ISO 4628-8 Assessment of the Degree of Layer Segregation and Corrosion Around a Scratch or Other Artificial Defect

The standard “ISO 4628-8 Paints and varnishes – Assessment of deterioration of coatings – Determination of the quantity and size of defects and the intensity of uniform changes in appearance – Part 8: Assessment of the degree of delamination and corrosion around a scratch or other artificial defect”, developed by the International Organization for Standardization (ISO), describes a method for assessing delamination and corrosion around a scratch or other artificial defect on a coated panel or other coated test specimen, resulting from a corrosive environment. This standard does not cover the assessment of pitting corrosion or pitting depth.

Examples of corrosive environments include artificial atmospheres such as salt spray used in the test method specified in the ISO 9227 standard, and seawater immersion used in the test method specified in the ISO 15711 standard. Natural environments can also be used.

The extent of other defects can be determined simultaneously with delamination and corrosion. The methods are as follows:

Blistering according to ISO 4628-2 standard
Rust according to ISO 4628-3 standard.
Cracking according to ISO 4628-4 standard
Exfoliation according to ISO 4628-5 standard
Filamentous corrosion according to ISO 4628-10 standard.

ISO 4628-8 is part of the broader ISO 4628 series of standards, providing a standardized system for evaluating and determining the degradation of paint coatings and similar protective layers.

This standard provides a precise method for evaluating two basic forms of degradation around an artificial defect (most often a linear line mark, but other shapes such as holes or cuts) on a coated test panel or specimen after exposure to a corrosive environment (e.g., salt spray, humidity, or weather resistance tests):

Degree of layer separation (loss of adhesion between coating and substrate or coating layers)
Degree of corrosion (formation of rust, pitting, or other corrosion products emanating from the defect)

This method is particularly useful in accelerated corrosion tests (such as ISO 9227 salt spray or cyclic corrosion tests) and in evaluating the performance of protective paint systems for steel structures, automotive coatings, marine applications, and industrial coatings.

This standard includes significant improvements in its latest update:

Rather than relying primarily on pictorial references, priority was given to directly measuring and calculating the affected areas (although illustrative figures are still provided for guidance).
Besides straight linear scratches, artificial defects (e.g., circular or irregular shapes) are included.
Clear procedures have been established to measure the extent or breadth of separation and corrosion on either side of a defect.

The evaluation works as follows:

Preparation: A controlled artificial defect (scratch) is created from the coating to the substrate on a test panel.
Exposure: The panel is exposed to a corrosive environment for a specific period of time.
If necessary, loose coating material is carefully removed.
The maximum distance from the original defect edge to the furthest point of visible separation or corrosion is measured (usually perpendicular to the line).
As mentioned, average or maximum values are calculated.
For separation, it is typically measured as the width of the peeled/removed area.
Corrosion is defined as the spread of rust or corrosion products.
Results are reported using standardized notation from the ISO 4628 series of standards (e.g., maximum width in mm or a grading scale).

The entire ISO 4628 series of standards covers different types of degradation with consistent nomenclature:

Chapter 1: General introduction and naming system
Section 2: Rising
Chapter 3: Rusting
Chapter 4: Cracking
Chapter 5: Flaking
Chapters 6 and 7: Chalking (tape and velvet methods)
Chapter 8: Delamination and corrosion around a line.
Chapter 10: Filamentous corrosion

The practical application areas of this standard are as follows:

Quality control and certification of coatings.
Comparison of paint formulations or protective systems
Compliance with standards such as ISO 12944 (corrosion protection of steel structures with paint systems).
Research and development in the coatings industry

In short, corrosion propagation (rust propagation, subfilm corrosion propagation, scratch-induced propagation, or under-corrosion erosion) refers to the gradual lateral spread of corrosion beneath a protective coating, starting from a point of damage (such as a scratch, scuff mark, cut, impact, or other defect exposing the underlying metal surface).

When a coated surface (e.g., painted steel, galvanized steel, or coated metal in automotive, marine, industrial, or structural applications) is damaged, the exposed metal becomes vulnerable to a corrosive environment (such as salt spray, moisture, road salt, or marine atmosphere). Moisture, oxygen, and electrolytes penetrate through the defect. Corrosion begins in the bare metal (e.g., rust in steel). Corrosion products exert pressure, or electrochemical processes weaken coating adhesion. This causes the coating to detach (lift or peel) around the defect. Corrosion then propagates laterally under the film, creeping along the metal-coating interface, often in either direction from the original defect. The result is a gradual loss of adhesion, blistering near the edge, rusting under the film, and ultimately, coating degradation over a wider area.

The ISO 4628-8 standard states that protective coatings (paints, varnishes, and similar systems) on metal surfaces are frequently exposed to aggressive environments (salt spray, humidity, road de-icing salts). In real-world damage inevitably occurs (scratches, stone chips, impacts, edges, welds), creating points where corrosion can begin and spread laterally beneath the coating (corrosion creep).

Without the ISO 4628-8 standard to assess this specific type of degradation, the following critical issues arise:

Lack of comparability and repeatability.
Inability to reliably compare coating performance.
Compliance with broader sector and regulatory requirements.
Objective, quantitative data instead of just visual judgment.
Relevance to the real world

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services compliant with the ISO 4628-8 standard to businesses.