Inorganic zinc-rich primer, organic zinc-rich primer technical requirements
1, zinc-rich primer evaluation
1.1 Anti-corrosion mechanism of zinc-rich primer
Primer on the corrosion resistance of the metal generally has three mechanisms.
One is to isolate the role of the environment (also known as physical coverage), it is painted on the metal surface to form a coating to prevent the metal corrosion caused by oxygen, moisture and harmful ions invasion;; the second is the role of corrosion inhibitors.
The second is the role of corrosion inhibitors, such as reddish primer, which reddish PbO and oil-containing media oxidation derived from low molecular acid will react to generate a strong corrosion inhibition of lead soap; and then for example, chromate primer, which pigments and water contact that is dissolved chromate ions, and its strong oxidation of the metal surface passivation, resulting in a very good corrosion inhibition effect.
Another rust prevention mechanism of primer is the cathodic protection of zinc-rich primer. As the name suggests, zinc-rich primer in the zinc powder content is very high can be as high as 90% (weight ratio). So in its dry film between zinc powder particles, zinc powder and steel substrates are in contact with each other, in this state, when the water intrusion into the film, zinc powder and steel substrates that is the formation of microcells, because zinc in the electrochemistry is more than the iron active metal, so from the zinc powder to the steel flow through the corrosion-resistant electric current, so that the steel to play a cathodic level of protection, which is zinc-rich paint corrosion protection mechanism. Theoretically, aluminum powder and magnesium powder can also formulate such primer in fact, only zinc powder is in line with the necessary conditions of cathodic protection in the paint. The surface of aluminum and magnesium powders tends to form an oxide film with low electrical conductivity, which excludes the metal/metal contact that is essential for cathodic protection.
A high content of zinc powder in the dry film may not result in excellent corrosion protection. The zinc powder content varies depending on the corrosive environment and the combination of coating systems.
The results of one year’s exposure of various zinc-rich primers in the tide differential zone in the Florida ocean by D.E. floyd et al. As can be seen from the graph, the corrosion protection is good at 80% zinc powder content.
In addition, in the water wetting serious place, the coating film in the dissolution of zinc powder is more powerful, this time the zinc powder content of large easily lead to paint film blistering, therefore, should be used in zinc-rich paint with a low content of zinc powder, generally not less than 70 %.
It must be pointed out that zinc is an anode for iron at room temperature, but at a certain temperature range (76.7~104.4)°C, zinc becomes a cathode for iron, which in turn corrodes the iron and “protects” the zinc.
This situation is often encountered in the heat insulation pipeline, this pipeline if coated with zinc-rich paint, plus the presence of moisture, not only can not protect the steel pipe, but will make the steel pipe to occur very deep hole corrosion.
Translated with DeepL.com (free version)
