Zinc(III) powder-based paints

by: Carlos A. Giudice and Andrea M. Pereyra

In the last edition we dealt with the uses of coatings and primers based on zinc in the protection of iron or steel surfaces. This time we will see the influence that the type of pigment used has on the properties of the coatings, as well as how to avoid a critical concentration of pigment.

Influence of pigment morphology on general properties
Regarding the critical concentration of pigment in volume, the first anticorrosives exhibit the best behavior at PVC values equal to or slightly lower than the CPVC. It is observed that those based on laminar zinc have significantly lower critical values than those made with spherical zinc (15 to 20 % depending on the type of binder); the higher rate of oil absorption of laminar particles (higher specific area) justifies these results so the consequent cost reduction of the use of this type of pigment in replacement of the spherical can be significant.

Likewise, the first formulated with spherical zinc and pvc lower than the CPVC show an abrupt drop in their anticorrosive performance while those with laminar zinc maintain their efficiency. This can be explained because the corrosion products of zinc on spherical particles not only increase electrical resistance but decrease the effective zinc content since the center of the particles would be electrically isolated (polarization).

Visual observations from experimental trials, particularly in areas adjacent to cutting, indicate that laminar zinc-based primers generally exhibit more white zinc corrosion products than those formulated with spherical. Thus, it is suggested that laminar morphology provides an overly active anode even at volume pigment concentrations much lower than the critical one and is consequently unnecessarily worn.

Considering the resistance to blistering, formulations with laminar zinc as the only pigment (for all binders considered), show better performance, particularly to PVC slightly lower than critical. This disadvantage is a consequence of the high galvanic activity of this type of pigment (large amount of soluble salts) that leads to osmotic phenomena responsible for blistering in the 100% Relative Humidity Chamber.

In summary, the primers based on laminar zinc can be formulated to PVC significantly lower than those with spherical zinc, with excellent corrosion inhibitor performance but with reduced blistering resistance. These results support the study of primers that, in addition to having laminar pigment in their composition, contain inert pavers, in order to increase resistance to blistering without affecting the excellent anticorrosive capacity. In addition, it has been found that in general the use of this type of loads reduces the corresponding CPVC and consequently the appropriate PVC. The magnitude of the decrease in CPVC in modified primers depends on the type of paver (more significantly as the oil absorption rate increases) and the level of zinc replaced (lower laminar zinc/extendedr ratio leads to lower values).

On the other hand, a significant improvement in the results obtained in the aforementioned trials is obtained with the incorporation of inhibitory pigments in the formulation of the primers. Thus, the replacement of zinc by 10 or 15% by volume of zinc phosphates, metal thanates, zinc phosphomolybdate and barium metaborate reveals a higher efficiency compared to paints with single pigment.

The tripigmented systems in general present the best performance considering experimental results obtained in Salt Spray Chamber and 100% Relative Humidity Cabin.

It should be noted that resistance to blistering involves an average of the rating corresponding to size and the following values can be assigned to the frequency (no blistering, 10; little, 7.5; medium, 5.0; medium dense, 2.5 and dense, 0.0).

Finally, the first ones with the best performance (higher sum value) in the Salt Spray Chamber and in the 100% Relative Humidity Cabin have the following composition:

- Binder: ethyl silicate and sodium or potassium silicates (of similar efficiency), plasticized chlorinated rubber, epoxy resin, vinyl copolymer and unsaturated polyester, in that order.
- Type of zinc: laminar-shaped particle.
- Inhibitor pigment: zinc phosphate, metal thanates, zinc phosphomolybdate or barium metaborate.
- Zinc / understandr / inhibitor ratio: 65 / 25 / 10 in volume.
- PVC range: 25.0 to 32.5 %.

Finishing paints
The paint film formulated with metallic zinc, both rich in zinc and modified with expanders and/or inhibitory pigments, exposed to strongly aggressive media, needs a finishing paint to control the reaction of metallic zinc with water, water vapor, oxygen, carbon dioxide, etc. from the environment.

Zinc-based film has, at the initial moment of exposure, occluded air; if the finishing paint does not have adequate surface tension in the wet film stage or if it dries too quickly, the concatenated air causes small craters usually called "pinholes" when it comes out. In this case the film is not yet contaminated with aggressive products and the application of the finishing paint requires only a slight cleaning with steam or pressurized water.

On the other hand, the former with a long period of exposure in service present a degree of contamination that is difficult to evaluate, particularly in superstructures in which a high amount of soluble zinc salts and atmospheric pollutants may be arranged inside their porous structure.

Conclusions
In summary, the best performance for each binder using laminar zinc is obtained by selecting a high oil absorption index extender as a "spacer" of zinc particles and a suitable corrosion inhibitor pigment; for this it is very important to carefully define the PVC and the composition of the pigment (zinc / understandr / inhibitor pigment ratio).

Very low levels of laminar zinc calculated on the dry film (approx. 20% by volume in the CPVC) are sufficient to achieve an effective cathodic protection of the substrate and also a controlled galvanic action that prevent the formation of large amounts of soluble zinc products causing the formation of blisters through osmotic phenomena.

Likewise, the reduced sedimentation capacity of laminar zinc in the container, from its incorporation and until the moment of its application, allows an easy and fast homogenization by agitation avoiding the generation of films of heterogeneous composition that generally lead to a poor performance in service, due to the presence of areas of PVC superior to the CPVC that present poor film properties and areas with PVC lower than the CPVC that generally exhibit blistering.

Finally, the results indicate that the first ones based on laminar zinc and modified with suitable pavers and inhibitor pigments are more economical and very versatile since they can be formulated in a wide range of PVC without modifying their properties.

Thanks
The authors thank CONICET (National Council for Scientific and Technical Research), CICPBA (Scientific Research Commission of the Province of Buenos Aires) and UTN – FRLP (National Technological University Regional Faculty La Plata).

Bibliography
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