Ballast Tank Protection – Black Magic or Black Hole?

Introduction
With the introduction of double hulled tankers, the area of ballast tank to be coated has increased considerably. The capital invested in such vessels has also increased considerably, with the tendency now for the initial purchaser to retain the vessel for much of its useful service life. Certain factors, for example: reduced scantlings, higher operating temperatures in certain areas of the tank, greater vessel steel flexibility and new types of steel however, all combine to potentially restrict ballast coating lifetime. With major repair or refurbishment of ballast tank coatings likely to be prohibitively expensive, the importance of “getting it right” at newbuilding cannot be overstated.

This paper discusses a means to achieving the most cost efficient anti-corrosion strategy available from coatings and sacrificial cathodic protection systems in ballast tanks. Get it right or suffer a huge financial drain – literally a Ballast Black Hole.  

The protection current requirement of different coatings from anodes has been measure both in the field and the laboratory using a number of novel experimental techniques. The rate of coating breakdown with time which can be obtained from standard and premium coating systems has been estimated from field inspections. This data has then been combined with information from coatings and anode manufacturers on material costs, together with figures from repair yards for anode installation.  

Laboratory Work

Measurements were carried out in our laboratories on two series of coated panels which had been immersed in sea water for six and eighteen months, prior to testing. These 12”x6” panels had a standard defect introduced onto one face, before tests commenced.  

The ability of a coating to interact successfully with a cathodic protection system results from a combination of good barrier properties and good resistance to cathodic delamination (i.e. its ability to resist the adverse effects of the CP system).  

Barrier properties were determined using two different techniques; Electrochemical Impedance Spectroscopy (EIS) and polarisation current measurements. Both methods involved the use of a three-electrode test cell, where the coated panel was the working electrode. The reference used was a saturated calomel electrode (SCE) and ferritic stainless steel was used for the counter electrode.  

In the EIS method, the test panel is subjected to a spectrum of voltage sine waves in the frequency range between 10 kHz and 0.1 Hz at an amplitude of less than 100mV. The resulting current sine wave is compared with the original and analysed in terms of its real and imaginary components of the impedance. For coated panels, this usually results in a semi-circular plot, where the diameter of the semicircle can be related to the barrier properties of the coating. Larger semicircles are associated with better coatings. A typical ? is shown below:    

The coating impedance for the standard (modified epoxy) and premium (pure epoxy) coatings after six months immersion are given in table 1, for both one and two coat schemes, at a dry film thickness per coat of 150µm.  

Table 1. Coating impedance after 6 months immersion.

It can clearly be observed that the resistance component of the impedance is approximately four times greater for the pure epoxy coating when full scheme thicknesses are applied. It is also noticeable that the number of coats is also crucial for achieving excellent barrier properties.

Data shown in table 2 is taken from one coat panels after eighteen months immersion. Three separate areas were measured on each panel, on the front close to the original artificial defect, over the defect and on the rear face of the panel.  

Table 2. Coatings after 18 months immersion.

If can be seen that the undamaged back of the panel (as would be expected) performs over an order of magnitude better than the panel face. The ratio of the impedances taken on the defect to that from close to the defect is an indication of the resistance of the coating to cathodic delamination. The ratio between the results from the front and back faces can also be used to provide information on the ability of the paint to resist localised breakdown. In all cases, the premium (pure epoxy) coating performed better than the standard coating (modified epoxy).

Cathodic protection current measurements were carried out at two applied potentials, -850mV and -1050mV. These correspond to the potentials found respectively, at a distance and close to, a zinc anode. The currents obtained are shown in table 3, where it can be noted that again, the premium (pure epoxy) coating is superior in performance by a factor of around 5 at both potentials.

Table 3. Protection currents and coating types after 2 months.  

However, it can be observed that the values for both paints are considerably less than the 5mA/m² that is taken as a design guide by both the anode manufacturers and the Classification Societies.

Time vs. protection current requirement measurements for the coated panels after a six month immersion period without cathodic protection, are shown in figures 1 and 2.

Figure 1 (above) shows how the initial current drops very rapidly over a period of some 350 hours as the pH falls at the defect area and calcareous deposits form as a result of the presence of the alkali.

Figure 2 (below) shows the longer term behaviour, where a gentle increase in the protection current required occurs with time. Part of this current passes through the coating and part travels along the interface between the paint and the steel. Increases in current are therefore a result of cathodic delamination and a reduction of the barrier properties of the coating. The slope of the graph may be taken as an indicator of the service life of the coating. Once again, the slop of the graph for the premium (pure epoxy) coating is shallower than that for the standard (modified epoxy) coating, showing the superior properties of the former.

Vessel Examinations

Examination of coatings in ballast tanks has shown large differences in the initial breakdown rates between standard and premium coatings. High levels of performance have been shown to be btainable from premium pure epoxy coatings after three years in service. Estimates of the level of breakdown of the coating are <0.01%, without maintenance. Standard modified epoxy coatings would be expected to show some breakdown at cut edges after this time period. A breakdown level of 10 times this would be typical after a similar service lifetime.

Interactions of Coatings and Anodes

From the laboratory work, it can be noted that coatings are susceptible to degradation from the “protection” current from the anodes. In service, the calcareous deposits build up under the coating at any areas of damage and can lever intact coating from the steel, thereby accelerating the rate of coating loss. It could be argues that the compact rust scales that also form at sites of coating damage, could have been much more protective that the calcareous scales. From a series of vessel inspections, it has been observed that cathodic protection systems can either cause or enhance coating blistering.  

Both the blistering and calcareous deposit leverage of the coatings are much more severe in their effects during the first few months of the life of the paint. During this period, the coating remains plasticized by any retained solvents and begins to develop electrochemical stability. The introduction of sacrificial anodes into ballast tanks when coating breakdown has begun to occur naturally, would therefore be a better option, from both and economic and corrosion protection viewpoint. Anode posts could be installed at new construction for later use or clamp on anodes may be preferred. In either case, capital is not tied up on the vessel unnecessarily during the early part of this ships service life.

Cost Benefit Analysis for Coating & Anode Combinations in Ballast

A dynamic spreadsheet model of the factors which relate directly to the costs of coating and maintaining ballast tank coatings has been developed. The Cost Benefit Analysis (CBA) model has been designed to allow a series of scenarios to be considered, in order to find the most cost effective route for the protection of ballast tanks.  

Data on anode size and cost can be entered directly, together with the data on the current which will pass through the paint itself. In the Classification Society guidelines and the sacrificial anode manufacturer’s literature, it is assumed that a “good” coating will require a protection current of mA/m². Uncoated steel will need around 110mA/m² for protection.  

Recent measurements which have been made on a number of coating types suggest that these figures are an overestimate of the current passing through coatings, as shown previously in table 3. It can be seen from the table that all the coatings allow a much smaller current to pass through them than is suggested in the calculation guidelines and that the pure epoxy is over an order of magnitude smaller in its protection current requirements that the other two coating types.  

These actual current requirements have been entered into the CBA and a significantly reduced number of anodes per give year interval was calculated from this data. In practice, many owners and operators have stated that the sacrificial anode consumption in ballast thanks, when the coating is in good condition, is not as great as the Class and anode suppliers calculations predict. The above data explains the reason for the difference.  

The Amtec/International Paint CBA model also permits the coating type to be changed to reflect the coast, coverage and anti-corrosion performance differences between different paint types to be included, as coating types vary in their corrosion prevention capabilities.  

Additionally, the costs of maintenance of the ballast coatings have been incorporated into the model. When the coating costs are combined with the sacrificial anode costs and maintenance costs, a picture of the relative financial benefits of different coating/CP options over the proposed lifetime of the vessel can be constructed. As the costs have been discounted forwards (i.e. the costs are those which would be payable in the future), informed decisions can be taken regarding the ballast coating scheme and anode combination to be used.
 

Cost of Sacrificial Anodes

Tables 4 and 5 are examples of calculations of the cost of zinc anodes and their installation into ballast tanks during a dry docking period. Table 4 shows the calculations based on the Classification Societies recommendations and table 5 shows the calculations based on measurements taken from a pure epoxy coating.  

  Table 4. Anode requirements based on Class recommendations.

  Table 5. Anode requirements, based on values recorded using a pure epoxy paint.

 
Comparative scenarios are shown in tables 6 onwards. All costs are in US$ and assume that a Bank interest rate of 5% will prevail over the 25 year lifetime of the vessel. These figures can be changed in the spreadsheet.

On Board Maintenance Costs

In the examples in tables 6 and 7, it has been assumed that the crew will carry out any ballast coating maintenance necessary and that the level of breakdown will be held constant. The area to be touched up therefore, (which includes an allowance for overlapping onto sound coatings) will remain constant, in this example.  

Pre epoxy coating (0.1% breakdown per 5 years)  

  Table 6. Future cost of pure epoxy coating.
 

Modified epoxy coating (0.5% breakdown per 5 years) 

  Table 6. Future cost of pure epoxy coating.

Anode and Coating Costs, Discounted Over Vessel Lifetime

Table 8 compares the potential anode requirements for two coating types and compares the costs of adding anodes into the ballast tanks at different stages of the coatings service lifetime. At year zero, the pure epoxy costs are for the coating only. The modified epoxy costs include the installation of sacrificial anodes.

Table 8. Projected cost in ballast tanks for two coating types.
 

Paint, Anode and On-board Maintenance Costs, Discounted Over Vessel Lifetime

Table 9 (below) summarises the total costs of coatings, sacrificial anodes (including costs) and the costs of on-board maintenance of the ballast coatings. At year zero, the pure epoxy costs are for the coating only. The modified epoxy costs include the installation of sacrificial anodes.

Table 9. Projected total costs of coating, sacrificial anodes and on board maintenance

It is apparent from table 9, that the application of a most costly coating with superior anti-corrosive performance and with low protection current requirements (such as a pure epoxy) can be more cost-effective than a standard (modified epoxy) coating type which would require a much larger number of anodes over the service life of the coating. However, it should be noted that for both of these coatings, the measure anode requirement is less than that which is currently calculated for vessels at new construction.

Conclusions

A combination of laboratory work and experience from ship examinations has shown that a premium quality, pure epoxy coating out-performs the best modified epoxy paints in terms of barrier properties and delamination resistance.

The current requirements of both coatings were significantly lower than those suggested by the Classifications Societies, with the pure epoxy Intershield Newbuilding, being an order of magnitude lower.

Cost benefit analysis demonstrates that the application of a high quality coating such as Intershield Newbuilding is technically and economically favourable over the lifetime of the vessel, when compared to standard modified epoxy systems, when maintenance and anode consumption costs are included in the calculations.

Anodes should be installed in ballast tanks when they are needed, rather than as a matter of routine at Newbuilding. The choice of ballast tank coating at Newbuilding (or reblast/major refurbishment) is of critical importance.

The author would like to thank International Paint for their help in compiling this paper.

Content Ends

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