurethane coating to fall off;
3. The hot-dip galvanized layer is thicker and the zinc layer crystals are coarser, which affects the normal production of the phosphating film;
4. As the zinc layer AL melts, AL3+ accumulates in the phosphating treatment tank, making the crystals of the phosphating treatment film coarse;
5. Metal zinc has strong specificity and reactivity, which reduces the viscosity of the coating;
6. The secondary reactant of zinc (basic double salt) dissolves in water. During painting and baking, the water in the coating reacts with the salt on the wall or material, which affects the adhesion;
7. Zinc, like micro-charged batteries, can produce rusty substances, causing the volume to expand and affecting the adhesion;
8. Zinc is relatively stable at a PH value of 9 to 11 points. If it is below or exceeds this range, the zinc will dissolve and dissolve, affecting the adhesion;
9. Zinc reacts with the oleic acid in the coating to form metal soap salts that are soluble in water, making the coating brittle or detached;
10. After hot-dip galvanizing, the surface layer is too smooth and the surface roughness is insufficient, which reduces the adhesion of paint.
To do hot-dip galvanizing spray painting, the process is first passivation treatment, phosphating treatment, surface cleaning, spray paint topcoat, and spray paint. These steps are all necessary. Failure to do any step well will affect the adhesion of coatings such as powder or paint.
Both passivation and non-passivation after hot-dip galvanizing will cause poor adhesion after painting. Because the film layer caused by passivation is water-containing, non-passivation will cause zinc oxidation problems. These two The problem is caused by poor adhesion after painting. The solution is hot-dip galvanizing and then phosphating or pottery and then painting.
1) It is stipulated that the galvanizing factory shall not passivate stainless steel after hot-dip galvanizing;
2) Spraying begins within 24 hours after hot-dip galvanizing;
3) Before painting, use P80-120 sandpaper for rough polishing, as long as the surface is rough (be sure to use a pneumatic grinder, as the drilling capacity of pneumatic tools is too large);
4) Appropriate pre-treatment (we choose ultrasonic cleaning to remove oil, pickling and phosphating) is very strong
If hot-dip galvanizing is cost-effective, it will strengthen the fine grain of the wall or material. If it is carbon steel, if you take a cross-section from the surface to the inside, it should be: 99% zinc - aluminum alloy of 96% zinc + 4% iron - aluminum alloy of 90% zinc + 10% iron - --Aluminum alloy of 75% zinc + 25% iron
The quality of hot-dip galvanizing is affected by the following factors only in terms of the degree of fine grain strengthening:
1. The quality of the dissolving agent after pickling and passivation. The dissolving temperature is generally around 60 degrees.
2. The temperature of the zinc pot. The temperature of the zinc pot is generally between 450 and 470 degrees. However, because zinc pots are very expensive and ultra-low temperatures will increase the service life of the zinc pot, manufacturers generally control it around 430 degrees. The lower the temperature, the worse the fine grain strengthening.
3. The hot-dip time is shorter. The longer the hot-dip galvanizing time, the rougher the surface layer, the greater the zinc content, and the lower the fine grain strengthening.
4. Refrigeration. Of course, cooling is good, but manufacturers usually use water to chill. This meets the needs of the production rhythm and makes the surface smoother, but the fine-grain strengthening will also be terminated.
From the perspective of project practice, most residential owners or paint dealers require sand sweeping and sand sweeping. They cannot accept hot-dip galvanizing and spray paint directly without any treatment. The reason is very simple, no treatment has been done. The quality of the coating cannot be guaranteed afterwards.
It is understandable that the worn hot-dip galvanized surface should be sanded and then painted, because stains and oxidation will affect the adhesion. But in any case, this kind of metal surface treatment cannot be at the expense of affecting the anti-corrosion ability of the normal hot-dip galvanized layer. For the new hot-dip galvanized surface, there is no such problem, so sand sweeping is not required for metal surface treatment.
Hot-dip galvanized parts are generally passivated using chromate solution, which affects the adhesion of the subsequent coating. For prefabricated components that need to be painted after hot-dip galvanizing, it is best not to do stainless steel passivation.
In addition, before painting hot-dip galvanized parts, it is best to use non-metallic wear-resistant materials to sweep sand to improve the adhesion of the subsequent coating.
It is important to note that if the pH value of the solution is high, the adhesion will also be relatively reduced with strong alkalinity.
