Fellow’s Corner

In this series of articles by practitioners who have made a significant contribution to the field of corrosion protection, the editor discusses paint technology.

Over the past 18 months this column has concentrated on topics relevant to the corrosion engineers, however, there is a need to address the part that protective coatings play in the corrosion protection of structures.  In an attempt to address this imbalance, this issue will feature an introduction to paint technology, and how protective coatings fit into the overall corrosion protection scenario.

Paints and coatings are used to protect and decorate, however, before we consider the properties of paints and how they work, it is necessary to consider “what is a paint”. 

All liquid paints are composed of three basic ingredients, resins, pigments and solvent. The resin is the film forming portion of the paint – it holds together the pigment particles and binds the paint to the surface. The resin plays the main part in contributing to the durability, strength and chemical resistance of the final film.  Paint types are often referred to by the type of resin in the formulation, so when we talk about an alkyd or epoxy for example, we are referring to the main resin used to make the paint.

The second ingredient in a paint is the pigment. This is a relatively insoluble finely divided powder, or more commonly a mixture of powders. The pigment(s) primarily provide hiding power (opacity), and colour, but they also improve corrosion and weathering resistance, increase paint adhesion, decrease moisture permeability and control gloss. The final ingredient, the solvent, “carries” the resin and pigment(s) and controls the viscosity, such that the paint can be applied to a surface. The chemical ingredients in each of the components vary widely from one generic type of paint to another, in addition each of the components (resin, pigment and solvent) are also usually mixtures of different materials. For example, a paint formulation may contain three or four solvents – one solvent dissolves the resin, while some are used to control evaporation, and others are used to dilute the solution (control viscosity). It is not important for a user to know all the ingredients in a paint, suffice that he knows the properties.

The words, paint and coating, are used interchangeably – they mean virtually the same thing. However, it is necessary to distinguish between a coating system and a coat of paint. A coating system is more than just the material applied, it also refers to other factors such as the surface preparation requirements, the application of a number of coats of paint, in a specific order, and the thickness of each coat of paint. A coat of paint is a single layer, applied to form a coherent film when dry.

The common designation of a series of coatings applied to a surface is primer, intermediate or build coat, and top coat. Normally each coat contains properties that contribute to the success of the total coating system.

Function of each coat

The primer is the first coat applied to the surface. The main function of the primer is to provide adhesion to the substrate – if the primer doesn’t stick, then the whole coating system will fail. The primer also provides a key for the rest of the system.

The intermediate coat is required in many coating systems to provide one or more of the following functions; increase film build, improve chemical resistance, or serve as an adhesion or tie-coat between primer and topcoat where they are not compatible.

The topcoat is intended to be the last coat applied. This provides the weather and/or chemical resistance and also imparts characteristics such as colour, gloss wear resistance, abrasion resistance.

Considering the two main reasons for painting – protection and decoration, this article will concentrate on the protection properties.  A paint can protect against, amongst others, abrasion, chemicals and fire, but probably the most common protection use is to prevent corrosion of steel. 

There are three recognised ways that coatings protect steel against corrosion, providing a barrier, inhibition and sacrificial action.

Barrier protection is just as the name implies, the dried paint film blocks moisture from reaching the steel surface. All coatings do allow moisture and oxygen to penetrate them to some extent, this is called permeability. Coatings which protect by a barrier mechanism have very low permeability. Typical barrier coatings are 2-pack epoxies and polyurethanes, although there are additives which can reduce permeability further (see below).

Coatings that protect by inhibition contain active pigments to inhibit or interfere with the corrosion reaction on the steel surface. Typical traditional inhibitive pigments were lead compounds and chromates.  However, concerns about toxicity and environmental pollution have led to their replacement with so called non-toxic anticorrosion pigments such as phosphates, and many proprietary materials. As moisture passes through the film, the anti-corrosive pigments slowly dissolve and depending on their chemistry interfere with either the anodic or cathodic reaction and thus retard corrosion.

The third mechanism is sacrificial action and is the way that zinc rich primers protect steel.  These primers are highly loaded with zinc, such that the zinc is in contact with itself and the steel surface.  As zinc is more active than steel, and if the elements necessary for corrosion are present, then the zinc will corrode in preference to the steel (i.e. sacrifice itself), and hence protect the steel. Zinc rich paints are classified into two types, inorganic and organic. This classification refers to the resins used in the formulation and not the form of the zinc.  The binder (resin) in inorganic zinc rich coatings is a form of silicate, and organic zinc rich paints are nowadays typically epoxy based.

Returning now to the paint system. This is designed to give optimum protection to the steel or metal substrate by combining the properties of the various coats. Thus for very long term protection, an inhibitive primer, or more particularly a zinc rich primer, would be combined with a barrier intermediate coat and topcoat.  In this way, two protective mechanisms are used to give long life protection.

The permeability of a paint and hence its barrier properties are related to the resin used, with oleoresinous and alkyd paints having high permeability and epoxy and polyurethanes having lower permeability due to their highly cross-linked structure.  Within each generic class of paint, permeability can be further reduced by formulation, and in particular the use of plate-like pigments such as micaceous iron oxide (MIO) and aluminium flakes. These special pigments orientate themselves parallel to the surface when the paint dries and provide an extremely low permeability film (they effectively increase the path length moisture has to take to reach the metal surface). In a similar manner, permeability can be reduced by increasing film thickness although there is a limit to this before other properties start to suffer.

No matter which type of paint is used, if proper surface preparation is not carried out then vastly inferior performance will be obtained.  Surface preparation is essential in two important areas, it provides an anchor for the coating and it allows intimate contact between the coating molecules and the metal surface, and this will be the topic
for a future column. 

Ask the Expert – Part 2


What is the best approach to detect and monitor CUI ?  CL


The most reliable method to detect and monitor CUI is a full strip of the insulation and visual inspection, although once CUI is detected with this method it is rarely monitored. The key disadvantage of this approach is the high cost and required resources associated with access, stripping, possible refurbishment of the coating, and reinstatement of the insulation. The need for a reliable screening technique to focus CUI inspection efforts has been recognised for many years. Various non-destructive techniques (NDT) are available to detect CUI directly without removing the insulation, such as real time radiography (RTR) and pulsed eddy current (PEC). However, the drawback of these techniques is the uncertainty around probability of detection (POD), despite significant technique improvements in recent years and development of technologies through joint industry projects such as those done by HOIS (Harwell Offshore Inspection Service). Possible presence of CUI can also be detected with indirect techniques which are based on detection of water or humidity in the insulation system. Techniques such as thermography, neutron & x-ray backscatter, and more recently various types of sensors are all capable of detecting water or humidity but similar to direct methods there is still uncertainty around the POD and questions about the reliability. Another drawback is that detecting water does not necessarily mean that CUI has occurred at the points where water is located. Much development work continues especially with sensors which may offer better monitoring capability including direct detection of corrosion. Improving CUI predictive capability through greater sharing of data and analysis can also help focus where to inspect, but the CUI still needs to be located and there are many instances of CUI “surprises” in the industry especially if 100% stripping of insulation is not done during service of the facility. It should also be recognised that it can take 15 to 20 years to fully validate CUI technology developments and therefore compromises will inevitably be sought. A complementary approach involving direct NDT, water detection, sensor monitoring and the application of better data analysis and CUI prediction is probably the optimal way forward to focus efforts with CUI inspection planning, but full strip and visual inspection remains the most reliable approach.

Steve Paterson, Arbeadie Consultants

Ask the Expert Part 1


“How do you know when the pot life of a 2-component paint has expired ?  Can you extend pot life, if so, how?”  PS


The reaction or cure of a two-component paint or coating is initiated when the two parts are mixed together. The manufacturer will state a pot life at a given temperature on the product data sheet.

Generally speaking the reaction rate doubles for every 10 degrees C increase and halves for every 10 degrees C reduction in temperature, this can be used to estimate the pot life at different temperatures if this is not stated on the data sheet. The age of the product can affect the pot life, depending on the chemistry of the particular product this could produce a longer or shorter pot life. Furthermore, the type of pump, fluid friction and pressure in the pump also have an effect and therefore it is also good to know the signs of when the material is towards the end of it’s pot life.

There are other variables in material chemical composition and properties such as the viscosity, lubricity and level of fillers but generally speaking a sudden increase in blockages, viscosity, reduction in fan pattern and atomisation are a sign the material is past its best, also most reactions are exothermic therefore the product will start to get hot.

One method that can be used for extending pot life at higher temperatures is cooling the product, or cooling the pump. Care should be used when cooling the product as it could be below dew point and take on moisture during mixing and atomisation.

Some products have a pot life inhibitor available to increase pot life in hot climates, this is mixed into the product prior to adding the Part B and applicable to products cured by free radical polymerisation. Some products are also available with tropical or winter grade hardeners.

Any attempt to extend a pot life after mixing will affect the final cure and other properties of the material and could poison the reaction completely, for example mixing in additional solvent. Of course solvent should only be added when it is recommended by the manufacturer and only up to the maximum percentage stated.

Phillip Watkinson, Corrocoat

CorrosionRADAR launches StarterPACK™ to unlock the value of predictive corrosion monitoring

CorrosionRADAR launches StarterPACK™ to unlock the value of predictive corrosion monitoring

CorrosionRADAR has launched StarterPACK™ which allows industry to try out predictive moisture and corrosion monitoring in a specially designed evaluation project. It is a quick and easy way for companies to weigh up the benefits of Predictive Corrosion Management (PCM) in tackling the challenges of Corrosion Under
Insulation (CUI).

It enables:

• Continuous CUI detection, localisation and monitoring

• Understanding on how data is transmitted, integrated and visualised

• Understanding on how data is analysed for effective decision support

• Reporting on business cases and ROI calculations

And at the end of the project, companies will have generated, a system performance report, a business case and a recommended field deployment plan for scale-up, concluded the company.

New Passive Fire Protection Coating

New Passive Fire Protection Coating

Global coatings manufacturer Hempel has launched Hempafire Pro 400, a new passive fire protection coating that maintains the stability of steel structures in the event of a fire for up to 120 minutes and has been optimised for maximum efficiency in the loadings for a 90-minute duration.

Passive fire protection (PFP) coatings insulate steel during a fire, which extends the steel’s load-bearing capacity and gives valuable extra time for evacuation and emergency response. According to the company, their Hempafire Pro intumescent PFP coatings are known for their low loadings, which can improve project efficiency.  These coatings can be used for all steel profile types – for both in-shop and on-site applications. They can be used in exterior conditions and corrosion environments up to C4 according ISO 12944. The Hempafire Pro 400 is available in two versions, standard and fast drying. Other attributes according to the company are, excellent aesthetic appearance when correctly applied, excellent application properties – designed with applicators in mind, and it can be applied up to 1,600 microns DFT in one coat to minimise the number of coats required.