Anticorrosive paints based on metallic zinc powder (II)

by: Carlos A. Giudice and Andrea M. Pereyra*

In issue No. 13 -6 we learned the generalities of zinc-based paints and the first zinc-based metallic paints. On this occasion, the authors delve into the use of these coatings for the protection of surfaces such as iron and steel.

Remember that paints rich in zinc and those modified with pavers and / or pigments inhibitors of metal corrosion, have a greater efficiency in relation to other types of coatings. Considering the concept of sacrificial anode (cathodic protection), paints have been formulated consisting of high concentration dispersions of high purity zinc powder in organic and inorganic vehicles. In these materials, when they are applied in the form of a film, there is an intimate contact of the particles with each other and with the base or metal substrate that you want to protect.

Testing of the first ones based on metallic zinc
Determining the in-service behavior of the dry film of zinc-rich primers requires accelerated laboratory testing. Formulations are usually applied on SAE 1010 steel panels of approximately 100 x 150 x 1 mm, sandblasted to grade ASa 2 1/2 (SIS 05 59 00/67) and degreased with toluene vapour; the maximum roughness ranges from 40 mm. The thickness of the only layer of the first should be approximately 75 to 80 mm and is made with a brush or torch, being convenient the protection of the edges by immersion in a solution of a stretched acrylic resin.

The evaluation of the behavior of protective paints requires that the properties valued are representative of the requirements to which the product in service will be subjected and that they also present an increasing and decreasing evolution depending on the PVC (concentration of pigment in volume) to optimize the formulation. Thus, for example, corrosion resistance and blistering tests in suitable chambers can be selected for metal zinc-based primers.

Corrosion resistance: degree of oxidation
After the application of the first on the panels, an X-cut is made on the painted surface and they are exposed for different periods, depending on the formulation, in the Salt Spray Chamber, ASTM D 117 (35 ± 1 ºC; pH 6.5 to 7.2; continuous salt spray of a sodium chloride solution at 5 ± 1 % by weight), Figure 4.

Method A consists of the evaluation of the degree of corrosion in the X-cut; rating

Resistance to blistering: degree of blistering
The painted panels are displayed for variable periods in 100% Relative Humidity Chamber (ASTM D 2247) to assess resistance to the formation of blisters of osmotic origin.

The size of the ampoules is also arbitrarily described on the numerical scale 10 to 0, in which 10 represents absence of blisters. The frequency is defined by the letters D (dense), MD (medium dense), M (medium) and F (little). The pattern photos present in the Standard allow the comparative qualification of the experimental panels, Figure 7.

With regard to the relative behaviour in the salt spray chamber of the first zinc organics based on different binders and pigmented in the same way with PVC values close to the CPVC, tested during the period necessary for signs of corrosion to appear, it is observed that the best results correspond in general to plasticized chlorinated rubber, followed by epoxy and vinyl and finally unsaturated polymers. On the other hand, in the cabin of 100% relative humidity and adopting a criterion similar to that used to evaluate the anticorrosive capacity of these primers, the experiences with respect to the formation of blisters indicate in general a better behavior of the epoxy products followed by the three organic binders considered.

Critical values of PVC
The critical concentration of pigment in volume can be estimated through the oil absorption index or by evaluating different properties of the film such as density, tensile strength, adhesion stress, porosity, permeability, corrosion, blistering, wet abrasion, staining and various optical properties (light scattering, opacity, dyeing power, brightness, etc.).

The highest CPVC values are usually obtained with chlorinated rubber (polar binder), followed by epoxy and vinyl resin (partially polar) and finally unsaturated polymer (non-polar). Considering as a reference the partially polar binders, the rubber exhibits a CPVC approx. 20% higher while the unsaturated polymer 10% lower.

For inorganic binders, the higher critical concentration of pigment by volume is generally obtained with organic silicate as alkalines have lower wetting properties. Taking chlorinated rubber as a reference, the first ones based on ethyl silicate have a similar CPVC while sodium silicate exhibits a lower value, approximately 5%.

One of the most distinctive features that make attractive the use of paints based on both organic and inorganic silicates with respect to those that include resins in their composition are the excellent resistance to exposure to light and water, which gives it a longer service life (greater resistance to chalking, cracked, cracked, scaling, etc.) due to the mineral characteristics of the film formed. On the other hand, paints formulated with metallic zinc and based on organic silicates have a higher cost than those with organic binders while inorganic silicates make up the most economical film-forming material.

A general disadvantage of the former that include silicates as a binder is that they exhibit a lower ease of application than those containing organic binders; particularly alkaline silicates require a higher demand in the preparation of the surface due to their reduced wetting capacity of the metal substrate.

* UTN-FRLP, Universidad Tecnológica Nacional Facultad Regional La Plata. CIDEPINT (CIC-CONICET), Center for Research and Development in Paint Technology