Corrosion protection is a challenge to the humankind and lot of work is in progress to improve long term corrosion resistance by developing efficient coating system in a cost effective manner. Efforts on monocoat systems are getting profound interest and importance in industries over conventional primer, topcoat based two coat system or primer, high build MIO and topcoat based three coat system. Higher productivity, fast return to service, lower manpower and inventory cost make monocoat system highly desired. This research is aimed at developing a new and improved coating that employs silane modified hydrophobic nano zinc oxide coated micaceous Iron oxide as the active pigment to achieve synergistic corrosion resistant and barrier properties in a single coat that can combine the effect of primer and high build MIO coat. In the present paper synergistic effect and performances of silane modified hydrophobic nano zinc oxide coated micaceous Iron oxide were studied on different type of metallic substrates.
Nano Zinc oxide, anticorrosive properties, Micaceous Iron oxide, Epoxy Monocoat, Direct to metal
THE global economic losses due to metal corrosion is over $1.5 trillion each year, about 7-8 times more than due to earthquakes, fire and other natural disasters combined. Therefore, development of excellent long term metal corrosion protection is critical.
Micaceous Iron Oxide (MIO), a naturally occurring ferrous oxide (Fe2O3) with lamellar structure is the most important barrier pigment used in coating for metalwork in corrosion protection. MIO is insoluble in water, organic solvent and is slightly soluble in strong acid at elevated temperature. Lamellar MIO align parallel to the substrate and create a tortuous path for diffusion of corrosion agent like water, oxygen and chlorides ions. The ISO standard (ISO 10601) states that the basic criteria for the quality of MIO-based products should have high content of lamellar structures (min. 65% ), minimum Fe2O3 content of 85% and a maximum grade distribution of 0.1% at >105 µm. Anti-corrosive performance of MIO can vary depending on the percentage of lamellar structures as well as minimum Fe2O3 content.
In addition to MIO-type barrier pigments, application of nanostructured materials in corrosion protection has attracted ample interest due to their small size (1–100 nm) and novel structures that exhibit significantly improved physical and chemical properties compared to their bulk or molecular precursors. Nanostructure metal oxides, in particular the transition metal oxides, are more interesting in that scenario as they are chemically more stable, easy to scale up and contain numerous edge or corner and other reactive surface sites, which can be easily functionalized with diverse functional groups for the desired applications. For a better efficiency of corrosion protection, the key point is to design and construct such active metal oxide nanostructures, among which ZnO remains an attractive candidate.
One effective approach to enhance the anticorrosion property of MIO is to put a homogenous thin layer of coating on the MIO surface. When MIO is modified with thin layer of nano zinc oxide (ZnO), the performance of the coating system can exceed the performance of conventional MIO based paints. This nano ZnO can improve the anticorrosion properties significantly along with the barrier properties of the existing MIO.
Furthermore, nano ZnO-MIO surface can be made highly hydrophobic through silane modification which in turn can improve the water barrier effect thus increasing the cumulative anti-corrosive properties.
The aim of the current study is to prepare silane modified hydrophobic nano Zinc oxide coated MIO and incorporate it into two pack epoxy direct to metal monocoat system as an alternative system to conventional epoxy primer and epoxy high build based system and study their performances on different metal substrate.
Preparation of Nano Zinc oxide (ZnO) coated Micaceous Iron oxide (MIO)
Two types of micaceous iron oxide were dispersed in water under stirring in a 500 ml beaker under vigorous magnetic stirring. In a separate vial, Zinc sulphate (ZnSO4) was dissolved in water solution and was added drop by drop to the MIO solutions. After complete addition, it was stirred for another 2 hrs. Solid calcium oxide (CaO) was added to the solution followed by stirring for another 1 hr. It was filtered and dried at 100ºC to achieve free-flow powders of nano ZnO coated MIO.
Hydrophobic modification of nano-ZnO-MIO
Nano ZnO coated MIO was dispersed in water under magnetic stirring. Hydrophobic silane monomer (MTES) was added drop by drop and stirred for 24 hrs at room temperature. After completion of the reaction, the solid was filtered and washed with water and ethanol to remove impurities and unreacted monomers. The solid powder was dried at 100ºC to achieve a free-flowing silane modified nano ZnO coated MIO.
Two pack epoxy monocoat MIO paints were prepared by high speed disperser using combination of bisphenol –A- based liquid epoxy resin ( EEW= 180-200) and medium molecular weight bisphenol A- based epoxy resin (75 % solution in xylene, EEW =480 -550 gm/mol).
All the base formulation was prepared keeping the following ingredients.
Eight numbers of variation done with Standard and Modified MIO keeping all other ingredients same.
Hardener part contain Polyamide 125, 2,4,6 DMP 30, and solvents . Hardener was kept same for all the bases. Base and hardener are mixed in the 5:1 (V/V) ratio and applied by conventional spray method over following four types of metallic substrate.
a) MS Non Blasted manually prepared substrate b) Galvanised manually prepared substrate c) Stainless Steel manually prepared substrate d) Aluminium Stainless Steel substrate
The coated panels were air-dried for seven days at ambient condition (i.e. at 30°C). The coating dry film thickness was maintained at around 100-125 Micron.
SEM & Water Vapour Permeability
In order to evaluate the performance of modified and unmodified MIO, the mixed paints were applied on manually cleaned Mild Steel, Galvanised Steel, Stainless Steel and Aluminium panels around dry film thickness (DFT) of 100-125 micron also were casted on polyester foil at around DFT of 100 micron for SEM and testing of water vapour permeability. Water vapour permeability of painted film was measured as per ASTM D 1653, dry cup method. Painted films of around 100-125 micron dry film thickness were prepared over polyester films. The test specimen is sealed to the open mouth of a disc containing desiccant. The assembly placed in a test chamber with controlled atmosphere. The weight of the cups were checked at 24 and 48 hours interval and recorded.
Contact Angle Measurement
To determine contact angle, mixed paint were applied on glass panel at around 100-125 micron dry film thickness and cured for seven days. Hydrophobicity of trial samples were measured through contact angle measurements.
Pull off Adhesion Test
Pull off adhesion strength was determined as per method described in ASTM D 4541. Measurements were carried out using a universal Testing Machine (Positest ATA). Different trial formulations were applied at DFT of 100-125 micron onto 100 mm x 75 mm x 5mm specimen of manually cleaned Mild Steel, Galvanised Steel, Stainless Steel and Aluminium surface.
Salt spray resistance Test
Salts spray resistance was carried out as per method described in ASTM B 117. Different trials were applied at a dry film thickness of 100-125 micron onto 100 mm x 75 mm x 5mm specimen of manually cleaned Mild Steel, Galvanised Steel, Stainless Steel and Aluminium surface. The procedure consists of constant spraying with a 5% NaCl solution in a testing cabinet in which the temperature is maintained at 35 ºC. The total duration of the test was 1000 hours, and specimens were withdrawn for assessment after testing times. These test panels were evaluated by measuring the total delamination of the paint film from the substrate in millimetres, on both sides of the scribed line (loss of adhesion between paint film and steel).
Cathodic disbondment Resistance: Cathodic Disbondment of these eight coatings system was tested according to ASTM G95-87 (reapproved 1992) method. One intentional holiday of 3.2 mm diameter made in each specimen to be tested. The holiday was drilled so that the angular cone point of the drill will fully enter the steel where the cylindrical portion of the drill meets the steel surface. The electrolyte for this test was prepared separately by dissolving 3% (wt/wt) of sodium chloride in distilled water. A plexiglass acrylic cylinder of 15 cm diameter and 10 cm height was glued to the coated panel using a moisture cured silicone sealant so that the intentional holiday fell at the centre of the cylinder. This formed an electrochemical cell in which the metal panel was the working cathode. Anode shall be of the platinum wire type, 0.51 mm (0.020 in.), 24 gauge diameter, suspended inside the test vessel so that the tip of the anode assembly remained closest to the holiday. A saturated calomel reference electrode (SCE) were inserted into the cell. A potential of (-) 3 VDC with respect to the reference electrode was impressed across the test cell. Tests were done for a period of 90 days at 25°C.
The potential drop was adjusted, caused by current change throughout the testing period and any loss of electrolyte during the test due to evaporation was adjusted by adding fresh to the electrochemical cell. At the end of the 90 day test period, cell was disassembled and panels were removed and wiped with warm tap water. After overnight maturation, a crosscut was made across the holiday and, by using a sharp knife, a delaminated portion of the coating was removed. The average distance between the circumference of the intentional holiday and intact portioned of the coating was taken as the disbonding radius.
Humidity resistance and DM water resistance Test
It was carried out as per method described in ASTM D 2247 and ASTM D1308 respectively. Different trials were applied at a DFT of 100-125 micron onto 100 mm x 75 mm x 5mm specimen of manually cleaned mild steel, galvanised, stainless steel and aluminium surface. The edges of the coated panels were sealed using molten wax before exposure. The test panels were exposed for 1000 hours in the humidity chamber and in DM water.
FESEM studies were done using JEOL JSM-7600F. The FESEM pictures show that MIO is covered with nano structured spherical zinc oxide.
Results & discussion
Summary of the results
Following are the summary of results of all the trial. The details has been discussed subsequently. (Table 3)
Contact angle measurement
Higher is the contact angle, greater will be the surface tension difference of water droplet and coated film. Water will not properly wet the surface and this accounts for some degree of hydrophobicity. Mean contact angle of water droplet over painted surface are tabulated in fig 3.
Among the all trials Silane modified nano Zinc oxide coated MIO based coating system (Trial 3, 4, 7 and 8) shows superior hydrophobicity due to presence of hydrophobic silane layer than the conventional MIO based coating (Trial 1, 2,5.6). Trial 4 and trial 8 contains higher dosage (12%) of modified MIO and shows the best hydrophobicity followed by trial 7 and 8 where hydrophobic silane modification done over indigenous MIO (red – brown).
Water vapour permeability
Water vapour permeability of painted film of eight trials were measured as per ASTM D 1653, dry cup method. The weight of the cups were checked at 24 and 48 hours interval and recorded. Following are weight gain in gms per 24 Hours and 48 Hours.
Water vapour permeability is lowest for Tr- 4 followed by Tr - 8 where the Silane modified hydrophobic ZNO coated MIO level is 12 %. Tr -3 and Tr- 7 shows moderate resistance to water vapour permeability, as the level of modified MIO level is slightly lower that is 6 % .
These proves surface modification increases hydrophobicity and effectiveness of MIO layer. Water vapour permeability is significantly high in case of indigenous unmodified MIO.
Pull off adhesion testing
Pull off adhesion testing of the eight samples on manually cleaned Mild Steel, Stainless Steel, Galvanised steel & Aluminium surface after 9 days curing of coating. Following results are obtained (Fig. 5)
Among these four trial adhesion properties of Trial 4 (Silane Modified Nano –ZNO – Grey MIO at 12%) shows the best results in all the substrate followed by Trial 3 (Silane Modified Nano –ZNO – Grey MIO at 6%), Trial 2 (unmodified Grey MIO - 12%) and Trial 1 (unmodified Grey MIO - 6%). Silane modification on MIO helps in improvement of adhesion on different substrate. Especially the degree of improvement is prominent in difficult to adhere substrate like Stainless Steel, Galvanised Steel and Aluminium. Increase in MIO percentage also improves adhesion as polarity of the system increases and its helps with the chemical bonding with the substrate.
The similar trend of results were observed with Silane Modified Nano –ZNO – Brown MIO. Pull of adhesion of Grey Modified & unmodified MIO superior than Brown MIO may be due to higher % Fe2O3 content in Grey MIO helps in polar interaction with substrate.
Adhesion properties on different substrate follows the order:
Trial 8> Trial 7 > Trial 6 > Trial 5 (Best to worst)
From the above data, it is clear that adhesion of the modified system is satisfactory in both ferrous and non-ferrous substrate.
Under an impressed cathodic current, coatings are likely to undergo delamination which is widely known as cathodic disbonding or cathodic delamination (CD). Apart from mechanical damage, holidays in the coating, lower dry film thickness (DFT), higher water and ion permeability of the coating also influence cathodic delamination. The disbondment radius were measured for the eight trials and found in the following order:
Tr4 (0.9 cm) > TR8 (1.1 cm) > TR 3 (1.4 cm) >TR 7 (1.6 cm) >TR 2 (2.6 cm) > TR6 (2.75 cm) > TR 1(3.8 cm)> TR 5 (4 cm)
Reduction of oxygen to hydroxyl ions changes the pH at the coating-metal interface changes from neutral region (pH 6) to highly alkaline region (pH 13.5). This higher alkalinity hydrolyses the chemical bonds of adhesion. Reduction of oxygen to hydroxyl ions also produces many reaction intermediates such as superoxide and hydroxyl radicals which have the capability to degrade a polymer. Delamination rate is also influenced by the oxide layer present on the metal surface. At sufficiently negative potentials, Fe3+ in the oxide film reduces to soluble Fe2+ or an intermediate composition of di- and trivalent iron oxides making the substrate more electroactive thereby facilitating delamination. Silane modification in MIO helps in substrate adhesion, increases hydrophobicity of the system and also makes the film less permeable to water and moisture. So the modification shows better CD resistance. As the modified MIO are also coated with Nano Zinc oxide the Fe3+ layer less susceptible to oxidation. As the percentage of Modified MIO increase the CD resistance also improves. Where as in unmodified MIO, Fe3+ in the oxide film reduces to soluble Fe2+ or an intermediate composition of di- and trivalent iron oxides making the substrate more electroactive thereby facilitating delamination.
Salt Spray Test Results
Silane modified hydrophobic nano zinc oxide coated MIO at 12 % level, Trial 4 and Trial 8 shows the best anticorrosive performances followed by Trial 3 and Trial 7 where the modified MIO loading are 6 %. Trial 2, Trial 6 shows moderately better anticorrosive performances than Trial 1, Trial 5 where unmodified MIO loading are 12 % and 6 % respectively and higher MIO loading slightly contribute to the barrier properties. These clearly shows that Silane modification and nano Zinc oxide coating on MIO increases the hydrophobicity and anti-corrosive performance of MIO along with the barrier properties.
On galvanised and stainless steel substrate, silane modified nano zinc coated MIO shows superior anticorrosion resistance and good adhesion while with unmodified MIO the adhesion and anticorrosion properties are limited. Adhesion and anticorrosion properties on aluminium substrate shows good and incomparable results. This may be due to ionic bond formation between aluminium substrate and modified and unmodified MIO.
Salts spray resistance of modified MIO and unmodified MIO on non-ferrous substrate:
Humidity resistance and Water resistance
Humidity resistance and water resistance properties of silane modified Nano zinc coated MIO on non-prepared MS , Galvanised , Stainless Steel and Aluminium substrate for 1000 hours shows almost same pattern and order that of salts spray resistance . Thus silane modification and nano zinc coated MIO (Tr4, Tr8, Tr7 and Tr3) shows better humidity and water resistance than unmodified MIO (Tr 6. Tr5, Tr2 and Tr1).
MIO surface can be modified with a homogeneous thin layer of zinc oxide. Such nano-zinc oxide modified MIO showed considerable improvement in anti-corrosive properties compared to normal MIO containing epoxy based coating. Anti-corrosive property of such coating was further enhanced when this nano-ZnO MIO surfaces was further treated with silane. This modification also helped to achieve adhesion over ferrous as well as non-ferrous substrate.
Such surface modified nano zinc oxide coated MIO can be used as a highly effective and efficient anti-corrosive pigment for high performance coating.
We owe our gratitude to our honourable Mr. Tapan Dhar, Mr. S.K Bakshi, Mr Somnath Roy and Dr. Tirthankar Jana for their valuable suggestions and constant inspiration.
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Debabrata Pal, Pradipta Sankar Maiti, Sandip Bera
Berger Paints India Ltd., 14 & 15 Swarnamoyee Road, Howrah
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