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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.