Sensible Improvement in Ballast Tank Corrosion Control

INTRODUCTION

Ballast tanks within the cargo area or adjacent to the cargo area must be protectively coated with a material compatible with environmental factors likely to be encountered. (e.g. chlorides, heat, chemicals, expansion, moisture, abrasion, fuel, oil) Inferior levels of metal hygiene in surface preparation and poor coating suitability are prime factors in coating breakdown and eventual corrosion in ballast tanks.

According to IACS rules, coating in the worst “area under consideration” must be “Good”, as a class notation. Any other notation will impair a ship’s ability to trade. Tank coatings must remain “Good” for life if ship operations are to remain profitable. However, actual performance ol coatings (in particular widely specified high-solid epoxies) falls short of the International Maritime Organization (IMO) 15 year target service life; the duration for remaining “Good” is 8 to 10 years or less in actual service.


Coating Condition IACS Class Notation

Minor spot rusting Good

Light rusting over >20% Fair

General breakdown >20% and hard scale >20% Poor

Epoxy Coating Nominal Dry Thickness # of Coats Expected Service Life

200 microns 1 5 ± 3 years

300 microns 2 10 ± 3 years

300-400 microns 3 15 ± 3 years

Note: expected service life ONLY if substrate surface salt content is 50mg/m2 before coating cures.

Coating Cracking

Over areas such as block joint areas and on butt, seam, and fillet welds, internal stress can cause cracking failures of high solid epoxies that may occur within a shipbuilder’s 12 month warranty, but may also take longer than a year to develop, resulting in unexpected repair costs for owners.

Wu describes the problems faced in mitigating the development of coating cracking: “Development of service cracks in epoxy-based corrosion protective coatings limits the life of the substrate structure. If cracks develop, corrosion protection is lost and costs of repair and re-protection of large marine structures can be crippling. Factors controlling development of cracks in the coating are poorly understood, and predictions of coating lifetime approximate.”1 Wu (2014).

Understanding main factors behind internal stress factors has made important strides in recent years. A recent study on internal stresses and mechanical properties of coatings points to adhesion as probably the most important factor in preventing cracking. “Adhesion however is not a fundamental property of the coating/substrate interface rather it is the consequence of the interaction between the polymer and the substrate. It is these interactions which must be understood to provide answers to the cracking issue.”2 Reed (2017).

Weld issues

One of the greatest challenges to welding is preventing contamination of the molten puddle. Welding over contaminants can form gases that cause metal to oxidize. When a metal oxidizes, it will not respond well, often resulting in weaker weldments. Heat-affected-zone (HAZ) adjacent to the weld are particularly vulnerable to corrosive attack, requiring extra attention in surface preparation and coating. The level of surface hygiene post-weld and over HAZ areas are a primary factor in a coating’s ability to bond with and protect these vulnerable areas. Pre-weld hygiene contributes mainly to the ease and likelihood of weld success.

Surface Preparation

Optimal surface preparation is essentially a matter of metal hygiene. To maximize adhesion and impermeability, coatings must perfectly and permanently match the surface and pores of the surface, with no embedded material or grit to introduce contamination or interfere with contact between surface and coating. The distance between the surface of the substrate and the coating should be as small as possible, with no contamination between substrate and coating to prevent perfect adhesion. More reliable surface decontamination in the field is therefore critical to creating an optimally receptive surface for coating.


Sulfides are extremely hygroscopic, ionically charged, difficult to remove, and ubiquitous in steel and other metals. In recycled metal, the presence of sulfides greatly accelerates corrosion. "In older vintage and low quality steels, hydrogen blistering is associated with dirty steel (i.e. high sulfur(S)) with highly oriented slag inclusions or laminations. These materials have produced large internal blisters in plate steels used to construct pressure vessels and tanks. In some cases these blister can reach a size of 30 cm (1 ft) diameter or greater"3 Burt (2015). Optimal surface preparation is, in essence, a matter of metal hygiene. Removing sulfide and sulfate contaminants logically increases resistance to cracking. Current technologies fail to address this important challenge to proper metal hygiene.

Testing for Sulfides and Hidden Salts

“Intergranular contaminants such as sulfides and chlorinated hydrocarbons are more elusive to quantify since they are more difficult to remove. There is no generally accepted standard for either chloride, sulfate or sulfide contaminant levels under coating and lining systems”4 Vincent (1998).


Kits are available to test for chlorides prior to coating, but not for sulfides (which are insoluble). Current field testing does not give accurate quantitative and qualitative measurements, as chlorides beneath iron sulfide films are undetectable. Accurate testing for intragranular contaminants requires scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDXA). Therefore, testing is unlikely to reveal the true state of metal hygiene.

New Technologies

Salt removal and rust removal products only remove soluble salts; they cannot remove sulfides or chlorides hidden beneath sulfide films. Indeed, it is difficult even to detect such hidden salts. “iron sulfide is insoluble, therefore water cleaning is not possible. Sulfides to penetrate into the intergranular crevices in metal substrate” and are difficult to remove.4 Vincent (1998).


Understanding that ensuring wholesale removal of sulfides and other microcontaminants is needed to promote maximum coating adhesion and consistent contact at the coating/substrate interface, a novel metal decontamination technology called Corr-Ze was developed to remove ionic and highly hygroscopic microcontaminants (i.e. sulfides, sulfates chlorides, nitrates and microbial by-products) from metal surfaces by penetrating the sulfide film, removing the ionic attraction, and rinsing away microcontaminant detritus to ensure more reliable and complete surface preparation outcomes. The product has been successfully tested in with small footprint portable CleanerBlast vapor abrasive blast equipment units that utilize cleaning at low pressure to prevent embedded grit and shearing of surface material. Corr-Ze is added to the blast tank to simultaneously decontaminate during blast cleaning. The resulting surface hygiene requires no additional processes or products (i.e.: salt removers, inhibitors, dehumidification, rust removers) before coating.10X engineered abrasive media is part of a total process approach that creates a synergy of new technologies and adds exceptional, rapid results in degreasing and blast cleaning to prevent debris or foreign material lodging on or in surfaces.

CONCLUSION

Corr-Ze and 10X used in the CleanerBlast equipment can easily address ballast tank surface preparation procedures to achieve improvements in metal cleanliness, as the contamination problems associated with ballast tanks are essentially the same as those in our summaries above, while the health and safety constraints are stricter. As CleanerBlast, 10X and Corr-Ze are able to lower health and safety risks significantly compared to traditional dry blasting and rust inhibitors, they may be considered especially suitable for enclosed space use. 10X contains no components that contribute to toxic dust and is recycled, recycleable and reusable in addition to costing less per project to use.

The modest cost of using this Total Process system is immediately recouped not only by eliminating the expense associated with rework attempts often needed to meet surface preparation standards but also because the Total Process system allows no dwell time, single-step cleaning and decontamination.


CleanerBlast equipment cuts downtime and wasted labor dramatically.

  • Fill & load time is reduced by 80% (compared to other vapor blast machines) via the built in fill station for easy, no-waste loading and dual pump system.

  • Accessible parts allow 10x faster field maintenance,

  • Rapid response rinse feature and long run time/low consumption rate eliminates need for a dedicated pot tending personnel.

  • Machine settings maintain consistent blast pressure, with no need for adjustment, shift after shift.

  • Most reliable and durable wet abrasive blast machine on the market.

  • Specially engineered for low wear on blast pump and hoses, 20x more service life of critical valves.

CleanerBlast equipment can also be used for other maintenance tasks (such as cleaning in place a variety of surfaces and gently removing delicate gel coat over fiberglass) for even greater ROI in the shipyard environment.

The suggested Total Process synergizes surface preparation technologies. Adopting this process for implementation in ballast tank surface preparation will prevent expensive additional inspections required by authorities due to poor coating performance, decrease operating costs and increase ship’s operating time and profitability.

Substantial improvements in surface preparation to properly addresses chlorides and iron sulfides can help reduce or eliminate premature coating failure. The suggested Total Process improvements offer a method for reducing maintenance, increasing adhesion reliability and improving resistance to cracking to increased ballast tank longevity and performance. Implementing these measures eliminates the need for expensive additional inspections required by IACS and Port State Control (PSC) authorities due to poor coating conditions increase operating costs and reduce operating time and profitability.

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REFERENCES

1. Wu, T.Y. et al., "Fatigue Crack Development in Epoxy Coatings on Steel Substrate: The Role of Coating and Substrate Properties in Determination of the Onset of Fatigue Cracks", Advanced Materials Research, Vol. 891, 2014. pp. 854-859.

2. Reed, C., & Eliasson, J. “Coatings Cracking in Water Ballast Tanks: A Different Look”, CORROSION 2017, paper no. 9088. Houston, Texas: NACE International. March 26-30, 2017. 8 pgs.

3. Burt, V. ed. Corrosion in the Petrochemical Industry, Second Edition. Materials Park, Ohio. ASM International. 2015. 426 pp.

4. Vincent, L.D. “Decontamination of Metal Substrates”. CORROSION 98, paper no. 98620. Houston, Texas: NACE International. March 22-27, 1998. p 2, 4.

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