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Coating Anomaly Case Analysis: Cracking observed after applying a zinc-rich inorganic primer on pipes!
Member "myarc" posted a message on the forum:
The zinc-rich inorganic primer on the pipes is exhibiting the following conditions, with an outdoor temperature of 31°C.
See the following image:
Some users on the forum have also provided feedback and analysis. The following representative responses are listed below:
User 1:
The primary cause of coating cracking is over-spraying, high ambient temperatures, and failure to implement the standard wet-on-wet process between layers. Controlling these factors can effectively prevent cracking defects.
User 2:
based on the crack appearance, this is not zinc-rich inorganic paint. Zinc-rich inorganic paint has very low shrinkage and sets quickly after spraying, resulting in immediate cracking due to inherent micro-cracks. The minimum surface preparation requirement for the substrate (e.g., steel) should be Sa2.5 or better. From the image, we can see:
1、The cracks are large and exhibit a chronic, tensile cracking pattern, which does not conform to the characteristics of inorganic zinc silicate paint. See icon 3-4.
2、From the cracked paint surface, it can be observed that the lower substrate has not been properly prepared. See Figure 1-2.
3. The white color at the crack opening indicates the presence of a steel substrate.
User 3:
The bottom color is uneven due to cracking. Was an inorganic zinc-rich paint applied on top of another coat of paint? Was it applied too thickly? Or was the mineral filler in the paint slurry added in excessive amounts?
User 4:
The humidity level should be 50% or higher. If necessary, water should be sprayed on the site to increase humidity. The thickness should not exceed 110-120 micrometers.
Ten Birds Analysis:
Cracking in inorganic zinc-rich primer coatings is primarily caused by shrinkage of the coating film during drying and curing. This cracking resembles the cracking of soil exposed to direct sunlight, and is commonly referred to as "soil cracking." The causes of cracking can include:
1) The paint film was applied too thickly, exceeding the manufacturer's specified maximum paint film thickness.
2) The surface roughness is too low, especially on welds, making it difficult to blast clean due to the high hardness, resulting in insufficient roughness.
3) When the relative humidity is high and ventilation is poor, the surface layer solidifies too quickly, which can also cause cracking of the coating film.
4) The product has exceeded its shelf life, which consequently compromises the coating quality.
The safe coating thickness range should be determined based on the formulation of the inorganic zinc-rich primer, which may range from 50 to 150 micrometers. Many coating manufacturers specify a maximum safe coating thickness of 100 micrometers or less. During actual application, most products are prone to forming cracks at structural corners. When attempting to achieve the minimum film thickness requirement, painters often apply multiple coats to the corners, resulting in an excessively thick coating. This, combined with uneven shrinkage of the coating at the corners, easily leads to cracking. In complex structures, frequent use of small nozzles exacerbates the problem. It is recommended to use larger nozzles to mitigate this issue. Additionally, if an excessively thick coating is observed, it can be quickly smoothed out with a brush.
How to prevent cracking in inorganic zinc-rich coatings?
1、The substrate should be cleaned to at least Sa2.5 level and have a certain degree of roughness (typically Ry5 = 40-100 microns) after spraying.
2、 Strictly control the coating thickness to not exceed the maximum dry film thickness specified by the coating supplier.
3. Wet-curing zinc-rich coatings require a hydrolysis reaction with moisture from the external environment to form a strong, continuous coating film. Therefore, a certain level of humidity (typically a relative humidity of not less than 50%) is required. In dry, hot environments, the risk of coating cracking increases.
4、While spraying, continuously stir. Otherwise, the zinc powder will settle to the bottom, altering the paint's mixing ratio. This can cause the resin and zinc powder to separate, leading to cracking in the coating.
5. It is not recommended to apply a second coat of zinc-rich primer. If the thickness of the zinc-rich coating does not meet the design requirements, it is not recommended to apply a second coat of zinc-rich coating to the specified coating thickness. Improper application can easily lead to layering and cracking between the two layers.
Article sourced from the WeChat public account "Industrial Coatings Engineer"