"Understanding Pitting Corrosion in Tank Containers: The Role of Material Compatibility"

1. Selection Of Material

• How do you select the best and most cost-effective material for the job - something that’s going to resist corrosion for a design life of say 20 or more years? In the past, for harsh applications such as corrosive environments, the answer always seemed to be ‘316’ - because it is a “stainless” steel and appeared to provide the most cost-efficient answer. But corrosion remains unchecked and continues to wreak havoc on infrastructure, posing both an economic threat and a human safety risk.
• Pitting and crevice corrosion are a major cause of corrosion failure of series 300 stainless steels in aqueous chloride environments such as offshore oil and gas platforms. Once a pit or crevice corrosion site is initiated it will continue to propagate rapidly, leading to failure of the component.

• PREN, CPT, and CCT stand for Pitting Resistance Equivalent Number, Critical Pitting Temperature, and Critical Crevice Corrosion Temperature. Understanding these acronyms takes you a long way toward choosing the best material for the job.
• The PREN (Pitting resistance equivalent number) numbers are useful for ranking and comparing the different grades, but cannot be used to predict whether a particular grade will be suitable for a given application, where pitting corrosion may be a hazard.
• 304L grade stainless steel has PREN of 18. 316L grade stainless steel has PREN of around 25. To resist sea water continuously, a PREN of around 40 is required.

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• Corrosion Resistance: Both 316 and 316L stainless steel contain molybdenum, which enhances their corrosion resistance, especially against chlorides and other corrosive agents. This makes them suitable for containing a wide range of liquids and chemicals without the risk of rust or corrosion.
• High Strength: 316 and 316L stainless steel offer good mechanical properties, including high tensile strength and toughness, which are essential for withstanding the stresses and pressures experienced during transportation and storage of liquids in tank containers.
• Temperature Resistance: 316 and 316L stainless steel have excellent temperature resistance, retaining their mechanical properties over a wide range of temperatures. This makes them suitable for containing liquids at both high and low temperatures without compromising their structural integrity.
• Longevity and Durability: Due to their resistance to corrosion, high strength, and temperature resistance, tank containers made from 316 and 316L stainless steel have a long service life and require minimal maintenance, resulting in cost savings over time.

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• Safety: Incompatible materials can lead to chemical reactions, leaks, spills, or even explosions, posing significant safety hazards to personnel, the environment, and nearby communities.
• Product Integrity: Certain substances can react with container materials, leading to contamination or degradation of the transported product. This can compromise the quality, efficacy, or safety of the substance, rendering it unfit for use or consumption.
• Regulatory Compliance: Many industries are subject to strict regulations governing the transportation and storage of hazardous materials. Ensuring compatibility between the container and the substance being transported is often a legal requirement to comply with these regulations.
• Environmental Protection: Chemical spills resulting from incompatible container-material interactions can have severe environmental consequences, including soil and water contamination, harm to wildlife, and ecosystem damage. Choosing compatible materials helps prevent such incidents and minimizes environmental impact.
• Financial Implications: Incompatible container-substance combinations can result in costly consequences, including clean-up expenses, fines for regulatory violations, product loss, and damage to equipment or infrastructure.

2. Understanding Substance Compatibility:

Tank container compatibility refers to the suitability of a tank container to safely store and transport specific substances without causing chemical reactions, corrosion, or other forms of material degradation. Compatibility is crucial to ensure the integrity of the container and the safety of its contents throughout the transportation and storage process.

Key Factors in Tank Container Compatibility:

Material Compatibility:

• The materials used in constructing the tank container (such as stainless steel, carbon steel, or specialized coatings) must be resistant to the chemical properties of the substance being transported.
• Incompatible materials can lead to corrosion, pitting, or structural failure of the tank.

Chemical Properties of the Substance:

• Corrosiveness: Highly corrosive substances require containers made from materials that can resist aggressive chemical reactions.

Physical Properties:

• Viscosity: Substances with high viscosity may need heating systems in the container to ensure proper flow during loading and unloading.
• Density: The container must be designed to handle the weight and density of the substance, especially if it differs significantly from standard liquids like water.

Environmental Conditions:

• The compatibility must account for the environmental conditions during transportation and storage, such as temperature extremes, humidity, and exposure to saltwater if transported via sea.

The important environmental factors that favor localized attack are higher chloride content, higher temperatures, lower pH, and more noble corrosion potentials.

IMDG code determine the portable tank instruction for each subatance but not the compability of substance it remain for operator and shipper to ensure that tank is compatible with respect of:

• Substance (Cargo)
• Tank shell and service equipment.

so this is the responsibility of the operator and shipper to review the SDS and corrosion chart carefully to ensure that the product is compatible with the material of the tank shell.

incompatible cargo could cause serious damage to the tank shell or spoil the substance itself.

3. Pitting Corrosion: An Invisible Threat: Cause & Propogation Failures:

• Pitting corrosion is a localized form of corrosion that leads to the formation of small, deep pits or cavities on and under the surface of metal. It occur s when specific areas on the metal surface become more susceptible to corrosion than others, resulting in the formation of these pits. Pitting corrosion can be caused by various factors, including exposure to corrosive substances, environmental conditions, and the metal's inheren properties. Problems with pitting corrosion attacks depend primarily on the chloride content, the pH value (the acidity), and the temperature. If pitting has taken place and if the environment in such is not too corrosive for the steel grade, a spontaneous repair of the passive layer will occur in the presence of oxygen.

• Tank are made of various grade of stainless steel. whereas it is resistant to corrosion, some substance, such as those contianing Chlorine,?Bromine,?Fluoride,?Fluorine,?or?salt,?and?corrosive are moslty incompatible. the graphic below shows contribution factors to corrosion. if we load product containing the chloride, in contact of mositure and elevated temprature it can cause corrosion and piting to shell.

• Type of StainlessSteel pitting: Corrosion can take many forms and is pitting, stress corrosion, inter-granular, cliningrevice. corrosive substance might require the tank shell and fittings to be specially lined with a protective linning or coating that is resistant to the substance.

three type of pitting as seen above.

Type A Clean, shallow pits – Commonly referred to as superficial pitting which is an open, clean pitting with a depth not exceeding 0.5mm.

Type B Crater, pin hole pits – these are deep shell defects and will get far worse if not actioned right away

Type C Pore, cavity pitting – this is one of the worst type of pitting to steel, on the surface it may be a small pitting hole, under the surface its far worse, any corrosive material still in this cavity will continue to eat away at the steel, the only real way to detect these is a shell thickness test, and then to drill into the steel to reveal the extent of the cavity and prep for repair

Consequences of pitting corrosion:

A. Pitting damage create safety issues for a tank both during operation and when in DEPOT.

B. Pitting damage can reduce the financial value of a tank.

C. piting damage can reduce the operation life of a tank.

D. pitting damage will increase M&R spending.

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The most common weld to stainless steel is TIG welding when repairing deep shell pitting, all tank manufactures will allow a corrosion allowance which is deemed suitable and still safe to transit, this can range from 0.20mm to 0.42mm depending on manufacture and steel.

This information can be found on the initial tank test certificate . The standard tank barrel thickness is 4.6mm and the dome ends are roughly 5.3mm.

Here is the diagram showing the cross-section of a stainless steel sheet with the specified thicknesses:

• Total Thickness (4.6 mm)
• Effective Thickness (4.18 mm)
• Corrosion Allowance (0.42 mm)

Deep Pit Repair: There are a number of methods for shell pitting repairs, with the most common being:

1. Drilling into the pit, taking care not to apply too much pressure until confident its reached the bottom and the pit area is fully cleaned out prior to any weld infills being applied.
2. Grind into the tank pit in a V shape, again taking care not to apply too much pressure until confident its reached the bottom and the pit area is fully cleaned out prior to any weld infills being applied.

Superficial Pitting Repair(Also Called Type A pitting): Buffing can be used to repair the pitting Corrosion Type A.

After any internal pitting repair the welds need to be ground back until a smooth finish, and the shell polished, if done correctly there should be no signs of any weld repair. The area should always be re-passivated to add the protective layer back to the steel.

Before conduct P&P (Pickle and Passivation)

Tank must be clean and dry condition - Free from any residues.

Apply 25% of nitric acid on the surface of tank barrel.

Leave it for approx. 30 minutes.

Flush it with clean cold water till free from pickle residues.

Steam the tank for approx. 45 minutes.

After conduct P&P (Pickle and Passivation)

Blow dry, using a blower.

Continue to blow air for approx. three hours for the passivation process to reform the stainless-steel shell

protection layer.

Passivation process:

It’is a legal, mandatory requirement any hot work (welding) to a pressure vessel shell that a hydro test to 6 bar is completed once all work finished to ensure the tank shell integrity.

Robo Grind: For superficial pitting/corrosion within the corrosion allowance the best repair method and finish is the robo grind, this removes an exact amount of steel from the tank shell as seen in the before and after photos below. There are not many depots around the world that operate this equipment and its not exactly a cheap repair, but it is guaranteed to removal minimal steel and not heavily effect the shell thickness with these more abrasive tank grind and polish repairs.

4. Best Practices for Preventing Pitting Corrosion:

Proactive Maintenance Practices:

• It is crucial to create a pitting tracker to determine the products that cause pitting in the tanks. By using this information, update the master data for each product to show its likelihood of causing pitting. This will guarantee that specific protocols are followed during loading, unloading, and cleaning to avoid additional damage.
• Tag products that cause pitting with a "CLEAN ASAP" label and emphasize the need for immediate cleaning to prevent long-term damage to stainless steel and avoid pitting corrosion. Inform recipients to ensure all openings are tightly closed after unloading to prevent moisture from entering. However, this may not always be possible due to the expertise of the unloading facility, unloading procedures, and safety considerations.
• Regular inspections of the internal shell of tanks are crucial. However, this is not always possible in some regions where local regulations prevent depot personnel from entering the tank. In the EU, depots follow the EFTCO standard, which allows for visual inspections from the manlid only. This method makes it difficult to detect pitting, resulting in tanks moving around with undetected pitting until they reach an area where internal entry inspections are performed. By then, the pitting often becomes larger and unacceptable per ACC manual standards, requiring costly repairs and a hydro test if the pitting is deep.

• Verify the purity of the chemical, specifically checking for the presence of chloride, bromide, or fluoride. Review previous transportation experiences with the substance.
• Eliminate Oxygen. Transporting corrosive cargo under an inert gas blanket, such as nitrogen (N?), is an effective strategy to minimize the risk of corrosion in tank container shells. Here's how this approach works to protect the tank:

Mechanism of Corrosion Prevention with Inert Gas Blanket:

• Oxygen Elimination:
• Corrosion is an electrochemical reaction that typically requires oxygen. By filling the tank with nitrogen, which is an inert gas, oxygen is displaced and prevented from coming into contact with the cargo and the tank walls. This significantly reduces the possibility of oxidation reactions that lead to corrosion.
• Maintaining an Inert Atmosphere:

• Nitrogen is non-reactive and does not support combustion or other chemical reactions that can lead to corrosion. Maintaining a blanket of nitrogen above the cargo ensures that the atmosphere within the tank remains inert, further preventing any oxidative corrosion processes.
• Reduction of Water Vapor:

• Corrosive reactions often require water or moisture. Nitrogen purging can also reduce the relative humidity within the tank, thereby minimizing the presence of water vapor that can contribute to corrosion.

Benefits of Using Nitrogen for Corrosive Cargo Transport

1. Extended Tank Life:By preventing corrosion, the structural integrity of the tank container is maintained over a longer period, reducing the need for frequent repairs or replacements.
2. Safety:An inert atmosphere reduces the risk of explosive reactions, particularly important when transporting flammable or reactive corrosive substances.
3. Cargo Integrity:Preventing corrosion also helps in maintaining the purity and quality of the cargo, avoiding contamination from corrosion products.

Visual Representation

To complement the explanation, here is a simple diagram illustrating the concept:

1. Tank Container with Corrosive Cargo
2. Nitrogen Blanket above the cargo to prevent contact with oxygen

Here's a visual representation of a tank container with corrosive cargo transported under a nitrogen (N?) blanket:

1. Tank Container Shell: The outer boundary of the tank, shown in black.
2. Corrosive Cargo: The blue section representing the cargo stored within the tank.
3. Nitrogen Blanket (N?): The yellow section above the cargo, illustrating the inert gas layer that displaces oxygen and prevents corrosion.
4. Applying the maximum allowable degree of fill in a tank container helps reduce agitation and oxygen exposure, which further prevents corrosion. Here's how this practice contributes to corrosion prevention:

Mechanism of Corrosion Prevention with Maximum Allowable Degree of Fill:

• Reduced Agitation:
• When a tank is filled to its maximum allowable capacity, the movement of the liquid cargo is minimized during transportation. Less agitation means fewer chances for the liquid to splash against the tank walls, reducing mechanical wear and the potential for air (oxygen) to mix into the cargo.
• Minimized Oxygen Exposure:
• A higher fill level leaves less headspace in the tank, which means there is less room for oxygen to be present. The reduced air gap limits the amount of oxygen that can dissolve into the cargo, thereby decreasing the risk of oxidative corrosion.

• Stable Cargo Environment:
• A tank that is nearly full creates a more stable environment for the cargo, as there is less free surface area for gas exchange. This stability helps maintain the effectiveness of the inert gas blanket, ensuring that the nitrogen layer remains intact and effective in displacing oxygen.
• Monitor the cleaning process and verify the compatibility of cleaning materials with the tank.
• Always follow the material compatibility chart to prevent chemical-induced corrosion.
• Effective cleaning practices minimize the risk of corrosion by preventing the accumulation of substances that can initiate or exacerbate pitting.
• Marked all products cause the pitting as "CLEAN ASAP" in system.
• If pitting or corrosion is discovered during inspections, arrange and carry out repairs as soon as possible to prevent further deterioration.

5. Safety data sheet and corrosion resistance Chart Review:

It is advised to review the Safety Data Sheet (SDS) and corrosion resistance chart before accepting any product for loading, as some contents may cause corrosion to stainless steel. If you're unsure about the product's compatibility with steel, it's best to seek advice from a third-party laboratory. There are numerous online services available to review SDS and offer guidance on compatibility. For example, the corrosion resistance chart from "Sandvik Material Technology" shows that Phenol is compatible with stainless steel.

Example of compatibility assessment.

Let's consider an example for transporting Phenol, Molten to UN2312. Before accepting the Molten Phenol, the operator should refer to:

1. Corrosion Resitance Chart: Looking at the corrosion resistance chart, you will see that this substance is compatible, free of impurities, and suitable for temperatures below 100 degrees Celsius.

1. Previous shipment: The shipper confirms that the previous shipment was transported without any corrosion to stainless steel.
2. Operator: Operator's tracker confirms that the previous shipment was transported to stainless steel without any issues.
3. Operator's procedure: The operator should provide the following operational guidelines:

• Maintain temprature below 50 degree C.
• Ensure heating temprature control in place.
• Clean tank promptly after discharge.

Service Equipment: Let's consider service equipment using our example of transporting Phenol Molten in a T11 Tank. While each T11 Tank would meet the IMDG code requirement, there are several equipment specification items that are outside the scope of the IMDG code. The tank operator should determine these requirements at the outset and make preparations within the transport plan. The following factors should be considered.

• The loading and discharge terminal trains people to load and offload tanks and work at heights. Procedures include inspecting the tank internally before it leaves the facility to ensure there is no excess residue inside. This is important because corrosive residue can cause stainless steel to corrode if it remains in contact with the tank for an extended period.
• The facility should be equipped with filling and offloading pipe connection.
• seals and gaskets are in good condition.
• Heating facility.
• filling measurement equipment.
• Shipper pre-tripconnections inspection procedure.

In order to ascertain whether a product has the potential to induce pitting or corrosion in a tank container, it is advisable to examine Section 10/2/9/14 of the Safety Data Sheet (SDS).

Section 10, which is titled "Stability and Reactivity." This section provides information on:

1. Reactivity: Describes specific test data for the substance or mixture that indicates its reactivity hazards.
2. Chemical Stability: Indicates if the chemical is stable or unstable under normal conditions of storage and handling.
3. Possibility of Hazardous Reactions: Lists conditions under which hazardous reactions may occur.
4. Conditions to Avoid: Identifies conditions like heat, moisture, and incompatible materials that might cause a hazardous reaction.
5. Incompatible Materials: Details materials that could cause a dangerous reaction if they come into contact with the substance.
6. Hazardous Decomposition Products: Lists the dangerous substances that could be released due to decomposition.

In addition to Section 10, it is also prudent to check Section 2 ("Hazards Identification") and Section 9 ("Physical and Chemical Properties") of the SDS for further information on the chemical’s corrosive properties.

Section 2: Hazards Identification: This section gives an overview of the primary hazards associated with the chemical, including any warnings about its corrosive nature.

Section 9: Physical and Chemical Properties: This section provides specific details such as pH, which can indicate corrosive potential, especially for acids and bases.

Section 3 is crucial for understanding what chemicals are present in the product, it does not directly provide information about the product’s potential to cause pitting or corrosion of tank containers. However, knowing the specific chemicals and their concentrations can help you cross-reference other sources of information, such as compatibility guides or chemical resistance charts, to determine if the product might be corrosive to certain materials.

6. What is corrosion?? Corrosion is a natural process that occurs when metals react with substances in their environment, leading to their gradual deterioration or destruction. It's most commonly associated with metals, but other materials can also corrode under certain conditions.

There are many forms of corrosion including:

• Dry/Wet
• Localised
• Crevice
• Microbial
• Stress corrosion cracking
• Corrosion Fatigue
• Galvanic

Pitting corrosion is highly localized and is generally associated with materials such as SS316L that obtain there corrosion resistance from a protective passive layer.

Cause of corrosion:

• Local penetration of the local oxide film results in direct exposure to the corrosive environment.
• Crystal Dislocations, Secondary phase, grain boundary, and chemical segregants can limit Cr2O3?formation.
• Mechanical damage can remove the Cr2O3?layer.
• Excessive fluoride and chloride concentrations can cause Cr2O3?degradations.
• where the Cr2O3?film is compromised, these locations act as favorable pit sites.

Pitting corrosion presents a significant threat to the integrity of tank container shells. Understanding the materials used, the types of pitting, and the causes behind this form of corrosion is crucial for effective prevention and repair. By implementing rigorous maintenance practices, regular inspections, and appropriate passivation techniques, the longevity and safety of tank containers can be significantly enhanced.

Moreover, continuous advancements in materials science and corrosion prevention methods offer promising solutions to combat pitting corrosion more effectively. Future research should focus on developing more resistant materials, improving detection technologies, and refining repair methodologies. Through concerted efforts in these areas, we can mitigate the risks associated with pitting corrosion and ensure the reliability and safety of tank containers in the long term.