COATINGS PERFORMANCE UNDER MARINE ENVIRONMENT Z. M. Muntasser, M. M. A1-Darbi, M. R. Islam,

Dalhousie University, P.O. Box 1000, Halifax, NS B3J 2X4

ABSTRACT

Marine atmosphere with its high level of salinity and humidity is very corrosive. It has been estimated that the direct cost of marine corrosion worldwide is between 50 and 80 billion dollars every year. However, coating the industry is responsible for almost 40 % of this cost. While polymer-based coatings have been used successfully to prevent corrosion in other parts of the world, few such coatings appear to have success in the marine environment. In this paper a new type of coating (properties of both a high performance epoxy and acrylic polyurethane) was tested for preventing corrosion in the marine environment.

To have a better understanding of mechanisms of attack and the long-term effects of coatings, salt fog corrosion tests were conducted to ascertain the corrosion protection capability of various coating systems in different thickness including the new coating. Using scanning electronic microscope (SEM) and profilemeters the onset and growth of corrosion were observed. A protocol was developed to identify performance and efficiency of these systems, which can help suppliers and engineers in developing a long life coating systems for marine structure.

INTRODUCTION

Marine environments are severely corrosive. The degree of severity however, depends on several variables such as humidity, temperature, chloride content, wind and sunlight. Saline particles in marine atmospheres accelerate metallic corrosion processes as chloride increases the solubility of the corrosion products. Marine chlorides dissolved in the layer of moisture also considerably raise the conductivity of

(2) the electrolyte layer on the metal and tend to destroy the passive film existing on the metallic surface . The all reaction that can happen when steel is immersed in seawater can be written as"

2Fe+2H20+O2 ~-,.- 2Fe+Z+4OH - ,,-~" 2Fe(OH)2 ( 1 )

On the other hand ferrous hydroxide (corrosion products) reacts with salt to form ferrous chloride:

Fe(OH)2+2NaC1 ,,-" FeC12+2NaOH (2)

In later stages of corrosion ferrous chloride reacts with water to form hydrochloric acid:

1

FeC12+2H20 ~ Fe(OH)2+2HC1 (3)

Ingress of chloride, which is abundant in marine environments at discontinuities in the rust is the single most cause of pitting. However, it was observed in many studies that the pH inside pits could reach as low as 2 because of the effect of HCL (3).

Offshore structures are often exposed simultaneously to five different zones of corrosion, the first zone is subsoil zone, the structure then passes up to submerged zone area, tidal, splash zone ending with marine atmosphere zone. The most sever attack occurs in the splash zone because of alternate wetting and drying and also aeration. Figure 1 illustrates the corrosion rates of steel structure through these zones.

In recent years the construction of offshore structures has significantly expanded and the techniques of coating methods and anti-corrosive materials have reached a very high stage of development. Since most of these structures are made of steel and designed to operate in a very aggressive environment for a long period of time, the coating systems must give a good performance with minimum maintenance cost (~). In addition, the cathodic protection, which is the normal protection method combined with coatings is not reliable in the splash zone. The reason in this zone is the surfaces are intermittently wetted so the cathodic protection system used for the rest of the structure will not work during the non- wet period. Resistance to water, easy of application, impact resistance, long term stability and resistance to bacteria and other microorganisms are the most important factors that need to be taken into consideration in choosing a coating in marine environment.

Many researchers have undertaken studies of marine corrosions behavior of different coatings. Some have developed indoor techniques (Lab) to reduce the exposure time frame, while others have carried out a long outdoor exposure tests, which can take a minimum of 5 years to get reliable data (2-5). Appleman (6) has compared several lab-accelerating tests of coatings with' exterior marine environment. One of the remarkable conclusions he addressed that a reliable accelerated laboratory test method for predicting field performance and durability of coating systems is still missing. Bone III (7), developed a comprehensive method for the selection of coatings in offshore environments. In the same study a correlation between accelerated lab tests and actual field performance was drawn. In another study performed by Chengde (8), the characteristic, fabrication, construction technique and quality control requirement for external pipelines coatings in seawater was discussed. He has reported also that some coating systems have been used successfully in this harsh environment. Szokolik (9), carried out experimental work to evaluate using a single inorganic zinc coat for oil and gas facilities in marine environments. It was found that the performance of single coat of zinc performed significantly better that most multi-coat systems. However, the sensitivity of zinc to acid and alkali makes use of a top coating very important. Munger (~), described the use of various coating systems in the five different zones of corrosion in marine environment.

Salt spray test according to ASTM B 117 is the most widely used coating evaluation test. Salt solution is pumped into a nozzle where it meets a jet of humidified and compressed air, forming a fine droplet spray under high humidity and temperature. In this paper the corrosion behavior of four coating systems were investigated.

EXPERIMENTAL WORK

The four coating systems tested in this study are described in Table 1. These systems include inorganic zinc coating (two thicknesses), inorganic zinc coating/epoxy, inorganic zinc

2

coating/polysiloxane, and polyurethane. All the coating systems used in this study contain VOC concentration less than 2.8(lbs/gal), which is environmentally accepted, this can be seen in Table 2. The coatings were applied by a coating manufacture on IS 1020 steel panels with dimensions (6x4x0.13 inch). Pan abrasive G14 steel grid was used to achieve a SSPC-SP5 white metal blast with a surface profile of 2 mils. Coating thickness electronic gage was used to verify the coating DFT. The panel weights as well as the surface roughness were measured before the test exposure.

The aerated salt spray test was conducted according to ASTM B 117 standard salt spray test. Figure 2 shows the panels placed in the salt fog cabinet. The edges of the panels were sealed with silicon rubber before the exposure. To assess the statistical reliability, four panels of each coating system were tested. The results obtained are reported as an average of the four. The test was conducted for 3000 hours for all coating systems. A modified salt fog test was conducted also for the same period of time, and the same coatings system. A drying time of 1 hour was conducted by exposing the samples to strong light every 12 hours of exposure. The specimens were examined every 500 hours to study the growth of corrosion and the rate of deterioration. Before the examination the panels were cleaned with water, alcohol to remove salt deposits, which followed by warm air. Five different methods were used for evaluation of coating systems. The first method was used is the degree of rusting. The degree of rusting was obtained using standard ASTM D610, the rust degree in this method range between 1 and 10. One represents a poor coating and 10 represents an excellent one. The second method is the surface roughness. Using profile meter, the change in surface roughness of the specimens was monitored. The weight loss factor was obtained also. The weight of the panels was measured before and after the exposure to salt fog test. Morphological characterizations of the coatings corrosion products layer were performed using scanning electronic microscope. Using an optical microscope attached with imaging analyzer, localized corrosion such as pitting was monitored and quantified.

RESULTS AND DISCUSSION

The coated panel surfaces were rated for their degree of rusting using ASTM D610, "Evaluating degree of rusting on panted steel surfaces". Some of the coated panels showed several breakdowns at the edges, therefore the edge corrosion was disregarded. The results are summarized in Figure 3. Figure 4 shows the change of surface roughness of each system during the exposure.

After 3000 exposure hours to salt fog tests and modified salt fog tests, the best coatings performance were system D and E. They gave the lowest rusting degree, weight loss and minimum change in surface roughness. The above results and observations are in agreement with the results obtained by Novak et. al., (~ ~ which was performed on polyurethane.

Single coat of inorganic zinc (system B) received the next highest performance. However, the same single coat with thickness 2.5 to 3.5 mils was very disappointed. This result was expected according to the initial study, which was performed using electronic thickness gage and the electronic microscope before the exposure. Very rough and non-uniform surfaces throughout the surface as well as some imperfections on the surfaces were observed. These results indicate the important role that surface thickness plays in certain coatings in the coating industry.

Two of the four zinc/epoxy samples had some type of failure. This coating system suffered light to moderate degree of pitting and blistering. This can be seen in Figures 5-b and 6-b. Once the epoxy top coating ruptures, a fine, white powder was detected under the coating and which was washed out during the exposure, leaving a bare steel to rust. Possible explanation to this phenomenon can be the formation of an oxygen-deficient cell at the topcoat and the primer interface caused by slow permeation of water

3

through the film. This water, with time, results in the formation of alkali, which dissolves the zinc primer, further accelerating the deterioration of coating system. Fultz (~2) has noticed the same phenomena in different coating systems including chlorinated rubber, alkyd, and aliphatic polyurethane.

Figure 5-c and 6-c show that the surfaces of zinc/polysiloxane system were highly protected with no signs of either blistering or pitting. The inherent compatibility of polysiloxane with inorganic zinc primer essentially improved the properties of this coating system, which was missing with a traditional epoxy.

Another purpose of this study was to determine if the 100% solids coating such as polyurethane will increase corrosion resistance to marine environment. When comparing the average of all panels coated with coated systems and polyurethane, the latter coating provides some improvement in corrosion resistance. However, some limitation still exist encountering this system due to the process of application and curing.

It was very important to correlate the data obtained by these two tests with the real marine exposure. The accelerating factor, which can be defined as the ratio of corrosion rate obtained from uncoated steel by an accelerated corrosion test to, that obtained by a long field performance test was obtained. Table 3 shows that accelerating factor for each test. Modified salt fog test however, improved the accelerating factor to 47 percent. Nevertheless, lots of work needs to be done in this field to reduce the time exposure frame of the available lab tests.

Localized corrosion growth, pitting, was monitored during the exposure using imaging analyzer. As can

be seen from Figure 7. It was interesting to see the pit was growing from area of [ 11766.44 ~m 2 ] to

[17403 ~m 2 ] in 10 days of exposure. This result shows how sever the environment was.

CONCLUSION

The new generation of coating system (polysiloxane) topcoated IOZ received the highest overall performance rating.

The single coat IOZ of 3.5 to 4.5 DFT performed significantly well. Polyurethane with 0% VOC gives very good results, which indicates that the 100% solid coating will

give very good results. The accelerating factor using modified salt fog test was improved.

ACKNOWLEDGEMENT

The authors would like to acknowledge funding from NSERC Operating Grant and the NSERC/ACPI CRD program. Special thanks are due to Ameron International Company, Mr. Danies Agnew, for their assistance and support.

REFERENCES 1. N. Rengaswamy, R. Vedhalakshmi and K. Balakrishnan, ACM&M vol.42, 3(1995):p.7 2. C. Giudice and B. Amo, Corrosion prevention and control, April (1996): p.43. 3. Z. Ahmed and F. A. A1-Suliman, British Corrosion Journal, 28, 2(1993): p.112. 4. K. Baldwin and C. Smith, Aircraft Engineering and Aerospace Technology Journal, 71, 3(1999). 5. H.H. Man, H. C. Man, L. K. Leung, Journal of Magnetism and Magnetic Materials, 152 (1996): p.

40.

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6. B.R. Appleman, Journal of Protective Coatings & Linings, October (1992): p. 134. 7. L. Bone III, Material Performance, November (1989): p.31. 8. Z. Chengde, SPE 29972, the international meeting on petroleum engineering, Beijing, China, 14-17

November (1995): p.215. 9. A. Szokolik, Journal of Protective Coatings & Linings, March (1992)" p. 22. 10. C. G. Munger, Material Performance, June (1992: p. 36. 11. H. L. Novak and Klotz, J.M., Proceedings of the American Chemical Society Division of Polymeric

Materials- Science and Engineering, 68 (1993)" p. 105. 12. B. S. Fultz, Journal of Protective Coatings & Linings, December (1988): p.46.

Atmospheric zone

Splash zone

Tidal

Submerged

Subsoil

Relative loss in metal thicl~ness

Coatings Steel

Figure 1 - Corrosion rates of steel and coating in marine environment

Figure 2. Coated panels placed in the salt fog cabinet

5

Excellent

Poor

A B C D E

- - " Salt fog - - - Modified

Figure 3 - Degree of rusting using ASTM D610

6] 5-I 4

# 3

14m

A B C D E

Q Before exposure

DAfter

Coating systems

Figure 4. Surface roughness change during the exposure

6

Figure 5 - pictures taken by imaging analyzer, (a) zinc coating, (b) epoxy (c) polysiloxane

....... ~ dN

~'~' i i ~ . . . . .

i

Figure 6. SEM photographic for coating systems

7

Figure 7 - monitoring of pit grow, (a) original pit, (b) the pit area (c) the pit area after 10 days of exposure.

Table 1 - Coating systems

System Primer Thickness in mils

Topcoating Total DFT in mils

A IOZ 2.5-3.5 Non 2.5-3.5 B IOZ 3.5-4.5 Non 3.5-4.5 C IOZ 3.5-4.5 Epoxy 9.5-11.5 D IOZ 3.5-4.5 Polysiloxane 9.5-11.5 E Polyurethane 6-7 Non 6-7

Table 2- VOC concentration of coatings

Coating IOZ Epoxy Polysiloxane Polyurethane C continent 0 VOC continent (lbs/gal)

2.7 2.3 1.0

Table 3- Accelerating factor

Accelerating factor

Salt spray test

17

Modified test

21

8

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