Performance of ‘Hybrid' Coatings in Marine Environment

Manoj Bhuraria and A.S. Khanna
Corrosion Science and Technology (IDP)
Indian Institute of Technology Bombay, Powai, Mumbai-400 076, India

Abstract

Paint coatings are an integral part of most corrosion control measures. In this study, performance of ‘hybrid' coatings - comprising of thermally sprayed Zn/Al sealed, with an organic sealer, has been assessed using various characterization techniques, mechanical and environmental exposure tests and electrochemical experiments. Although not a new concept, its capabilities have not been fully realized in India. The use of sealed Zn-Al alloy coatings is capable of providing extended corrosion protection by combining the long-term protection of Al and the cathodic protection efficiency of Zn, along with the barrier effect of the sealer coat. It could well be a most effective, efficient, long term corrosion protection answer for most heavy duty engineering structures and vessels operating in marine environment. The paper, however, caters more to the problems faced on ships and its allied structures.

Introduction

The marine environment encompasses a great diversity of sub-environments and atmospheres. The marine atmosphere extends from areas where sea water directly contaminates a structure, to remote areas where the sea salt contamination is carried by the winds. It includes indoor and outdoor atmospheres on ships and off-shore platforms, as well as piers, bridges and on-shore structures. It is also one of the most corrosive of all natural environments; and, of all the anti- corrosive measures, coatings of various origins and formulations, are the oldest and most widely accepted means of controlling corrosion.

Today, a number of heavy-duty paint coatings like, High-build chlorinated rubber (HBCR)-, Self-polishing copolymer (SPC)-, Solvent-free epoxy- and poly-urethane based paints are in use to combat diverse and aggressive environmental conditions prevailing on vessels/structures operating in marine environment. However, the best of these polymeric paint systems are not capable of guaranteeing a corrosion-free life of more than 5-7 years. Definitely, ships and submarines, bridges, off-shore platforms, and such heavy-duty structures are designed for a useful service life upwards of 20 years. Therefore, continuous periodic replenishment, sizeable regular monetary input and loss of useful service hours are some of the fall-outs of maintenance painting in the purely organic coating regime. So, in the context of achieving unhindered, long term corrosion protection, the concept of ‘hybrid' coating is pertinent. ‘Hybrid' coatings - a combination of thermal spray (metalizing) coatings topped with an appropriate organic sealer and top-coat - have the potential to effectively last for more than 20 years in single application in marine environment. However, marine corrosion is a vast subject, but this study is limited to the problems faced by ships and its allied structures. On any military or merchant vessel, besides underwater hull, critical areas like bilges, fore-peak tanks, void spaces, machinery and wet compartments and weather decks are also prone to a variety of physical, chemical and operational degradation. Quite often, it is these areas which corrode rapidly and cause great difficulty in repairs/re-painting due to their inaccessibility or dense equipment-fit and outfitting. In order to contain this problem, various trials were conducted by the U.S. Navy [1]. It was observed that most of the corrosion problems could be overcome to a great extent by application of sprayed Aluminum coating on the substrate. Three general categories of ship systems were identified for protection against corrosion by using Al/Zn metal spray -

Thus, the objective of this study is to propose an alternative coating scheme which provides extended corrosion protection, and is viable, too. With this backdrop, various tests and techniques have been used to assess mechanical strength and corrosion behavior of the coating systems (mentioned below), which could be taken as preliminary pointers towards judging the efficacy of these hybrid paint schemes.

Experimental Work

Salient features of the substrate and coating types used in the study are mentioned below:

The samples under investigation are designated as follows for ease of identification -

Table 1:  Sample Identification

*(dft) = dry film thickness

In order to evaluate the performance of the sealed metalized coatings with respect to their corrosion behavior and mechanical strength in marine environment, various experimental techniques and accelerated tests have been carried out starting with microstructural inspection.

The characterization of thermal spray coatings in samples A, B and C was done using optical and scanning electron microscopy (SEM) after due metallographic preparation. Energy-dispersive analysis of X-rays (EDAX) for elemental identification and quantification and X-ray diffraction (XRD) for identification of phases in the metal spray coatings were also carried out. Various important properties like, surface porosity content, bond and impact strength of the coating systems were evaluated using standard techniques. A combination of accelerated environmental exposures and electrochemical tests were employed to assess the corrosion behavior of the coating systems.

Results and Discussions

Coating characterization
The inspection of the microstructure helps to detect cracks, coating delamination and other defects like porosity. Figures 1-a,b,c show optical micrographs of cross section of samples A, B and C at 150x magnification. The uniformity of the coating thicknesses, the interface bonding, size and distribution of pores and phases can be seen. The sprayed coatings can be seen to be comprised of splat quenched particles, more so in sample A due to lower melting temperatures of zinc (fig. 1-a). The lamellar structure as seen in fig. 1-c, sample C, shows thin dark lines which are oxide layers. The larger dark areas are pores within the coating [2]. The micro-irregularities of the substrate surface anchors the initial layer of spray particles and also re-distributes the residual stresses at the interface so that the coating is less likely to spall. Mechanical inter- locking is the primarey mechanism for the adherence, but some physical and chemical bonding forces may also come into play [3].

SEM micrograph in fig. 2-a shows sample A (pure zinc) to be more dense than the alloyed ones as seen in figs. 2-b and 2-c. Average surface porosity evaluated from a number of micrographs of each type corroborates the above fact.

Sample A shows least porosity (3.5%), sample B, 5.2% and sample C, 7.2%. However, the occurrence of porosity per se, is not much of a deterrence, in fact, pores allow the organic sealer coat good adhesion.

The EDAX results confirmed 99.48 wt. % Zn in sample A, signifying high purity zinc in the spray. Proportionate elemental distribution was obtained for samples B and C as well, indicating efficient spray procedure. XRD diffractographs indicated presence of Al2O3 in alloyed coatings, more so in sample C with 55% Al, which is expected since air/oxygen is involved as compressed air in the process.

Mechanical Strength
Table 2 gives the average adhesive/cohesive strength of various coating systems in dry and wet conditions. The test was conducted as per ASTM C-633 on a Tensile Testing m/c by applying uniaxial force through properly aligned aluminum dollies. The wet bond strength values were obtained after the samples had been subjected to 1000-hour salt spray test.

Table 2:  Bond Strength Values

* - adhesive failure


The bond strength values obtained are quite satisfactory, particularly in the sealed panels. Most failures recorded were cohesive which signifies that the adhesion to the substrate is still better. In wet conditions, unsealed samples recorded adhesive failure mainly due to the initial ingress of corrosives through the unfilled pores of the metal spray coating.

The Falling weight Test (BS 3900-E3) [4], was carried out to assess the impact strength of the coatings. The results re-confirmed good cohesive strength of the unsealed coatings, showing no cracking or disbandment near the impacted area. The sealed coatings did show few hair line cracks around the impacted area.

Corrosion Tests - Accelerated/Exposure Tests
Assessment of corrosion behavior of the coating systems was done using environmental exposures, accelerated tests and electrochemical tests. Sealed and unsealed panels were put under continuous immersion in natural sea water in an aerated aquarium tank for 13 weeks. At the end of this period, some whitish corrosion product marks were seen on sample A (pure Zn), and to a lesser extent in sample B, but no corrosion marks were seen. Other samples (sealed) were only covered with slime which could be washed off. Sea water was changed periodically in the tank and its pH was monitored weekly.

Figures 3-a and 3-b depict the condition of sealed samples after 1000-hour exposure to accelerated Salt spray (fog) Tests, done as per ASTM B117 [5]. It is clear that although sealed (AE, AP), the zinc metals activity builds up sizeable amount of corrosion products viz., oxides, hydroxides, carbonates/chlorides, which have significant volume expansion, leading to blistering after prolonged exposure, as seen in sample AP. The epoxy sealed samples performed better than the pipcothane (PU) sealed ones.

The results of 10-cycle (80 hours) exposure to accelerated weathering test in a QUV-B weatherometer are shown in figures 4-a and 4-b.

Although not a corrosion test, it is still an effective indicator of the degradation of coating under the combined effects of ultra-violet (UV) radiation and condensation. Here, the pipcothane (PU) coated sample showed no visible degradation, while there was some discoloration observed on the epoxy coating. NO other deterioration was seen.

Electrochemical Tests
Tabulated in Table 3 are E-corr (vs. SCE) and i-corr values of unsealed test sample A, after 1- hour pre-exposure; sample B and C, after 24-hour pre-exposure in 3.5% NaCl solution. The curves obtained from potentiodynamic polarization are shown in figures 5-a,b,c. Fig. 5a clearly represents the electrochemical dissolution tendency of Zn in sample A, while sample C (fig. 5c) shows certain degree of passivation effected due to 55% Al in the spray coating. Al content in the coating helps in forming a tenacious film of aluminum hydroxide on the surface, thereby increasing the IR drop and reducing the corrosion current, i-corr. Thus, it is clear that the protection accorded by Zn spray is purely sacrificial in nature and its life is directly proportional to the thickness of the coating applied. Increase in Al content in the Zn-based metal spray reduces the electrochemical activity of pure Zn coating, thus increasing the longevity of the coating by combining the effects of passivity and sacrificial cathodic protection.

Table 3:  Potentiodynamic polarization results

The A.C impedance spectroscopy is regarded as the most effective technique for an objective assessment of coating performance in a given medium [6]. Unfortunately, the difficulty in exact electric circuit modeling for complex electrochemical phenomena of electrolyte-coating-metal interfaces and certain ambiguity in interpretation of its data there after, having not helped the use of this technique as a popular standard tool for performance evaluation. Nevertheless, an attempt has been made here, using CMS 300 software (Gamry Instruments, Inc., USA) interface, to evaluate the comparative corrosion performance among the various coatings systems under consideration here. The coating systems were tested in 3.5% NaCl solution with graphite counter electrode and saturated calomel (SCE) as the reference electrode. The measurements were done at open circuit potential with an amplitude of 10 mV and a frequency range of 0.2-50,000Hz.


Figures 6, 7, 8 and 9 depict the Nyquist plots for samples B, BE, CE and BP respectively, as on day-01 and day-42. In fig. 6-a, the porous nature of unsealed sample B is evident from the low overall resistance value on X-axis. It is clear from the trend that initially, the pores in the coating allow the electrolyte to penetrate; hence, the dissolution reaction starts and at a low impedance value the curve begins to take a downward turn. However, as the system stabilizes, the corrosion products fill the pores, the impedance value rises and the curve straightens, indicating diffusive, but protective mode. Corrosion processes controlled by diffusion can occur in case of formation of insoluble corrosion products that block the pores in the coating [6]. Similar effect is clear in figs. 7-a and 8-a; also, the higher impedance values (~10 to the 6th power ohms), signifies a good barrier effect by the sealer coat. However, it is difficult to get exact coating resistance value so long the trend stays like this. The coating resistance value will go on reducing as the water uptake in the coating increases (fig. 7-b), until the curve tends towards a semi-circle. Sample BP, in fig. 9, signifies a coating with minor defects. The relatively low value of resistance (~10 to the 4th power ohms) indicates the sealer coat also suffering from some porosity. The high frequency component is probably due to the coating itself; whereas, at lower frequency, the straight line close to 45 degrees is representative of the classical Warburg type diffusion process. It is pertinent, however, to mention that no visible corrosion marks had appeared on any of the samples yet. A more comprehensive evaluation will be done on completion of the test.

Conclusions

Performance of pseudo-alloy thermal spray coating (sample C) is better than pre-alloyed (B) or, purely metallic (A) coatings; and, its sealing further yields a synergistic upgradation in the coating performance, both in terms of corrosion behavior and mechanical strength.
Pipcothane (PU) sealer shows excellent resistance to ultra-violet radiation while, Lapox epoxy shows good overall efficacy, particularly with respect to corrosion protection, as indicated by Salt spray and A.C impedance Tests.
Hybrid coatings are not only effective for critical areas of ships, as mentioned earlier, but also for drydock components and equipments like drydock gates, gratings, drain tunnels, access doors, handrails as well as marker buoys and all such critical carbon steel components, as have been successfully proved in Europe and the USA [7].

Acknowledgment

The authors express their gratitude to Mr. Dhirendra Kumar, Scientist "F", Head of Paint Section, NMRL, Ambamath; and Dr. R. V. Bhave, Sc. ‘F', Head, Paint Lab., Naval Dockyard, Mumbaie and RSIC, IIT Bombay for extending their help and facilities in carrying out certain experiments related to the study. We gratefully acknowledge the contributions of M/s Modi Metallizing, Mumbai, M/s Metallizing Equipment Co. Pvt. Ltd., Jodhpur and VCM Polyurethanes, Mumbai in preparation of the samples.

References

1. Vincent J. Lanza, Coatings for Corrosion Control of Navy Ships, Proceedings of the 37thMeeting of the Mechanical Failure Prevention Group, National Bureau of Standards, Maryland, USA, 1983, p-82.
2. Barbara A. Shaw, P. J. Moran, Characterization of the Corrosion Behavior of Zn-Al Thermal Spray Coatings, Material Performance, Vol. 24 (4), 1985, p-24.
3. R. P. Krepski, Thermal Spray Applications in Chemical Process Industries, 1994, p-11.
4. British Standards - BS 3900:Part E3, 1966.
5. The Annual Book of ASTM Standards - 1994, (03.02), (13.01).
6. B. S. Skerry, D. A. Eden, Electrochemical Testing to Assess Corrosion Protective Coatings, Technical Report (Sherwin-Williams Co., Chicago), 1998.
7. S. W. Vittori, James D. Herbstritt, Thermal Spraying for Maintenance of Naval Shipyard Facilities, Proceedings of NTSC 87, Orlando, USA, 1987, p-383.

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