New Zinc-Free Anticorrosive Pigments
Modern electrochemical research methods were successfully used to accelerate work on a new zinc-free pigment with improved anti-corrosive properties, and these results were confirmed using conventional tests.
New Zinc-Free Anticorrosive Pigments
Identifying Specific Synergies Using Electrochemical Corrosion Research:
The development of anticorrosive pigments for different coating systems is extremely time-consuming and expensive due to necessary climate tests such as salt spray exposure.
Modern electrochemical research methods were successfully used to accelerate work on new zinc-free pigments with improved anticorrosive properties, and these results were verified using conventional tests [1].
Mode of Action of Conventional Phosphate-Based Anticorrosive Pigments:
The use of appropriate anticorrosive pigments has a significant effect on the protective properties of formulations created for metal surfaces. The mode of action of the anticorrosive pigment is based on the following factors [2, 3]:
• Increased resistance of the paint film,
• Prevention of rust spread in defective areas and corrosion beneath the paint film,
• Delaying corrosion,
• Cathodic and/or anodic passivation of the metal surface.
Figure 1: Mode of action of phosphate pigments
In the case of zinc phosphate, very low solubility in water causes the incorporation of secondary phosphate ions into the coating, which are responsible for the formation of corrosion-inhibiting adhesive structures on the metal surface and are associated with anodic passivation. Another theory of the mechanism of action describes the formation of tribasic iron phosphate mixtures in a weakly acidic environment [4, 5]. As an ampholyte, zinc or zinc hydroxide after hydrolysis exhibits solubility behavior in both acidic and alkaline environments, providing advantages in corrosion protection. Modified zinc orthophosphate pigments and zinc polyphosphate pigments established in the market demonstrate serious high chemical and electrochemical efficiency, providing excellent protective properties when compared with conventional zinc phosphate. In addition to aluminum, molybdate and organically modified types (ZPA, ZMP, ZPO), the universal WSA pigments ZCP PLUS and ZAM PLUS are noteworthy.
Beyond economic concerns, ecological and regulatory factors are playing an increasingly important role in innovative coating system formulations today. This is why it is not surprising that the need for zinc-free anticorrosive or labeling-free pigments has been continuously increasing in recent years. Zinc-free technology is not new; numerous pigments based on calcium, strontium, aluminum and magnesium phosphate have been on the market for a long time. However, the real problem is that the combination of very good corrosion protection with universal application, as in the case of modified zinc phosphate, is possible only in rare circumstances.
While there may be other reasons, the primary reason for this situation is that the solubility of the relevant compounds differs from zinc phosphate when compared with zinc phosphate.
Although the periodic table offers many alternatives to zinc that do not contain heavy metals, only a few metals can be used as alternative cations. In the selection process, the focus is therefore on possible positive interactions between calcium and magnesium phosphate compounds. The required properties for the new zinc-free pigment are as follows:
• Zinc-free pigment technology,
• High-efficiency anodic corrosion protection in solvent and water-based systems,
• Stability and universal application,
• Easy dispersion properties,
• Low cost.
Even in initial trials using newly developed pigments with different magnesium-calcium ratios, significant improvements in corrosion prevention performance were observed. This result also proved positive in the salt spray test.
Electrochemical Testing in Paint Dispersions
After comprehensive preliminary tests, test conditions could be established for two electrochemical methods that enable comparative statements on the protective effects of anticorrosive pigments used in water-based binders. The first is rest potential analysis and the second is electrochemical noise analysis. For all electrochemical tests, a round unalloyed steel (C55, material no.: 1.1203) rod sensor or working electrode was used. An aqueous organic paint dispersion in which the anticorrosive pigment was dispersed was used as the electrolyte. Since organic dispersions are water-based but of high viscosity, they were diluted in a 50:50 ratio with deionized water.
Table 1 shows electrolytes consisting of four different anticorrosive pigments or pigment combinations with different Ca/Mg ratios with a water-based binder. Variable B here has a higher magnesium content than variable A. Pigment P2 (CMP) shows the highest solubility and the lowest pH value in conductivity. This makes it difficult to predict the actual efficiency of the pigment combination in real paint systems. However, it is also the first indication of altered electrochemical behavior.
Table 1: Electrolytes used consisting of binder and anticorrosive pigment
Electrochemical Rest Potential Analysis
Rest potential analysis (RPA) is based on rest potential measurement using two-electrode arrangements with an Ag/AgCl electrode as the reference electrode and unalloyed steel C55 as the working electrode. During measurement, the continuously stirred water-based paint dispersion (see Table 1) was used as the electrolyte to prevent pigment settling. At defined time intervals, a one-molar sodium chloride solution was added to the electrolyte as a corrosion stimulator using a computerized pump device. As an important measurement parameter, the voltage curve was observed and the amount of chloride at which significant voltage drops occurred was determined and evaluated. For all measurements, the voltage curve was recorded for 60 minutes before chloride was added.
Figure 2 shows the rest potential curve as a characteristic example of rest potential analysis for each anticorrosive pigment from P1 to P4. Using the voltage curves, differences in pigment behavior can be observed. As a result of the addition of a certain amount of chloride, a noticeable voltage drop is observed from P1 to P3. For P4, it is noted that the rest potential increased at -400 mV at the very beginning of the test. At this voltage, after 60 minutes and as a result of chloride addition, there is strong metal dissolution that will accelerate further. A slight rise in voltage around -250 mV is attributable to corrosion or secondary products on the metal surface and is not a result of inhibitory mechanisms.
Figure 2: Rest potential curve for unalloyed steel measured in aqueous-based binder containing anticorrosive pigments from P1 to P4
Pigments P2 (CMP) and P3 are pigment combinations in which pigments P1 and P4 are combined in different proportions. Pigment combinations primarily experience voltage reduction around -400 mV and subsequently experience voltage increase depending on the amount of P4 pigment added. As higher amounts of P4 are added to the pigment combination (100% magnesium composition), it takes longer for the metal to develop a passive surface and correspondingly for voltage to increase.
Figure 3: Critical chloride amount in the examination of anticorrosive pigments from P1 to P4 dispersed in water-based binder
Figure 3 shows the critical chloride amounts defined for characteristic voltage drops for anticorrosive pigments from P1 to P4 dispersed in water-based binder. The average value calculated using a minimum of three separate measurements is shown for each pigment. Pigment P2 (CMP) shows the highest critical concentration value, and pigment P1 and especially P4 show the lowest.
Electrochemical Noise Analysis
Electrochemical noise analysis (ECN) is a highly sensitive method used to record localized wear processes and material changes and has been successfully used for many years in many applications [6], including the paint industry [7]. A three-electrode arrangement was used to perform currentless measurements to investigate the dissolution behavior of unalloyed steel in water-based dispersions. For this purpose, two working electrodes made of identical C55 were short-circuited with a zero-resistance ammeter and connected to a high-ohm voltmeter and an Ag/AgCl reference electrode.
The measured noise signals were filtered through a bandpass filter. This process allows them to be separated from their steady-state components (current and voltage) and for their power to be amplified separately. After a 20-minute experimental period conducted without chloride addition, 0.04 ml of one-molar sodium chloride solution is added to the electrolyte at five-minute intervals. Voltage and voltage noise are measured and evaluated together with current noise between the steel electrodes. By calculating the noise charge quantities and noise resistances specified in comprehensive preliminary tests as characteristic values for the protective effect of pigments, more detailed differentiation in the results is obtained since charge quantities show a direct relationship with pigment effect.
Figure 4 shows the noise current-time curve for unalloyed steel in water-based binder containing anticorrosive pigments from P1 to P4 for the full 180-minute experimental period.
Figure 4: Noise current-time curve for electrochemical noise analysis in water-based paint dispersions
To exemplify the relationships and processes occurring on the metal surface, Figure 5 shows the cumulative charge quantities detected in the noise current-time curve.
Figure 5: Cumulative charge quantities in two test intervals from the noise current-time curve determined by electrochemical sound analysis in water-based paint dispersions
Clear differences between pigments were demonstrated. While P1 showed low noise activity at the beginning of the measurement and therefore low metal dissolution, strong initial activity could be identified for P2, P3 and P4 pigments in the first test interval due to the magnesium composition. During the test period between 120 and 180 minutes, noise activity in P2 and P3 pigment combinations decreased while P1 showed increasing dissolution. In fact, pigment P4 shows a sharp decrease in noise current and cumulative charge quantity, but active metal dissolution continues as evidenced by the detected noise resistance.
Verification Using Conventional Corrosion Testing
Conventional corrosion testing was used to verify the validity of electrochemical examination methods. Cold-rolled steel sheets were coated with a practical water-based styrene acrylate formulation containing anticorrosive pigments from P1 to P4. After aging for 408 hours in salt spray (DIN EN ISO 9227), the results are presented in Figure 6. The combination of calcium and magnesium composition in the anticorrosive pigment (P2) demonstrated high wear resistance.
Figure 6: Test results after 408 hours of salt spray exposure, base: water-based styrene acrylate
To investigate the performance properties of P2 pigment combination (CMP) in other binder systems, salt spray tests were performed with formulations based on solvent-based short-oil alkyd resin (see Figure 7) and solvent-based epoxy resin (see Figure 8). For comparison purposes, pigment combinations were tested against a control sample without anticorrosive pigment, magnesium phosphate, calcium phosphate and a zinc-coated reference sample. The dry film thickness in applications was set to 70 μm. To evaluate the degree of rusting and rust spread in cross-section, the lower half of the paint film was removed after the salt spray test.
Figure 7: Test results after 408 hours of salt spray exposure, base: solvent-based short-oil alkyd resin
As a result of salt spray exposure, the use of CMP was observed to give better results compared to the use of magnesium phosphate. In fact, the use of CMP proved to give even better results than the zinc-containing reference sample. The application containing magnesium phosphate showed serious problems with adhesion, rust and swelling.
References
Figure 8: Test results after 504 hours of salt spray exposure, base: solvent-based epoxy resin
In this system, significant improvement in cross-section adhesion and rust spread can be achieved using CMP1).
Summary
A new, highly effective zinc-free pigment was developed. Investigations using rest potential analysis and electrochemical noise analysis showed that pigment composition and the effects of these compositions on substrate dissolution can vary. The amount of magnesium in different pigment combinations from P1 to P4 has a significant effect on the anticorrosive performance of the overall system. Using rest potential analysis, it was clearly observed that an increase in the amount of magnesium composition in the anticorrosive pigment causes a decrease in the critical concentration value, which is an indicator of the anticorrosive effect. A 100% magnesium composition concentration in the anticorrosive pigment leads to active metal dissolution and loss of anticorrosive properties. As a result of this investigation, P2 (CMP) was identified as the most suitable pigment for this system with a specific Ca/Mg ratio.
With respect to electrochemical noise, a trend was identified in which initial noise activity increased with increasing magnesium concentration in the time interval from 0 to 60 minutes. This behavior indicates increasing substrate dissolution in the first 60 minutes of measurement at increased magnesium concentration. After extended test duration and chloride addition, P2 pigment combination showed the best anticorrosive properties. Pigment P4 with 100% magnesium composition actually shows a decrease in noise activity, but active metal dissolution continues, as evidenced by specific noise resistances.
Results at a Glance:
• It is helpful to identify and use synergistic interactions when developing new, highly effective anticorrosive pigments.
• Results from electrochemical testing support the selection of appropriate synergistic combinations and significantly reduce the time required to conduct comprehensive preliminary tests.
• The use of modern electrochemical testing such as rest potential analysis and ECN was successful.
• Conventional wear testing confirmed the high performance properties of the new CMP1) pigment in electrochemical tests.
Note
1) Pigments used: CMP, HEUCOPHOS® CMP (calcium magnesium orthophosphate) manufactured by Heubach GmbH, Langelsheim, Germany.
Author: Dr. Lars Ludwig Kirmaier - Anticorrosives Product Manager - Technical Marketing Department - HEUBACH GmbH
Translator: Olçun Ekinci - Marketing and Business Development Manager - HEUBACH COLOR
[1] S. Bender, M. Babutzka, L. Kirmaier: "Moderne elektrochemische. Korrosionsuntersuchungen gezielt eingesetzt", Farbe und Lack, (2014), published.
[2] L. Kirmaier, Farbe und Lack, (2009), 115, p. 120-123.
[3] Vogelsang, J., Basics about Anticorrosive pigments and Corrosion Inhibitors and Possibilities for their Usage, European Coatings Conference, Berlin, 2000.
[4] Ruf, J., Organischer Metallschutz, Vincentz Verlag, Hannover, 1993, 260.
[5] Yongsheng Hao, Fuchun Liu, En-Hou Han, Saima Anjum, Guobao Xu, Corrosion Science, 2013, 69, p. 77-86.
[6] Heyn, A., Göllner, J.: "Analysis and Monitoring of Corrosion using Electrochemical Noise - 5(th) Part", Materials and Corrosion (Vol. 64), No. 8, 2013, p. 663.
[7] Plagemann, P.; Yezerska, O.; Brinkmann, A.: [2] L. Kirmaier, Farbe und Lack, (2009), 115, p. 94-97.
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