Use of inorganic zinc in construction

by: Abel De la Cruz*

Showing the constant dynamics in the development of new products in our region, we focus our attention on one of the first of the widest application and use in high durability painting systems for the corrosion protection of surfaces exposed to environments of high atmospheric corrosivity (C5M: ISO 12944-2) and immersion (Im1, Im2, Im3: ISO 12944-2).

These are inorganic zinc coatings, such as solvent-based zinc ethyl silicate, which presents known application difficulties to achieve wide use and coverage in the anticorrosive treatment of steel surfaces in new construction works.

However, thanks to the launch in Peru, four years ago, of a new generation of coatings called modified Inorganic zinc, hybrids or "Reinforced" equivalent in performance to zinc ethyl silicate, but with friendly application characteristics, its diffusion through specifications, use and application in new construction projects has been progressively increased, under the objective of generating greater return on investment, an increase in the useful life cycle of the infrastructure, lower corrosion losses and cost savings in maintenance costs over time.

Inorganic zinc-based coatings are considered polymers that contain zinc powder as a pigment in an inorganic vehicle. This type of paints protect by galvanic action, zinc is highly active and protects at least active as carbon steel, making the function of sacrifice; for this to happen, an electrical continuity is necessary, which requires a high level of cleanliness to ensure an intimate contact between the Zinc and the steel substrate.

Types
According to the SSPC Paint Specification N° 20, the first rich in zinc of the Inorganic type defined there are three:

1. Type I-A: Post Healing Inorganic Vehicles, Water-based Alkali Silicates.
2. Type I-B: Auto Healing Inorganic Vehicles, Water-based Alkali Silicates.
3. Type I-C: Auto Curing Inorganic Vehicles, Solvent-based Alkyl Silicates.

Due to the content of volatile organic compounds (VOCs), conventional Alkyl Silicates release large amounts of solvents in drying and curing (by evaporation of solvents and by the hydrolysis reaction), also presenting difficulties to be applied in confined spaces. However, this characteristic has been modified and improved in the "reinforced" inorganic zinc making it compatible with the environment due to its low level of VOCs, almost 20% less emissions than conventional zinc ethyl silicate. As for the yield, this has been increased due to the higher content of solids in volume (80% approx.) in its formulation, improving its price for each m2 of coated surface compared to conventional inorganic zinc.

The first inorganic zinc are used in the different industrial sectors, mining, oil, gas, energy, transport, chemical, etc. They are also employed in the marine industry to protect the exterior of vessels, buried platforms and in applications where high abrasion resistance is required. To protect carbon steel, it is normally applied to a dry film thickness of 3.0 mils, presenting an excellent and proven performance over the years, depending on the degree of corrosivity of the exposure environment.

The presence of the silicate-based inorganic matrix among the individual zinc particles improves the chemical resistance of the first zinc against galvanizing. Exposure to strong acids and alkalis requires an adequate finishing barrier layer with high chemical resistance and low permeability, they can usually be of the epoxy or epoxy-siloxane type. The resistance to solvents and organic chemicals, including oils, fuels, is high, so it is successfully used to coat the outside and inside of storage tanks.

Other features

Good practice requires that before cleaning with abrasive jet a cleaning with tensile-actives or biodegradable detergents and water must be applied to ensure the absence of any residue of grease or oil and achieve adequate adhesion. The specified roughness must be between 1 and 2.5 mils anchor profile.

Since an adequate application begins with the proper mixing of the components, one of the difficulties presented by conventional inorganic zinc (Ethyl silicate) consists in the lack of ease for its homogenization and mixing of zinc powder and resin, generating agglomerations and the lack of an intimate moisturizing contact between the binder and the pigment, which causes application defects such as dry spray. On the other hand in many formulations its application to dry film thicknesses greater than 5.0 thousands is critical and generates the potential risk of cracking ("mudcraking") or forming "crocodile skin", which is a superficial phenomenon and the defective film must be completely removed, before repainting.

The new formulation of the inorganic zinc type "Reinforced" on the other hand, is constituted by two liquid components (resin), in one of which the zinc powder is incorporated, its mixture is easier and faster, as well as its homogenization, minimizing the losses of zinc powder. The wetting capacity of the resin on the pigment is superior and the risk of cracking is minimized when it is generated on film thicknesses of 6.0 mils or 8.0 mils during its application.

If during the curing of the first inorganics rich in ethyl silicate type zinc there is a low relative humidity, for example less than 60% RH, there is a risk that the product cures insufficiently, so it is necessary to control the repainting time, by means of the resistance test to rubbing with solvent: ASTM D4752, Test Method for Measuring MEK Resistance of Ethyl Silicate Zinc Rich Primers by Solvent Rub, to know if inorganic zinc is suitable to receive the finishing layer. If the finishing layer is applied on a first rich in zinc that has not cured sufficiently, this finishing layer prevents contact with the moisture of the medium, necessary for the proper curing of zinc ethyl silicate, causing the loss of adhesion of the finish.

In the case of the new "reinforced" inorganic zinc the chemical reaction mechanism for the constitution of the polymer matrix in the film curing process does not require the presence of moisture consequently it can be repainted in less time (1 1/2 to 2 hours at 21°C) without risk of detachment of the next finishing layer, compared to zinc ethyl silicate which requires a minimum curing time of 24 hours when the ambient temperature is 21 °C and 60% RH with good air circulation.

Conventional inorganic zinc film (Ethyl silicate) is porous and this porosity may vary depending on the environmental conditions during application and the application technique. The porosity causes air to be trapped when another layer of paint is applied and when evaporated it can generate bubbles or pinholes just after application.

To avoid this, it is recommended to apply a thin layer, called mist or "mist coat": First the thin layer will be applied to fill the pores of the zinc silicate and a few minutes later the general layer is applied to the recommended thickness. In the case of "reinforced" inorganic zinc, the excellent wetting of zinc powder by the vehicle means that no air spaces are formed in the structure of the paint film so it does not require "mist coat" or another technique to prevent the formation of bubbles.

Finally this innovation of inorganic zinc-rich paints has allowed not only that the discovered parts or damage of the dry film can be repaired ("touch-up") with the same coating but also can be repainted with an additional layer of inorganic zinc in case the specified thicknesses have not been reached, unlike zinc ethyl silicate, where the retouches or repairs are recommended to be made with the organic zinc epoxy-polyamide to achieve adequate adhesion.

* INSTRUCTOR authorized by ASTM in Protective Coatings. General Manager- Principal Consultant AmericanConsultPerú, Quality Auditors. E-mail: [email protected] Http://www.americanconsultperu.com