WSA - wide spectrum anticorrosives

The corrosion of metallic structures has a significant impact on the economy, including infrastructure, transportation, utilities, production and manufacturing. Every second one-ton of steel is destroyed globally through corrosion. 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].

Corrosion protection of metallic materials has for a long time been one of the key roles performed by organic coatings. However, today it is known that the performance of protective coatings can be significantly improved by the utilization of anticorrosive pigments. These pigments can be divided by the way that they work including chemically and/or electrochemically (Active Pigments) or physically (Barrier Pigments) (Fig. 1).

Fig. 1. Different paths of inhibition

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.

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.

Chromate based pigments have long been utilized as anticorrosives for protective coatings. Their efficiency is directly linked to their of water-soluble chromate content within the pigment. As most of the paint films are permeable to water, chromates have been successfully used in the past. They are either effective in the cathodic or anodic area. Their high oxidizing potential and thus the degree of reduction in the cathodic areas determine their ability to passivate the substrate surface.

The utilization of chromate anticorrosive pigments has shrunk significantly in the last several 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 as well as 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. The extensive development of corrosion protection pigments based on zinc phosphate and modifications thereof has led to products providing optimized particle size and structure. For example, highly modified zinc orthophosphate and zinc polyphosphate pigments have been designed to provide improved chemical and electrochemical effectiveness.

Protection of solvent borne systems

Typically, non-chromate pigments are specific to the different resin system and are affected by factors like pigment conditioning, surface pre-treatment, substrate material and environmental conditions. With the development of modern Wide Spectrum Anticorrosives (WSA) we are able to offer the chromate-like wide applicability while providing excellent performance properties. This protection is exhibited not only in conventional solvent borne systems but also in modern high-solids and particularly waterborne resin systems.

Fig. 3 Salt spray test results

Fig. 3 provides the salt spray test result of three different corrosion protection primers based on different coatings systems:

• H316:  Solvent borne short oil alkyd primer
• H325: DTM-primer based on High-Solid Polyaspartat
• H341:  Water soluble primer based on Styrene/Acrylic
• Substrate: Cold rolled steel

The primers containing two different modified zinc orthophosphates and the wide spectrum anticorrosive were compared to a control panel not utilizing any anticorrosive pigment. The visual assessment of blistering, rust at scribe and overall rusting leads to a calculative rating figure between 0 (low performance) and 100 (outstanding performance), which allows the evaluation of the overall performance of the different anticorrosive pigments.

The test results demonstrate the outstanding performance of the different zinc orthophosphates designed for the protection of the specific coating systems, but also show the outstanding performance of the wide spectrum anticorrosives in all three coating systems.

For the first time, The WSA anticorrosives allow for the utilization of corrosion protection pigments with outstanding corrosion inhibition properties and broad-based applicability.

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. Waterborne coatings have quickly taken hold in different coating market segments. The most significant advantage waterborne coatings offer are low VOC levels, which typically enable compliance with 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 following test results demonstrate the performance of WSA anticorrosives in two different waterborne primer systems.

 1. Primer based on water reducible 2-part polyurethane including a WSA pigment (Fig. 4)

Anticorrosive Pigment:	Control Competition (25 Vol.-%) WSA (25 Vol.-%)
PVC/CPVC:	0.6
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. 4. Water reducible 2-part polyurethane

2. Primer based on waterbased acrylic emulsion including a WSA pigment (Fig. 5)

Anticorrosive Pigment:	Control Competition (38 Vol.-%) WSA (38 Vol.-%)
PVC/CPVC:	0.5
Substrate:	CRS ST 1205 (DIN 1623)
Application:	Compressed air spray
DFT:	60 microns
Drying conditions:	4 weeks RT, 2h 50°C
Test method:	Salt Spray Test (ASTM B 117-03 / ISO 9227) (600 h)

Fig. 5. Waterbased acrylic emulsion

The utilization of WSA anticorrosive leads to improved adhesion and undercutting even on un-treated cold rolled steel panels during salt spray exposure in different kind of waterborne primers.

Utilization of synergistic effects for further improvement of the corrosion protection performance

So-called synergistic effects in solventborne and especially waterborne protective coatings can be observed when using an inorganic anticorrosive pigment like WSA in combination with specially designed organic corrosion inhibitors.

By taking advantage of synergies when combining modern modified phosphate pigments like WSA with a small portion of a specially 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 an improved overall substrate protection behavior of the coating system.  

3. Primer based on waterbased acrylic emulsion including a WSA pigment in combination with RZ (Fig. 6)

Anticorrosive Pigment:	Control Competition (37 Vol.-%) WSA (33 Vol.-% / 4 Vol.-%)
PVC/CPVC:	0.4
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. 6. 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 waterborne primers.

Conclusion

The protection of metallic substrates is one of the key requirements for organic coating materials. A major focus has been put on waterbased technology for protective coatings due to the need to produce lower and lower VOC coatings. Today it is possible by the utilization of modern Wide Spectrum Anticorrosives (WSA) to offer broad application latitude while providing excellent protective behavior which was only demonstrated by chromate based pigments in the past. Moreover, taking advantage of synergies by the 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 protective coatings world.