KUP 40 Ability to state the causes of corrosion in cargo spaces and ballast tanks and how corrosion can be identified and prevented

Corrosion poses a persistent and costly threat to the structural integrity and operational lifespan of ships, particularly in cargo spaces and ballast tanks where exposure to harsh environments and varying conditions is most pronounced. Understanding the root causes of this corrosion is paramount in developing effective strategies for its identification and prevention.
This KUP aims to shed light on the multiple factors contributing to corrosion within these critical ship areas. We will explore the chemical reactions, environmental influences, and operational practices that accelerate corrosion, as well as the telltale signs that indicate its presence. Additionally, we will delve into a range of preventive measures, from advanced coatings and cathodic protection systems to meticulous maintenance routines and material selection considerations.

Corrosion in cargo tanks

Cargo tanks are essential revenue generators for tankers. Corrosion or structural failure in these spaces can result in substantial financial losses, expensive repairs, and environmental contamination. For chemical and product carriers, choosing the wrong coatings can lead to cargo contamination, potential rejection by charterers, costly repairs, and significant downtime. Selecting the right cargo tank coating is crucial for ship operators, as an incorrect choice can severely limit a vessel’s cargo capacity and trading capabilities.

Oil tankers

Corrosion in oil tanker cargo tanks typically falls into four categories: general, local, pitting, and weld metal corrosion.

1. General corrosion often appears as a crumbly scale on uncoated surfaces, exposing fresh steel upon removal. This type is accounted for in the tanker’s design, with an estimated in-service wastage of roughly 0.1mm annually. Classification societies generally expect structural members to last 20-25 years based on their corrosion allowances.
2. Local corrosion targets highly stressed components that flex during operation. Rust or scale dislodges from these areas, exposing bare steel to further attack. As the material thins, stress increases, accelerating corrosion. Grooving corrosion also occurs at structural intersections where water collects or flows.
3. Pitting corrosion is localized and commonly found on tank bottoms, horizontal surfaces, or details where water gathers. A protective layer of black rust and residual oil often shields bare steel plates. However, when this natural coating breaks down, severe pitting can occur in areas like cargo bellmouths or where cleaning fluids impact, especially due to electrolytic or microbial action. Extensive pitting can weaken the structure, necessitate costly repairs, and even lead to leaks and pollution.
4. Weld metal corrosion involves electrolytic action between the weld and base metal, resulting in pitting or grooving. This corrosion type can be particularly problematic due to the potential weakening of welds, which are crucial for structural integrity.

Various factors contribute to corrosion in cargo tanks, including:

• No Protective Coating: Uncoated tanks experience general corrosion, but certain areas may corrode faster due to factors like cleaning fluid impact or detached mill scale.
• Excessive Crude Oil/Water Washing: While crude oil forms a protective layer, frequent washing with seawater removes it, initiating corrosion. Increased crude oil washing also weakens this layer.
• High Sulfur Content: Crude oil with high sulfur reacts with seawater, forming acidic compounds that cause significant general and pitting corrosion. Sulfur also promotes corrosive cell formation.
• Poor Inert Gas Quality: Inert gas with oxygen content exceeding 1% increases corrosion, especially on overhead surfaces where moisture condenses. Sulfur and soot in unwashed flue gas can also accelerate corrosion.
• Inadequate Electrical Grounding: Ineffective grounding leads to stray currents in the steel, worsening pitting corrosion.
• Coating Defects: Localized coating breakdowns concentrate electrolytic action, accelerating pitting in those areas.
• Material of Construction: Modern high-tensile steels, especially those made with the Thermal Mechanical Control Process (TMCP), are a potential factor in accelerated corrosion compared to older vessels.
• Microbial Attack: Bacteria are present throughout oil production facilities, from the plant and pipelines to the cargo tanks on oil tankers. Many of these microbes produce corrosive acids, and a diverse population of interdependent bacteria, rather than single species, often leads to the most significant corrosion. Sulphate-reducing bacteria (SRB) are commonly linked to steel corrosion due to their production of sulphides, which can manifest as metal sulphides, dissolved sulphides, or hydrogen sulphide gas.
• Sludge/Scale Accumulation: The accumulation of sludge and scale in cargo tank bottoms is not uncommon. This residue from previous cargoes or dislodged corrosion products creates an ideal environment for bacterial growth and can conceal subsequent pitting damage. Moreover, it hinders tank drainage by blocking holes and creating uneven surfaces.
• Water in Cargo Tanks: Residual water in cargo tanks can come from various sources, including heating coil leaks, inert gas condensate, ballast water, wash water, poor drainage, entrained water in cargo, slop tank water, and leaks from adjacent ballast tanks. When this water separates from the cargo, it can trigger electrolytic or microbial corrosion, especially on the aft tank bottom near suction bellmouths where water accumulates due to the ship’s trim.
• High Humidity: Excess water in cargo tanks leads to high humidity, especially in the vapor space when loaded and throughout the tank during ballast voyages. This worsens both general and pitting corrosion.
• High Temperature: Double-hull tankers’ wings and double bottom spaces act as insulation, preventing cargo tanks from cooling down to sea temperature. This keeps the tank structure close to cargo loading temperature, even after discharge, until cooled by air or ballast water. This temperature difference can reach 15°C, significantly increasing general corrosion. For every 10°C rise, the corrosion rate reportedly doubles. High temperatures also boost bacterial growth and microbial corrosion. Wing tanks in single-hull tankers also provide insulation for center tanks.
• Structural Flexing: Optimized design and high-tensile steel use have reduced the stiffness of ship structures, leading to increased flexing. This causes scale to shed from vertical and inverted surfaces, exposing fresh steel to accelerated general corrosion.

Bulk carriers

Operational factors significantly influence cargo hold corrosion in bulk carriers. The frequency of transporting coal, more corrosive than iron ore due to sulfates and chlorides, is a major factor. The type of cargo also affects wear and tear on protective coatings in different areas. Environmental conditions like humidity, coal and seawater temperature, and deck heating influence corrosion rates, particularly for topside and double bottom ballast tanks. Ballast ratios also matter, as exposed steel corrodes when tanks are emptied. Therefore, predicting bulk carrier corrosion requires considering factors like coal/iron ore cargo ratios, loading and ballasting frequency, ballast ratios, and trade routes.

Coal’s corrosivity is highly variable due to the presence of impurities like chlorides and sulfates, along with varying leachate acidity. These factors, combined with operational variables, make bulk carrier corrosion rates highly unpredictable. Coal’s corrosiveness stems from its ability to produce acidic leachate and the contact of coal particles with steel. Free moisture containing leached impurities, along with galvanic and differential aeration effects at contact points, contribute to this. Carbon in coal forms a galvanic couple with mild steel, accelerating corrosion. Overall, bulk carrier corrosion rates are highly variable due to the combined effects of cargo type, operational factors, and environmental conditions. Understanding these factors is key to predicting and mitigating corrosion in bulk carriers.

Corrosion in Bulk Carriers and Operational Factors

The electrochemical corrosion process involves anodic oxidation and cathodic reduction, usually requiring oxygen and moisture. Temperature also affects the rate of this chemical reaction. In bulk carriers, these factors vary depending on operation and location within the ship. A summary of corrosion characteristics for primary cargo hold components, linked to operational influences:

Tank Top Plating:

• No protective coating, corrosion product layer is worn away by cargo handling.
• Corrosion occurs when loaded (with moisture from coal/iron ore) and unloaded if humidity is high.
• Corrosion rate depends on coal type.

Lower Stools:

• No coating, but corrosion product layer forms.
• Chlorides and sulphates leached from coal contribute to corrosion.
• Time of wetness (TOW) affects corrosion, similar to tank top plating.
• Corrosion rate varies with coal type.

Transverse Frames:

• Higher corrosion at lower regions, where coating wears quickly from iron ore contact.
• Wear depends on the number of voyages with iron ore.
• TOW depends on voyages with coal and humidity levels when unloaded or carrying iron ore.
• Moisture quantity varies with sideshell sweating, which depends on coal and seawater temperatures.

Upper Stools and Bulkhead Plating:

• Fully coated, no contact with iron ore.
• TOW similar to transverse frames, but critical humidity likely lower due to hygroscopic coal dust.

Double Bottom Ballast Tank (DBBT):

• Coating and sometimes cathodic protection used.
• Coating breakdown on underside of tank top due to grab damage.
• Corrosion occurs when unballasted due to ineffective cathodic protection and high humidity.
• Coating deterioration and anode consumption depend on ballast time, salinity, and temperature.

Duct Keel:

• Fully coated void space.
• Coating breakdown on underside of tank top, related to frequency of cargo changes.
• Corrosion occurs when humidity is high (over 70%).
• Corrosion rate is roughly proportional to TOW due to powdery rust offering minimal protection.

Topside Tank:

• Coatings and usually cathodic protection used.
• Coating and anode deterioration depend on ballast time, salinity, and temperature.
• Underdeck stiffener coating prone to failure, possibly due to deck heating.
• Corrosion at exposed areas depends on time unballasted when cathodic protection is inactive.
• Unlike DBBT, humidity isn’t high and surfaces dry out, so TOW is proportional but not equal to unballasted time.

Ballast tanks

Seawater ballast tanks are the most challenging areas for corrosion prevention on a ship. Due to their large, mostly wet surfaces, they provide ideal conditions for corrosion to occur, and unprotected tanks may require steel replacement within six to ten years. Ballast tanks are often overlooked due to the difficulties of onboard maintenance, leading to structural issues. However, recognizing their importance for the ship’s structure, classification societies and port state control have implemented strict maintenance standards to ensure their condition remains satisfactory.

Requirements for Corrosion Prevention in Ballast Tanks (Tankers & Bulk Carriers)

SOLAS Amendment Chapter II-I/Regulation 3.2 mandates an effective corrosion protection system for ballast water tanks. The coating selection, application, and maintenance must be approved by the Flag State administration based on IMO guidelines. Light-colored coatings and, where suitable, sacrificial anodes are recommended.

IACS Unified Requirement URZ8 specifies the standards for corrosion protection coatings in saltwater ballast tanks, while URZ10.2 requires enhanced surveys for these tanks. Coating condition is categorized as GOOD (<5% corrosion), FAIR (5-20% corrosion), or POOR (>20% corrosion). Ballast tanks with POOR coatings, no coating, or soft coatings undergo annual surveys.

Applicable to tankers and bulk carriers aged five years or older, enhanced surveys involve close inspections with a focus on corrosion. Coatings and tank corrosion prevention systems are thoroughly examined, and thickness measurements are taken to assess plate condition.

A common issue in ballast tanks is the premature failure of protective coatings. This quickly leads to severe corrosion of the unprotected steel. Additionally, once the coating has failed, it’s challenging to restore it to its original condition.

Specific types of corrosion commonly seen in ballast tanks include:

• General corrosion: More prevalent on the deck head due to exposure to moist, salty air. It’s also found on bulkhead plating and stiffeners, worsened by proximity to hot cargo or bunker tanks. Higher oxygen availability in the upper tank regions exacerbates this issue.
• Stress corrosion: The use of optimized structures and high-tensile steels leads to increased strain on local components. This causes protective coatings to crack, exposing steel to corrosion. Rust and scale, normally protective, flake off, and the thinning material accelerates corrosion, potentially leading to cracks and structural failure.
• Coating condition: Areas difficult to coat, like welds and edges, are the first to suffer coating breakdown, exposing steel to corrosive ballast water. This is often due to poor initial surface preparation. Galvanic effects from large intact coating areas can further accelerate corrosion on these exposed spots.
• Coating brittleness: Over time, coatings become brittle and lose flexibility, leading to cracking and disbonding at high-stress points, especially in deckhead structures. This is caused by cyclic temperature changes, evaporation of volatile components, and chemical changes.
• Bottom pitting: Electrolytic and microbial corrosion are common in ballast tanks, particularly under sludge or dirt on the bottom plating and horizontal surfaces. Stagnant, oxygen-deficient water intensifies this process.

Corrosion Protection

Corrosion prevention is crucial to avoid costly and time-consuming repairs, including the downtime associated with them. The cost of prevention is comparatively minimal. To combat corrosion, we need to break the electrochemical circuit represented by the corrosion triangle. This can be achieved by coating the anodic surface, depositing a protective layer, introducing a sacrificial anode, or using an impressed current cathodic protection (ICCP) system.

Several methods can reduce or prevent corrosion on a ship’s structure:

• Applying a suitable paint coating.
• Implementing a cathodic protection system with either sacrificial anodes or impressed current.
• Combining paint coatings with a cathodic protection system.
• Galvanizing or depositing zinc on exposed surfaces.

The appropriate protection method depends on the location of the steel structure. Deck structures can be categorized as:

• Fully immersed (hull below the waterline)
• Splash zone (hull above the waterline and exposed decks)
• Exposed to the atmosphere (superstructure and deck mountings)

Corrosion Preventive Paint Coatings

Coatings, typically 0.2-0.5mm thick, act as a protective film against corrosion through barrier effects, cathodic action, or inhibition/passivation. They can be categorized as:

Type	Nature	Use
Hard Coating	Epoxy, coal tar epoxy, polyurethane, chlorinated rubber, vinyl, zinc epoxy, zinc silicate. Hardens by curing.	New constructions
Non-Convertible	Air drying.	Maintenance
Semi-Hard Coating	Stays flexible yet firm enough for contact.	Tank internals
Soft Coating	Soft, easily worn off by touch or low impact.	Tank internals
Shop Primer	Temporary coat applied after surface preparation.	Surface preparation
Inhibitors	Rust preventives and arrestors. Retard corrosion, form protective layers, or reduce solubility of corrosion products.	Localized application

Choosing the right coating depends on the specific application and environment. For example, hard coatings are ideal for new constructions, while non-convertible coatings are better suited for maintenance. Paint coatings often fail prematurely due to a few key reasons:

Physical damage: Impacts or abrasion from contact with metal objects can break the coating, exposing the underlying surface to corrosion.

Inadequate surface preparation: The surface wasn’t properly cleaned or treated before painting, leading to poor adhesion and early failure.

Incorrect paint application: The paint was applied too thickly or thinly, unevenly, or under unsuitable conditions (temperature, humidity), compromising its effectiveness.

Unsuitable paint choice: The wrong type of paint was used for the specific environment or application, leading to premature degradation.

Altered or harsher operating conditions: The coated surface experiences unexpected changes in its environment, such as increased exposure to chemicals or higher temperatures, which the paint wasn’t designed to withstand.

Criteria for evaluating coating:

Coating Evaluation Criteria typically assess the degree of damage by measuring the percentage of coating breakdown and/or rust scales in the inspected area, usually the entire tank. It also includes details on coating damage at edges and welds. The coating condition is then categorized as follows:

• Good: Minor spot rusting is present.
• Fair: Localized coating breakdown occurs at edges and welds in approximately 20% of the area.
• Poor: Coating breakdown exceeds 20%, or hard scale is present in 10% of the area.

Common coating defects that can be noted on inspection reports include:

• Paint Cracking: Deep fissures in the paint that reveal the underlying material.
• Blistering: Raised, often dome-shaped areas caused by the loss of adhesion between the coating and the substrate.
• Pinholing: Minute, deep holes that expose the substrate.
• Sags: Excessive paint flow, also known as runs or curtains.
• Pinpoint Rusting: Localized rust formation at pinholes or holidays (small areas where the coating is absent).

Protection in ballast tanks

Several solutions exist for ballast tank corrosion:

• Coat the entire tank: This offers comprehensive protection against corrosion.
• Coat and add anodes: Combines coating with sacrificial anodes for secondary protection.
• Partial coating and anodes: Coat the overhead and upper sides, then install anodes to protect the lower sections.

The first two methods are preferred. Anodes alone are insufficient because they are ineffective when tanks are empty, and they mainly protect surfaces in direct line of sight. The ideal coating is a high-performance, tar-free epoxy system with contrasting colors for easy inspection during dry docking. The final layer should be light-colored for easier in-service checks. If combined with cathodic protection, the coating must be compatible. Anodes at the tank bottom prevent microbial-induced corrosion (MIC) pitting. Coatings on bulkheads bordering heated spaces must withstand temperature fluctuations without becoming brittle or losing adhesion. Consider the potential for poor edge coverage with high-solids coatings.

Maintenance involves:

• Pumping out mud
• Hand scraping loose scale (e.g., magnesium descaling)
• Phosphating pitted areas (with safety precautions)
• Freshwater rinsing
• Drying
• Surface preparation (based on damage and desired lifespan)
• Coating
• Anode protection

Coating selection considers various factors:

• Service conditions, maintenance, and costs
• Ballasting/deballasting frequency
• Tank location relative to heated surfaces
• Required surface condition and cleanliness
• Cathodic protection compatibility
• Proven corrosion protection
• Light color
• Tar-free composition
• High solids content to meet VOC limits
• Surface tolerance
• Damp tolerance

The chosen coating system should have detailed specifications, proven performance, and be recognized by classification societies. IMO’s coating standard and IACS Recommendation 87 offer valuable guidance.

Contents

OCIMF guidelines

IACS recommendation

IMO coating standard