Zinc-rich paints, Zinc Sprays, Zinc primers and "Zinc metal coatings" have all been around for many years. As with all products, some are better than others are and some are made more cheaply than others are. They do tend to share one particular trait, however, in that they have all used zinc-dust to provide both pigment and protection.
UNTIL NOW.
There is a new form of zinc pigment that provides at least comparable cathodic protection, with a lower zinc content, to the best of the conventional zinc products in the market. This factor alone can provide real advantages for the specifier, user and ultimately the end receiver of the finished product, the customer.
The pigment form (you have probably guessed by now) is zinc-flake. The benefits of its use may not be so immediately obvious. Following are listed some of the main advantages available from this new technology with some comparisons to the performance of conventional materials made with zinc dust.
The platelet-like structure of zinc flake provides a much larger surface area than spherical zinc dust (see diffusion route schematic). This leads to a greater binder uptake, which leads in turn to such favourable aspects as: -
- Higher current density (improved electrical conductivity)
- Greater flexibility
- Improved cohesion, combined with better substrate adhesion
- Low porosity and permeability
- Better overcoating properties
It has long been argued that to provide full cathodic protection, a zinc rich product had to contain minimum 90% zinc in the dry film (this being measured by weight not volume). In many respects this was true, particularly as this type of protection requires electrical contact between the zinc particles themselves as well as the substrate.
Zinc is one of the heavier metals, with a specific gravity of 7. Even so this level of zinc means that the amount of resin available for holding the coating both together and to the substrate is minimal. At this level there is no room for gap-filling between particles. This in turn leads to porosity in the coating that is easily penetrated by water and salts. Traditionally the resultant corrosion (zinc salts) was relied upon to provide the remaining protection. The zinc salts, however, tend to interrupt the electrical flow between the zinc particles and the substrate, although some protection by the barrier method may be maintained.
Once penetrated, the cohesive strength of the coating can suffer as the resultant corrosion takes up more space than is available within the confines of the film. If this has been overpainted the adhesion of the top coats will be affected by the mushrooming corrosion below, encouraging further penetration of the ruptured topcoats by water and salts.
With zinc-flake pigmentation many of the above problems are removed. The full two-dimensional contact between the zinc platelets give a higher current density which improves the cathodic nature of the product. Whilst this can lead to a higher erosion rate for unprotected flakes, anti-corrosion pigments can be added to the formulation as the level of zinc in the dry film is less critical.
Allowing for a greater amount of resin/binders in the film, the zinc particles are better protected from corrosive influences thus reducing the problem at source. The zinc flakes themselves tend to lay down and overlap each other, improving the barrier protection of the coating still further. The addition of corrosion inhibitors to the product also affects the potential for corrosive influences to impact on the coating.
This all leads to a product, which not only out-performs the leading products in the marketplace, but makes it capable of solving coating problems that have traditionally been reserved for products such as cadmium and nickel plating. Indeed there is already a British Standard for this type of application on fasteners and no doubt more will follow.
