How do you evaluate the extent of rust coverage?

Everyone understands that the result of a breakdown of a coating system is inevitably the formation of rust. Initially this may first become apparent as rust spotting, rust rashing or rust staining – and once it starts the rust coverage area continues to grow unless remedial measures are taken.

evaluating rust coverage

However, the formal evaluation of rust coverage as a result of the breakdown of a coating can be very subjective – and the estimating the extent of a rust problem with any degree of accuracy is fraught with problems.

For example, using the International Association of Classification Societies (IACS) method in the marine sector, condition assessments will need to be made on a ‘good’, ‘fair’, or ‘poor’ basis depending on the estimated percentage of coating breakdown. For any broader evaluations linked to coating failures or paint performance guarantee claims, more detailed methods and specific estimations will be required.

The IACS method of, for example, evaluating the extent of rust coverage in a ship’s ballast tank, cargo tank or hold, would require the space to be divided into designated sections with each individual ‘area under consideration’ being assessed. The poorest rating applied would then be used to determine future inspection requirements of the whole area.

IACS has a formal classification table of what determines ‘good’, ‘fair’, or poor coating conditions and photographic examples are also provided to assist in the estimation of percentage breakdowns.

Rust coverage 1

For surveys linked to potential claims against coating failure or poor performance, more precise estimations might be required.

In such cases diagrams and rust coverage scales are available, against which affected areas can be compared and estimated, included guidance on both scattered and localised coating breakdown.

Rust coverage 2

Another possible method to assist in the more accurate estimation of the percentage of an area affected by coating breakdown is to visually or digitally move all the breakdown areas into a corner of the photograph where a composite evaluation can be made more easily.

Rust coverage 3

A full description of coating breakdown estimation, with supporting examples, illustrations and helpful assessment diagrams, is provided in Fitz’s Atlas of Coating Surveys at Fitz’s Atlas.

Marine vessel coating surveys

Completing a marine coatings survey on a ship or complex offshore structure is a challenging task, requiring specialist knowledge, careful planning and thorough execution.

So why is a marine coating survey necessary? It can be carried out to assess the condition of a paint system and maybe required under a guarantee or to produce a remedial painting specification. It may also be due to a complaint, or to investigate a coating failure or claim – or simply to assess a coating’s condition.

Also, in the case of ships, annual, intermediate, special and bottom (dry-docking) coating surveys can be done to ensure compliance with classification society requirements for IACS Enhanced Survey Programme (ESP).

A shipowner may also request a voluntary CAP (Condition Assessment Programme), performed by class or independent organisations. The main aim here is to document the quality of older ships and assess them based on current and real condition, rather than age. The CAP covers tankers and bulk carriers of 15 years of age and over. However, at the shipowner’s discretion, it can be used for ships of all ages as well as for other vessel types.

Coating breakdown can occur on any type of vessel, and the typical areas where it can be found and should be thoroughly surveyed, include the outside shell, decks, superstructures, engine rooms, tanks and peaks.

It’s important to note that different areas of a structure may have different coating systems and/or exhibit different extents and/or types of coating breakdown. Further sub-division into smaller areas will enable a thorough and accurate assessment of the coating system’s condition to be completed. Ballast tanks also warrant careful attention. The division of ballast tanks into smaller areas for Classification Society surveys is referred to as ‘areas under consideration’, while the IACS method for assessment of coating condition divides them into specific areas.

Areas under consideration for ballast tanks and double-side skin spaces in ships other than oil tankers are defined in MSC.1/Circ.1330 – Guidelines for maintenance and repair of protective coatings, and IACS Recommendation 087 for topside tanks, hopper tanks, double bottom tanks, side tanks, fore peak tanks and aft peak tanks.

Despite it being only one portion of the tank, it’s important to note that when ‘an area under consideration’ is judged ‘fair’ or ‘poor’ all the tank is considered ‘fair’ or ‘poor’, even if most of the areas are assessed as ‘good’.

Only precise areas of cargo oil tanks on a crude oil tanker are specified to be coated as a minimum requirement. For guidance, IMO resolution 288 (87) should be consulted. It should also be noted that areas should be assessed individually during a survey. MSC.1/Circ.1339 offers further details.

For expert guidance and advice on how to carry out a marine vessel coating survey, see Fitz’s Atlas of Coating Surveys at Fitz’s Atlas


Identifying common coating defects

Any number of defects and modes of failure can impact on a coating, affecting its long-term performance, but the most common forms are detachment (variously described as flaking, peeling and delamination), blistering, cracking and rust formation.

Flaking involves adhesion failure, where the paint flakes away from the substrate and is often preceded by cracking before the coating edges curl away from the substrate. You will see this on timber substrates where there is differential expansion between the coating and timber – typically, small pieces will flak away compared to the larger detachment of coating seen in peeling.

Associated with soft and pliable coatings, peeling reflects a reduction in bond strength of the paint film through poor surface preparation, contamination between coats and excessive overcoating, or coating incompatibility.

Delamination is described as the loss of adhesion between coats of paint and can be caused by incompatible layers, coating contamination, poor separation and exceeding overcoating times. Dust, dirt, grease, condensation, exudations (amine blush) and chalking are typical signs of contamination, which can affect the adhesion of subsequent paint coats.

Pertaining to coating standards, only the estimation of the degree of flaking is covered by standards: ISO 4628-5 details the assessment of the degree of flaking in relation to quantity (density), size and depth, and differentiating it as either without preferential direction or in preferential direction. ASTM D772 spans the evaluation of the degree of flaking (scaling) of exterior paints through comparison with photographic references.

Blistering is another defect. This can be caused by osmosis, where water is attracted into the coating by the presence of water soluble or water miscible material within the coating or at the coating/substrate interface.

Alternatively, electroendosmosis (or non-osmotic) blistering sees small areas of coating breakdown, exposing the substrate. Cathodic disbondment around cathodic protection, cold wall blistering related to thermal gradients, and compressive stress blistering are also symptoms of blistering. A detailed examination of the location of the blisters will be required to establish the reason for the problem.

Cracking and rust formation can also inhibit performance. Cracking occurs where the dry paint film splits through at least one coat of paint, but usually down to the substrate. This can be caused by surface movement, ageing, thermal cycling, absorption/desorption of moisture or low molecular weight hydrocarbons and lack of coating flexibility.

Rust formation happens when a coating system breaks down and shows as either rust spotting, rashing or staining. Individual spots of rust, which increase in size and density over time, are described as rust spotting. ISO 4628-3, ASTM D610, SSPC VIS2 and the European Scale of Degree of Rusting for Anticorrosive Paints should be used for assessing/evaluating the degree of rusting.

More about identifying defects can be found in Fitz’s Atlas of Coating Surveys at

How to distinguish between coating flaking and peeling

A lot of terminology linked to the failure of a coating system may appear interchangeable at first sight, but the reality is that different terms are indicative of very different technical problems and need to be used properly. This is often the case in describing some common coating defects.

The failure of a surface coating can be caused by a variety of reasons at any stage in the lifecycle of a paint system.

Poor design or system formulation, inadequate surface preparation, poor application or inadequate curing can all cause problems, as well as exposure to particularly aggressive environmental conditions.


One of the most common defects associated with coating failure is detachment – more commonly recognised as flaking, peeling or delamination.

Flaking is the result of adhesion failure, causing the paint to become separated from the substrate. It is often preceded by cracking, with the edges of the coating gradually curling away from the substrate.

Flaking is particularly familiar on timber substrates where the differential expansion between the coating and the timber can cause the separation to begin. It can also be caused by the use of an incorrect paint system, poor surface preparation or ageing of the paint system.

Flaking is similar to peeling, but with the former the coating tends to be harder and brittle. Peeling is more often associated with softer and pliable coatings and is caused by a loss of adhesion between individual coats or between the first coat and the substrate.

Typically, peeling results from a reduction in bond strength of the paint film that is caused by inadequate surface preparation, contamination between coats, exceeding maximum overcoating times or coating incompatibility.


In many respects peeling is similar to delamination, particularly when there is contamination between coats in the form of dust, dirt, grease, condensation and chalking – all of which can disrupt proper adhesion with subsequent coats.

For those involved in paint inspections and coatings , an estimation of the degree of flaking is covered by the standards ISO 4628-5 and ASTM D772.

ISO 4628-5 assesses flaking in relation to quantity, size and depth by comparison with standard diagrams and differentiates the flaking as wither with or without a preferential direction. ASTM D772 covers the extent of flaking (or scaling) by comparison with four photographic references.

Although not designed specifically to do so, the standards can also be used as an aid to estimate the degree of peeling of delamination.

For more details see Fitz’s Atlas 2 of Coating Defects at

Comprehensive New Guide to Protective Coating Surveys

A new guide to protective coating surveys provides comprehensive practical advice for those involved in the inspection of paint systems used for corrosion protection applications.

Fitz’s Atlas of Coating Surveys incorporates thorough practical guidance for consistent protective coating assessments and visual evaluations, and the production of detailed coating reports. It is designed to assist engineers, surveyors, inspectors and asset owners to make the correct decisions on corrosion protection and coating maintenance programmes.

Written by industry experts with many years of coatings and corrosion control experience, the new publication will enable users to make informed decisions on the management of anti-corrosion systems and coating performance.

For ease of use, the extensive guide is broken down into separate covering the essential information needed by anyone involved in undertaking surveys to establish coating condition and remedial painting specifications, review coating guarantees, or investigate coating complaints, failures and claims.

With specific reference to the  marine, offshore, petrochemical, bridges and other industrial sectors, the handbook looks at common defects and explains how percentages linked to coating breakdown can be estimated, alongside full advice on approved field testing and sampling techniques.

Other technical guidance is provided on the European and International rust scale, dry film thickness, adhesion testing and porosity (holiday) detection, as well as coatings used as passive fire protection systems.

Fitz’s Atlas of Coating Surveys refers to ASTM, ISO, CEPA and SSPC standards for coating evaluation and includes the full Re Scale of Rust illustrations.

In addition, among the practical considerations covered, the guide also considers health and safety requirements, the role of photography, documentation and reporting, as well as the industry standards and test methods that should be referenced.

Fitz’s Atlas of Coatings Surveys is produced in the same pocket-sized style and format as the highly regarded Fitz’s Atlas of Coating Defects which has become a standard reference guide in the protective coatings industry.

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