Our research has led to the development of single component polymeric penetrant which can be applied with or without surface preparation over clean or lightly corroded steel/aluminium (no sand blasting or chemical treatment needed). The coating contains unique Nano-sized reactive materials which first penetrate the rust and scale to its morphological depth and then migrate to the non-corroded metal surface. Once the particles penetrate to metal surface, they polymerise and completely encapsulate and arrest the rust by forming a highly crosslinked network that is impervious to moisture and corrosive salts. The coating forms a long-lasting chemical and physical bond with base metal. Results over cleaned pretreated steel surfaces can exceed 10,000-hour salt spray with no blisters or scribe creep when top coated. Many coatings for use in refinery or off shore applications are comprised of three layers, which may include 75 to 150 microns of a zinc rich primer, 25 to 35 microns of an epoxy primer and 60 to 100 microns of a two-component urethane topcoat. The new innovative technology described herein provides a significant benefit in terms of 1) improved performance, 2) elimination of the epoxy primer coat and 3) material and labour cost savings as the primer is applied at 100 to 125 microns and the topcoat at 75 to 100 microns.

Introduction

Mild steel is one of the more widely used alloys for different kinds of applications because of its low cost, abundant supply and easy fabrication. But corrosion of steel is one of the major issues faced by transport (e.g. automobiles, aircraft, ships, etc.) and infrastructure (e.g. pipelines, buildings, bridges, oil rigs, refinery etc.) industry which directly affects its structural integrity, resulting in issues related to safety and maintenance of steel structures. According to the research published by NACE International[3], corrosion  is  responsible for losses over $2.5 trillions every year. There are different methods to counter corrosion such as, using corrosion inhibitive lining, electroplating, organic polymeric coating and chemical vapor deposition. Application of protective organic coatings on to metallic substrate, especially aluminium and steel, is an effective way to protect those substrates against severe corrosive environments. Organic coatings can minimise corrosion of metallic substrates by three main mechanisms such as barrier, sacrificial and inhibition.

In the field, we  often see early signs of corrosion on   a steel structure due to a variety of reasons such as poor surface preparation and application of protective coatings and some of the environmental factors which include acid rain, high humidity, temperature variations, condensation of moisture, presence of chemical fumes and dissolved gases   in case of structures submerged in water or soil. Among the  factors listed above, Improper surface preparation  is one of  the most important factors which contribute  to corrosion of steel structures and can lead to loss of structural integrity and structure before its  service life.  If there’s a way to protect the structures after observing initial signs of corrosion, without going through labour intensive tasks such as coating removal, cleaning, pretreatment and recoating application then it can  significantly increase  its service life more efficiently and economically.

Experimental

Materials, instruments and methods

One unique aspect of low molecular weight oligomers used in RA Exp1, is a prevalence of three types of reactive unsaturation on the resin backbone and low molecular weight reactive diluents. The three types of double bonds  offer  a  synergistic curing mechanism that  results in ancillary curing properties and high crosslink density that inhibits the penetration of soluble salts and moisture. Corrosion resistance is further improved when this resin blend is  coupled with corrosion inhibitor pigments such as organically modified zinc aluminium molybdenum orthophosphate hydrate and zinc-5-nitroisophthalate and unique conductive particles. Graphical representation of how RA Exp1 penetrates rust is shown in Figure 1. After penetrating surface of the substrate, low molecular weight monomers and oligomers having unsaturation, chemically bond/crosslink with other reactive sites, forming a highly crosslinked network which is impermeable to moisture and other soluble salts responsible for aggravating corrosion.

Hydrophobic and superhydrophobic variations of RA Exp1 were produced by adding superhydrophobic nano- textured silica[2] (Experimental name – SNTS). This additive itself is superhydrophobic in nature having both hydrophilic/ phobic sites and produces volumetric hydrophobic coating. Hence, even if the surface of the cured coating is abraded due to normal wear and tear experienced in the field, underlying layers will still repel moisture. A separate design of experiments was formulated for RA Exp1 (with and without SNTS) and 2-component polyurethane topcoat (with and without SNTS).

Characterisation

Salt spray – accelerated corrosion test

Variations of RA Exp1 with and without SNTS were applied on zinc nickel treated cold rolled steel substrate which was later top coated with 2k polyurethane coating with and without SNTS at 5 mils (125 microns) dry film thickness (DFT) each. Salt spray test was performed in salt spray cabinet in accordance with ASTM B117 standard, after all the panels were cured at ambient temperature for 7 days. Coated panels with artificial defect (scratch with a dimension of 4 inches     x 2 mm, created using a ‘1 mm’ scribe tool) were used to accelerate the corrosion process. All coated panels were placed in test chamber at an angle of 45° and exposed to the

5.0 wt.% NaCl solution at 40°C. The condensate collection rate and relative humidity were of at least 1.0 to 2.0 ml/h per 80  cm2 (horizontal collection area) and 95%,   respectively.

The protective performance of the coating was further investigated with the emphasis on size and distribution of corroded or damaged area on the coated sample surfaces after 10,000 hours of salt spray exposure.

Figure 2 shows 10,000-hour salt spray exposure, 3 of the 4 systems with RA Exp 1 as primer and 2K polyurethane topcoat show no scribe or face blister and/or corrosion.

Electrochemical impedance spectroscopy measurements

The barrier protection properties of RA Exp1 was investigated by performing EIS[1] on zinc phosphate pretreated cold rolled steel, results of which were compared with that of commercially available coatings based on conventional 2-component epoxy and moisture cured urethane system, experimentally named ‘2K Epoxy’ and ‘moisture cured urethane 1 & 2’. A three-electrode paint test cell (reference electrode: Saturated Calomel electrode (SCE), counter electrode: working electrode: steel samples in 14.6 cm2 area) was used to perform the EIS measurements. Impedance quantifications were made at open circuit potential (OCP) which were maintained potentiostatically in the frequency range of 0.1 Hz to 100 KHz and at amplitude sinusoidal voltage of ± 60 mV. The four samples (RA Exp1, 2k  epoxy and 2 moisture cured urethane samples) were immersed in 40 mL NaCl solution (3.5 wt.%) and EIS measurements were performed over a period of 40 days.

Initial Bode and Nyquist plots ( Figure 3a & 3b respectively) indicate that all coating variations show a capacitive behaviour with the high values of impedance.  RA Exp1 was found to have relatively lower impedance values compared to other “control” samples which could be attributed to conductive/anti-static nature of the coating due to addition of conductive nano-particles and additives   to enhance corrosion resistance.

Figure 6 shows a simplified equivalent circuit for a metal substrate protected by a semi-permeable coating layer, ignoring the coating resistance of negligible magnitude. The values of circuit elements in equivalent circuit networks can be used to characterise coating performance directly. Pore resistance (Rp) values extracted by fitting equivalent circuit model as a function of exposure time can be used    to compare performance and rank various coating systems. Plotted graph (Figure 5) containing logarithm of pore resistance (RP) Vs. exposure time (hours), which indicate that Rp of 2K epoxy shows a decreasing behavior with time whereas, RA Exp1, Moisture cured urethane 1 and urethane  2 are nearly constant for 1,000 hours of exposure to 3.5% NaCl solution.

After 1,000 hours of immersion, impedance values of moisture cured urethane samples 1 & 2 decreased significantly while RA Exp1 and 2K Epoxy were able to maintain their impedance values without showing a significant decrease. As shown in the Figure 4a & 4b, behaviour of moisture cured urethane 2 sample changed from 1 to 2 constant. This could be due to diffusion of electrolyte to coating and substrate interface; hence, a “double layer” could be formed below coating layer. For other samples including RA Exp1, no such behavior was observed which suggest better resistance of the coating layer against diffusion of electrolyte and soluble salts.

Physical and chemical tests

RA Exp1, 2K Epoxy and various moisture cured urethane systems were spray applied on clean zinc phosphate pretreated cold rolled steel panel at 5 mils (125 microns) dry film thickness (DFT) and were allowed to cure at ambient temperature for a period of 7 days before characterisation of physical and mechanical properties.

Thermogravimetric analysis

Thermogravimetric analysis ( TGA) was performed on RA Exp1, 2K Epoxy and moisture cured urethane 1 & 2. Results indicate that RA Exp1 has comparatively higher decomposition temperature of  463.74 oC,  whereas, decomposition temperature of other coatings are in the range of 430-440°C (Figure 7). This study confirms that  RA Exp1 can potentially be used in an environment where coatings are exposed to extreme conditions such as high heat. (i.e. boilers, chemical processing equipment, pressurised vessels etc.)

Results and conclusion

RA technology represents a dramatic enhancement in corrosion resistance of metal substrates such as: pretreated aluminum, zinc-Nickel treated cold rolled steel, lightly rusted steel and zinc phosphate treated cold rolled steel coated with RA Exp1. Results demonstrate better face blister resistance, scribe creep resistance and overall better corrosion resistance per ASTM B117 than all other systems tested in this scope of work. Higher decomposition temperature per TGA analysis indicate potential use of RA Exp1 for high temperature applications. Reaction kinetics of different vinyl polymerisation reactions and oxidative cure   of RA Exp1 are not fully defined and still remains a subject   of investigation.

In conclusion, this new generation of innovative protective coatings and superhydrophobic protective coatings provide the industry unsurpassed corrosion protection in a two-coat system.

Potential Applications

• High performance protective coatings for maintenance and repair application,
• Automotive refinish,
• Industrial application,
• Product finishing,
• Offshore application such as oil rigs, refinery,
• ACE industry,
• Boilers, chemical  processing  equipment, pressurized vessels.

References

[1] B.Merten. “Coating evaluation by Electrochemical Impedance Spectroscopy (EIS)” Report “ST-2016-7673-1” 2015.

[2]     J. Simpson et al. 2015 Rep. Prog. Phys. 78 086501.

[3]     NACE International – https://inspectioneering.com/news/2016-03-08/5202/nace-studyestimates-global-cost- of-corrosion-at-25-trillion-ann