Corrosion within cargo and water ballast tanks poses a substantial and ongoing problem in the maritime industry. Analysis conducted by DNV (Det Norske Veritas), a leading classification society, indicates that corrosion is the single most frequent cause of hull-related issues identified during surveys, reports DNV.
Rapid Acceleration
The actual rate at which corrosion occurs within ballast and cargo tanks on ships can vary considerably from the assumptions made in classification rules and the corrosion models that are integrated into them. Survey data collected by DNV reveals a notable increase in the detection of corrosion in both water ballast tanks and cargo tanks on oil tankers and bulk carriers, specifically at the third renewal survey. This trend continues to escalate, reaching its peak at the fourth renewal survey (as illustrated in Figure 1, which is not provided here). This observation suggests that for a considerable number of vessels, the effective lifespan of the protective coatings applied may only be approximately two-thirds of the duration mandated by the Performance Standard for Protective Coatings (PSPC).
Material experts at DNV highlight the fact that the process of deterioration due to corrosion is neither linear nor are its consequences straightforward. Once the initial breakdown of the protective coating begins, the rate at which corrosion progresses accelerates rapidly. This swift advancement leads to various forms of corrosion, including pitting, edge corrosion, and grooving corrosion. Further analysis of the DNV survey data indicates that a significant number of corrosion findings were documented in areas believed to be exposed to higher temperatures. In these locations, the rate of corrosion and its spread were observed to be even more advanced, underscoring the influence of temperature on the corrosion process within ship tanks.
Possible Causes
The deterioration and eventual failure of protective coatings within ship tanks are complex processes often driven by a combination of interacting factors. The rate of corrosion itself is influenced by a range of ambient conditions, including the salinity and acidity of the environment, humidity levels, temperature fluctuations, the duration of exposure to these conditions, and the initial condition of the coating. It’s crucial to understand that these degradation rates are highly non-linear, meaning they don’t progress at a constant pace, and they are significantly dependent on the amount of time the coating has been in service.
Interestingly, different stakeholders in the maritime industry often have differing perspectives on the primary causes of coating failures. Coating manufacturers frequently assert that the majority of failures can be attributed to inadequate surface pre-treatment prior to coating application and/or deficiencies in the way the coating was applied. On the other hand, ship insurance companies and charterers often contend that the primary reasons for coating failure lie in incorrect specifications, such as poor design or the use of substandard coating quality, and operational conditions that exceed the original design parameters of the coating system.
Examples of such detrimental operational conditions include maintaining higher cargo storage temperatures for extended periods, employing incorrect or overly aggressive cleaning procedures, and mechanical damage inflicted on the coating during cargo handling and ballast operations. These differing viewpoints highlight the multifaceted nature of coating performance and the various stages throughout a vessel’s life cycle that can contribute to coating degradation.
Impact Of Corrosion
Corrosion significantly compromises the structural integrity of ships. It causes a reduction in steel thickness, both locally (e.g., pitting, grooving) and generally. While localized corrosion might not immediately seem like a major safety concern, it can act as a catalyst for the formation of cracks, holes, or surface irregularities. These can lead to leakage into ballast tanks, resulting in environmental damage or contamination of cargo with ballast water (as shown in Picture 1, right, which is not available here). A dangerous “domino effect” can also occur: localized corrosion causing steel/welding wastage and cracking in multiple structural parts can lead to an overload of adjacent members, further weakening the vessel’s structure.
Beyond structural concerns, corrosion also drives up maintenance and operation costs if not addressed promptly. Incorrectly assessing the coating condition in areas experiencing accelerated corrosion can necessitate steel renewals even between scheduled class surveys. This leads to off-hire time for the vessel and potential breaches of contract due to unavailability.
Recommendations Given
Maintaining the original protective coating on the interior surfaces of ballast tanks stands as one of the most effective strategies for preventing corrosion. Any necessary repairs to this coating should be carried out by qualified personnel who strictly adhere to the coating manufacturer’s application instructions. Failure to do so significantly increases the likelihood of premature coating damage following the repair work.
Implementing a consistent and regular inspection and maintenance program for coatings is crucial. This proactive approach allows for the early identification and treatment of corrosion issues before they escalate into more severe problems that could compromise the vessel’s structural integrity or lead to operational disruptions.
Engaging DNV early in the process for an assessment of the coating’s condition can provide valuable insights into the current status of the tanks. DNV offers a customized hull survey plan designed to closely monitor the condition of tanks that have been reported with existing corrosion damage or whose coating condition has been rated as “Fair” or “Poor.” In addition to the regular periodical surveys, targeted surveys can be introduced to specifically include these identified tanks within the scope of the upcoming inspection, ensuring focused attention on areas of concern.
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Source: DNV
