Steel Renewal Criteria as per IACS Rules

Introduction

The International Association of Classification Societies (IACS) establishes unified requirements and guidelines for steel renewal on ships, aimed at ensuring the structural integrity and safety of vessels. These criteria are designed to address issues such as corrosion, fatigue, and other factors that may compromise the ship's structural components. The following article consolidates detailed information on steel renewal criteria as outlined by IACS and individual classification societies, including Lloyd's Register (LR), Bureau Veritas (BV), Nippon Kaiji Kyokai (NKK), RINA, Polish Register of Shipping (PRS), and DNV.

Classification Society Minimum Thickness Requirements

Each classification society provides specific percentages for minimum allowable thickness, often tailored to the ship type and location of the structural component. Below are some general guidelines:

• Lloyd's Register (LR): Minimum thickness is typically set at 75-80% of the original plate thickness for primary structures.
• Bureau Veritas (BV): Allows up to 20-25% reduction in original thickness, depending on the structural importance of the component.
• Nippon Kaiji Kyokai (NKK): Specifies stricter limits for components subject to high stress or located in corrosive environments, often requiring no more than 20% loss.
• RINA: Accepts a reduction of up to 20%, with variations based on ship type and age.
• Polish Register of Shipping (PRS): Typically aligns with IACS UR but emphasizes stricter tolerances for older vessels, allowing 15-20% loss.
• DNV: Sets minimum thickness at 75% for most structural elements and 90% for critical components like watertight bulkheads.

These percentages represent general practices, and specific values should be confirmed in the respective society’s rules.

Types of Corrosion and Differences

1. Uniform Corrosion Occurs evenly across a surface and is generally easier to detect and measure. Renewal is required when thickness loss reaches the allowable limit, as specified by the classification society.
2. Localized Corrosion (Pitting) Concentrated in small areas, forming pits that can penetrate deeper than uniform corrosion. More severe as pits may exceed the allowable depth (typically 15-20% of the original thickness).
3. Galvanic Corrosion Occurs when two dissimilar metals are in electrical contact in a corrosive environment. This type requires close monitoring and may necessitate renewal when it significantly weakens one of the metals.
4. Crevice Corrosion Found in shielded areas such as joints and overlaps where the environment is more corrosive. Inspection and mitigation are essential to prevent structural failure.
5. Rust Corrosion A specific form of oxidation where iron reacts with oxygen and moisture to form iron oxide (rust). Often a sign of neglected maintenance. Renewal may be necessary when rust leads to measurable loss in plate thickness or compromises surface integrity.

Steel Corrosion: The Fundamentals

Steel corrosion is an electrochemical process where iron (the primary component of steel) reacts with its environment, resulting in the gradual deterioration of the metal. This reaction involves the transfer of electrons from the iron atoms to other substances, typically oxygen and water. The iron atoms lose electrons and become positively charged ions, which then react with other elements to form various corrosion products, such as oxides, hydroxides, and sulfides.

Corrosion vs. Rust: A Key Distinction

While the terms "corrosion" and "rust" are often used interchangeably, there's a crucial difference:

• Corrosion: This is a general term encompassing various forms of metal deterioration, including rusting. It can affect a wide range of metals and alloys.
• Rust: This term specifically refers to the corrosion of iron or iron-based alloys (like steel). The reddish-brown flaky substance we commonly call "rust" is primarily composed of iron oxides.

Other Related Terms (as per IACS Perspectives)

1. Uniform Corrosion: This is the most common type of corrosion, characterized by a relatively uniform loss of material across the exposed surface. IACS members pay close attention to this as it can lead to a gradual thinning of steel structures.

1. Pitting Corrosion: This localized form of corrosion creates small pits or holes in the metal, which can be particularly damaging as they can concentrate stress and lead to structural failure. IACS rules emphasize the importance of detecting and addressing pitting corrosion.

• Crevice Corrosion: This type of corrosion occurs in confined spaces (crevices) where oxygen levels are low and corrosive substances can accumulate. It's a concern for IACS members as it can occur in areas like welds, joints, and under deposits.

• Galvanic Corrosion: This occurs when two dissimilar metals are in contact in a conductive environment (like seawater). The more active metal corrodes preferentially. IACS rules provide guidance on material compatibility to minimize galvanic corrosion.

• Erosion-Corrosion: This combines the effects of mechanical wear (erosion) and chemical corrosion. It's often seen in areas with high flow rates or turbulence. IACS members consider this in areas like pipelines and pump impellers.

• Stress Corrosion Cracking: This occurs when a combination of tensile stress and a corrosive environment causes cracks to form in the metal. IACS rules emphasize the importance of proper design and stress management to prevent this type of corrosion.

Hypothetical Scenario for Steel Renewal

Case Study:

• Original Plate Thickness: 15 mm
• Allowable Corrosion Allowance: 20% of original thickness = 3 mm
• Measured Thickness: 10 mm
• Conclusion: The actual thickness loss is 5 mm, exceeding the allowable corrosion allowance. Renewal of the steel plate would be required under IACS UR S31.