Waterborne coatings for industrial and protective applications performance properties of anticorrosive pigments and inhibitors in waterborne Primer Systems

By Lars Kirmaier, Heubach GmbH, Langelsheim, Germany

What are anticorrosives and how do they work?

Paint coatings remain the most widely used way of protecting steel structures from corrosion.  Corrosion leads to deterioration of metals by chemical or electrochemical reactions resulting from exposure to weathering, moisture, chemicals or other agents in the environment into which they are placed.

It is estimated that from 1.800 million tons of produced iron for structures, objects, etc., 800 million tons have been destroyed or better said converted to rust during the past 50 years [Glausch.R., European Conference Proceedings, Anticorrosive Pigments, Berlin, 2000].

The corrosion of iron is an electrochemically driven process of energy exchange. With the presence of humidity iron passes into solution at the anode and hydroxyl ions are formed out of water and oxygen at the cathode. Due to the existence of an electrolyte there is the possibility for the electrons to migrate to the cathode and react with the environment. The result is the formation of rust. It is known that the performance of protective coatings can be positively affected with the utilization of anticorrosive pigments. Those pigments may be divided by the way that they work, for example chemically and/or electrochemically (Active Pigments) or physically (Barrier Pigments) (Fig. 1).

Fig. 1 Different paths of inhibition

Because of their special properties various chromate pigments were often used in anti-corrosive primer paints. They include: basic zinc chromate/ alkali chromate; basic potassium zinc chromate; basic zinc chromate (zinc tetraoxy chromate); strontium chromate; calcium chromate; lead chromate. Their efficiency is directly linked to the content of water-soluble chromate within the pigment. The high oxidizing potential and thus the degree of reduction in the cathodic areas determine their ability to passivate the substrate surface (Fig. 2).

Fig. 2 Passivation

The utilization of chromate anticorrosive pigments has shrunk significantly compared to previous decades. While unrivalled in their performance and flexibility in inhibiting corrosion, they are classified as either confirmed or suspected human carcinogens and as "Dangerous for the environment". International legislation, political and ethical pressures have accelerated the search for viable commercial alternatives. As a result, ongoing development work starting more than 25 years ago has come up with an increasingly sophisticated and effective range of chromate-free alternatives. Modified zinc phosphate pigments represent an important class of non-toxic corrosion inhibitors that have been successfully applied to the development of high performance chromate-free coatings. Since first being introduced to industry the value of these unique materials has been demonstrated through considerable field and laboratory testing. Typically, non-chromate pigments are specific to the different resin systems and are affected by factors like pigment conditioning, surface pre-treatment, substrate material and environmental conditions. By utilization of modern Wide Spectrum Anticorrosives (WSA) it is possible to offer chromate-like wide applicability while providing excellent performance properties not only in conventional solvent borne but also in modern high-solids and especially waterborne resin systems.

Evaluation of waterborne systems

One major trend in today's protective coatings world is the growth of the waterbased paint area. The current state of waterborne technology allows for completion of the transition from conventional coatings to aqueous products in more and more areas. Recent enhancements in waterborne resin system polymers as well as the way they are formulated have produced paints with a set of barrier properties that are superior to their standard solvent borne analogs. Waterborne coatings have quickly taken hold in different coating market segments. The most significant advantage waterborne coatings may offer are low VOC levels, which enable to satisfy even the most stringent regulations. Typically, waterborne coatings have VOC levels well below 2.0 lb/gal.

In waterbased protective coatings modified zinc phosphates play a major role as replacements for chromates. Not fully elucidated it is well-known that they have a positive impact on the anodic reaction. Modified zinc phosphates are able to avoid the solution of iron at the anode due to formation of stable iron phosphate complexes on the metal surface. In this matter, especially WSA pigments have shown outstanding performance properties in waterbased protective coatings.

The anti-corrosion primer plays the active role in a protective coating system. Key factors for the performance of the primer are:

›       Type of resin

›       PVC to CPVC ratio

›       Type of anticorrosive pigment (physical, chemical and electrochemical properties)

›       Interaction with other pigments and fillers

›       pH-value, pH-stability during storage (especially when incorporated in water based systems)

›       Dispersing conditions

Application-specific investigations in waterborne primer systems

Primer based on water reducible 2-part polyurethane including a WSA pigment (Fig. 3)

Anticorrosive Pigment:         Control

                                    Competition         (25 Vol.-%)

                                    ZCP-PLUS         (25 Vol.-%)

PVC/CPVC:                           0.6        

Binder:                           Bayhydrol A145/ Desmodur XP 2410

Substrate:                           CRS ST 1205 (DIN 1623)

Application:                            Compressed air spray

DFT:                                    70 microns        

Drying conditions:                  5 weeks RT, 2h 50°C

Test method:                           Salt Spray Test (ASTM B 117-03 / ISO 9227) (336 h)

Fig. 3 Water reducible 2-part polyurethane

ZCP-PLUS helps to improve adhesion and undercutting as well as overall rusting in salt spray test.

Primer based on water reducible Epoxy-Dispersion including a WSA pigment (Fig. 4)

Anticorrosive Pigment:         Control

                                    Competition         (35 Vol.-%)

                                    ZAM-PLUS         (35 Vol.-%)

PVC/CPVC:                           0.4        

Binder:                           Beckopox EP 385w/ Beckopox EH 613w

Substrate:                           CRS ST 1205 (DIN 1623)

Application:                            Compressed air spray

DFT:                                    60 microns        

Drying conditions:                  5 weeks RT, 2h 50°C

Test method:                           Salt Spray Test (ASTM B 117-03 / ISO 9227) (192 h)

Fig. 4 Water reducible Epoxy-Dispersion

ZAM-PLUS provides very good adhesion properties even on un-treated cold rolled steel panels during salt spray exposure.

Using Synergies

Synergistic effects can be observed when using an inorganic anticorrosive pigment like WSA in combination with special designed organic corrosion inhibitors.

The requirements on common organic corrosion inhibitors are as follows:

›       High activity at low concentration levels (typ. 0,5 -2%)

›       Sufficient thermal stability

›       High activity in the range of pH 5-9 (preferably pH 2-14)

›       Compatibility with a broad variety of resin systems

›       Easy to add in

›       Low water-solubility

In the past, a lot of focus had been put on a group of barely soluble metal salts of carbonic acids with regard to their electrochemical activity in the corrosion process. A superb performance could be achieved with metal salts of nitro-isophthalic acid like RZ.

By taking advantage of synergies when combining modern modified phosphate pigments like WSA with a small portion of special designed organic corrosion inhibitors like RZ it is possible to improve the early substrate protection (RZ) resulting in increased long term rust prevention (WSA). The result is a reduced tendency of blistering and enhancement of adhesion, especially wet adhesion and thus improved overall substrate protection of the coating system.

Primer based on waterbased acrylic emulsion including a WSA pigment in combination with RZ (Fig. 5)

Anticorrosive Pigment:         Control

                                    ZCP-PLUS         (37 Vol.-%)

                                    ZCP-PLUS/RZ         (33 Vol.-% / 4 Vol.-%)

PVC/CPVC:                           0.4        

Binder:                           Avanse MV-100

Substrate:                           CRS ST 1205 (DIN 1623)

Application:                            Compressed air spray

DFT:                                    50 microns        

Drying conditions:                  14 d RT, 2h 50°C

Test method:                           Salt Spray Test (ASTM B 117-03 / ISO 9227) (336 h)

Fig. 5 Waterbased acrylic emulsion

By adding a very small amount of organic RZ to the WSA pigment (1:9 ratio by weight), it is possible to improve the overall performance of the acrylic emulsion primer.

Conclusion 

The protection of metallic substrates is one of the key requirements on organic coating materials. Not only due to the increased regulations to produce low VOC  a major focus had been put on waterbased technology for protective coatings in the past. Today it is possible by utilization of modern Wide Spectrum Anticorrosives (WSA) to offer broad application latitude while providing excellent protective behaviour which was only known for chromate based pigments. Moreover, taking advantage of synergies by combination of modern WSA pigments with special designed organic inhibitors like RZ has led to performance properties which fulfill even the highest demands of today's waterborne protective coatings world.