New corrosion inhibitor for high performance coatings

In mid-2013, NACE International conducted a study demonstrating the high cost of corrosion in the United States, which has risen to more than $1 trillion per year in the United States due to the costly and enormous challenges of corrosion. Corrosion is the largest single expenditure in the US economy and is estimated to exceed 6.2% of GDP. Corrosion has brought a very high price tag to the United States, second only to health care.1 A study funded by the US government in 2001 estimated that the cost of corrosion for military systems and infrastructure alone would be about $20 billion per year2. Most of the losses are due to corrosion of steel used in the manufacturing process of roads and bridges, pipelines, storage tanks, automobiles, ships, sewer systems. The cost of corrosion is money and life, which can lead to dangerous failures and increase the cost of each application, from utilities to transportation.

In addition to the main advantage of providing an improved appearance, organic coatings also play a vital role in preventing corrosion of substrates. Paint formulators use three basic strategies to provide corrosion protection to metal surfaces: 1) the coating acts as a barrier to prevent oxygen and water from reaching the metal surface; 2) the use of corrosion inhibitors and pigments to passivate the metal surface ; 3) Protected by sacrificial metal.

The barrier coating reduces permeability and prevents oxygen and water from reaching the surface of the film. Barrier properties are often imparted by the use of flake (mica) additives such as flaky talc, mica and micaceous iron oxide and flake metallic pigments such as aluminum pigments.

Zinc-rich primers provide a good example of corrosion protection, at least part of the cathodic protection coating. The metal zinc incorporated into the coating film serves as an anode and preferentially corrodes than the iron substrate because zinc has a lower oxidation-reduction potential than iron, thereby providing cathodic protection to the iron surface.

The use of anti-corrosion pigments is one of the most widely used methods to improve the corrosion resistance of coatings. The anticorrosive pigment reacts with the water absorbed by the coating film to release inhibitory ions which migrate to the metal surface and passivate the metal surface by deposition or adsorption of the inorganic layer on the metal substrate. Metal salts based on zinc, antimony and lead chromate have been used to suppress corrosion because their cations form insoluble deposits on the metal surface to add a protective layer to inhibit further corrosion. Because of their toxicity, environmentally safe, non-toxic anti-corrosive pigments have been developed to replace chromate. These non-toxic pigments are usually based on metal salts of phosphosilicates, phosphates, borates and metaborates. Unlike chromate, the pigment function of these anode parts is achieved by limiting the diffusion of oxygen to the metal surface, so the effect of the anode passivating metal surface may not be fully effective, resulting in poor corrosion resistance.

Regulatory issues will continue to be the driving force for some time to come, especially corrosion inhibitors. Recent notable developments include the European Union (EU), which will contain zinc oxide and phosphate products labeled with special hazard labels, and the Occupational Health and Safety Administration (OSHA) to reduce the allowable exposure limits for hexavalent chromium inhibitors.

The addition of organic corrosion inhibitors to coatings is another way to improve the corrosion resistance of coatings. These organic corrosion inhibitors are based on various chemical compounds including amines, aromatics, heterocycles, carboxylic acids, sulfur and nitrogen containing functional groups. The effect of these corrosion inhibitors is achieved by passivation of the anode or cathode on the metal substrate, or by forming a protective layer on the metal surface, which can disrupt the flow of corrosive ions on the substrate.

Some new corrosion inhibitors have been developed that impart excellent rust resistance in a variety of aqueous and non-aqueous high performance coatings. These new organic corrosion inhibitors can be used as primary corrosion inhibitors or in combination with environmentally friendly anti-corrosion pigments. They are compatible with a wide variety of resins used in primers and direct coating to metal coatings (DTM) used in a variety of industrial applications. This article describes some of our recent attempts to improve the corrosion resistance of various coatings using these new corrosion inhibitors.

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The current dominant trend in the coatings market is the absence of primers in high performance coating systems and the use of reinforced topcoats in DTM applications. Since no primer is used, this method saves the costs associated with primer construction and reduces overall costs. The disadvantage is reduced corrosion resistance and adhesion provided by the primer. The addition of non-toxic, anti-corrosive pigments to DTM coatings improves the corrosion resistance of the coating, but corrosion resistance is generally not as good as the primer system. The solution to this deficiency is to add an organic corrosion inhibitor to the DTM coating while adding a non-toxic anti-corrosion pigment.

In the past, organic corrosion inhibitors used in primer systems were based on phosphates, sulfonates and carboxylates. These chemicals work well, but they are often found to be defective when used in DTM coatings. The development of DTM coatings with improved corrosion resistance requires the use of stronger organic corrosion inhibitors that work synergistically with non-toxic anti-corrosion pigments.

In order to solve this problem, some new liquid corrosion inhibitors have been developed, which are metal salts based on complex organic acids. These inhibitors provide improved corrosion resistance and wet adhesion when used alone and in combination with non-toxic, anti-corrosive pigments. When applied to solvent-based two-component polyurethane systems in which acrylic or polyester polyols are crosslinked with aliphatic isocyanates, we have found these products to be particularly effective.

Two component polyurethane DTM coating

To demonstrate the efficacy of these novel corrosion inhibitors, they were used in various DTM coatings and compared to more conventional organic corrosion inhibitors. In the first formulation, a white two-component polyurethane coating was prepared from an acrylic polyol and crosslinked with a polyol with an aliphatic HDI trimer. The ratio of pigment to binder prepared by the system is 1:1. The system contains 5% (mass ratio) of antimony, zinc, and phosphosilicate anticorrosive pigments. The coating was modified with a 2% (by mass) conventional metal sulfonate corrosion inhibitor and 2% (by mass) of a novel metal complex inhibitor, NACORR® XR-424. Two samples were compared to a control plate containing only anti-corrosive pigments but no organic corrosion inhibitor. A coating having a dry film thickness of 1.5 to 1.7 mils was applied on a 1000-bond iron plate treated with phosphate and then cured at room temperature for 7 days. The panels were then placed in a salt spray box and exposed for 500 hours in accordance with ASTM Test Method B117. Immediately after removal of the exposed panels from the cabinet, scratches were performed using a metal spatula as described in 7.2 of ASTM D1654. As can be seen in Figure 1, the new corrosion inhibitor provides better wet adhesion and rust protection than conventional organic inhibitors and controls using only anti-corrosive pigments.

In the second formulation, a white two-component polyester polyol formulation with more pigment was formulated to achieve a pigment to base ratio of 1.5:1 while adding 5% (by mass) of rust preventive pigment. In this case, the anticorrosive pigment is a pigment of the calcium silicate type. The formulation was cross-linked with the HDI trimer again and modified with a 2% (by mass) conventional metal sulfonate corrosion inhibitor and 2% (by mass) of a novel metal complex inhibitor, NACORR 1389MS. Sex. Two samples were compared to a control plate containing only anti-corrosive pigments but no organic corrosion inhibitor. A coating having a dry film thickness of 1.4-1.6 mils was applied to the Bond board and then cured at room temperature for 7 days. The panels were then placed in a salt spray box and exposed for 500 hours in accordance with ASTM Test Method B117. The exposed plate was scratched with a metal scraper. It can also be seen that the use of new corrosion inhibitors can improve corrosion resistance compared to controls using traditional organic inhibitors.

Aqueous thermosetting system

Although these new corrosion inhibitors were developed for DTM coatings without primers, they can also be used in primer systems. An aqueous thermosetting iron red primer was prepared by crosslinking a dilutable polyester with hexamethylol melamine hexamethyl ether. The formulation uses iron oxide red as the primary rust preventive pigment and zinc silicate bismuth as the second anticorrosive pigment in an amount of 2.5% of the total formulation quality. The pigment to binder ratio is 1:1. The formulation was treated with 2% conventional organic inhibitor and 2% novel metal complex inhibitor, NACORR XR-419. The two samples were again compared to a control plate using only anti-corrosion pigments. The coating was applied to a 1000 Bond plate to give a coating sample having a dry film thickness of about 1.0-1.2 mils. The panels were then cured in an oven at 150 ° C (300 ° F) for 15 min. The sample was then placed in a salt spray box for 500 hours. The exposed sample was taken out and examined. As can be seen in Figure 3, the performance of the novel corrosion inhibitor is superior to the standard inhibitor compared to the control formulation.

in conclusion

This new generation of liquid organic corrosion inhibitors can be used to enhance the performance of non-toxic, anti-corrosive pigments in direct coating to metallic coatings (DTM). They work synergistically with these pigments to improve corrosion resistance, allowing formulators to use primers to meet less demanding metal corrosion protection applications.