Out, out black spot

Finishing Articles - Coating Contamination

Malcolm Griffiths looks at how contamination can come in many forms and can affect your finish. Since joining the paint industry in 1963, Malcolm has worked for several leading coating manufacturers. He is a graduate in chemistry, Fellow of the Institute of Metal Finishing and a Chartered Member of the Institute of Occupational Safety and Health. He now runs the independent coatings advisory service Ad Qual Castech

The old saying, that ‘a coating is only as good as the surface preparation,’ is as true now as it ever was. Generally, contaminants on the surface of a substrate can hinder any chemical conversion reaction or subsequent bonding of any paint that we might try to apply, impairing durability and performance of the overall coating system. Some contaminants can also give rise to noticeable faults in appearance, causing rejects.

The latter, more visible problems at least give the applicator some chance of putting it right before delivery but those defects that go undetected during manufacture can rebound with a vengeance once they reveal themselves after delivery and out in the field. Remedial work is always expensive but even more so when it involves paying teams to go out and work in situ. It is worth spending the extra time and effort in checking the quality of each stage of our process. The sort of processes that I am referring to here are those for cleaning and pretreatment of a substrate, including the general removal of debris, particulates, corrosion products, existing coatings, oil and grease. I also include the chemical conversion of a surface to enhance its adhesion or corrosion resistance.

Contaminants come in many forms. For example, the obvious ones are oil and grease; inclusions (i.e. bits; fibres); rust; millscale; oxidised layers from welding processes. I also include any unwanted residues from the pretreatment processes themselves, such as dust and grit left behind from abrasive methods or chemical residues from conversion treatments.

Chemical strippers for removal of old coatings are usually based on active ingredients such as dichloromethane, hydrogen peroxide, formic acid or sodium hydroxide. Some act by penetrating, solvating and swelling the film whilst others chemically attack the resin. With critical components, it is important to avoid attack on the substrate itself; for example some steel alloys can suffer hydrogen embrittlement and strong acids or alkalis can etch into the surface of aluminium alloys.

Degreasing of surfaces by hand wiping with solvents can - at best - be described more realistically as the redistribution of oil and grease over the substrate. It is only really even remotely practicable when the substrate is otherwise free of millscale or rust and the process involves wiping on with one cloth and wiping off the grease solution with another. The better, solvent-based alternative is vapour degreasing but, as we have been ‘informed’ lately, there is a great deal of confusion amongst coating applicators about what can and can’t be used now and what will be allowed in the future.

With the increasing restrictions set by anti-pollution laws and the Solvent Emission Directive in particular, aqueous degreasing systems have come much more to the fore. It is important that all residues are rinsed of the surface to avoid later adhesion problems with the applied coating.

In the right circumstances, grit blasting is a good method of removing millscale and rust in a single operation, providing a good key for primer. It is best to remove heavy deposits of oil and grease beforehand, to avoid contamination of the grit. The big advantages are that only particulates are emitted and dry waste residues are very much easier to dispose of than liquids. The process has always been seen as dusty and therefore more suited to heavy industry rather than general factory processes. Improved dust control methods and power-and-free conveyors allow the design of in-line blast rooms.

There is a wide range of specialist media to choose from, such as garnet, steel grit and copper slag, depending on the sensitivity of the particular substrate. There is also the option for plastic media stripping, widely used on very sensitive substrates such as aerospace alloys. Even more controlled is ‘Cryo-blasting’ using solid carbon dioxide, which then rapidly sublimes.

Acid pickling with such chemicals as hydrochloric or phosphoric acids will readily remove oxide layers from steel. Such a process is commonly used as a precursor to hot dip galvanizing. Talking of galvanizing, this is a good point to consider the methods available for treating zinc surfaces before painting. Many people swear by ageing of galvanizing but mordant /T-wash treatments are often used at onsite installations, to speed up the process. Interestingly, even in the last week, we came across a coating specification on a very well known and important landmark, which called for priming of galvanizing with zinc rich epoxy, overcoated with an alkyd finish. It well known that alkyds react with zinc to form soaps at the interface. In turn, this causes embrittlement of the film and catastrophic loss of intercoat adhesion.

There is a wide range of aqueous chemical conversion processes available now, mainly based on phosphate technology and often allowing mixed substrates to be pretreated. Many are based on novel technology, free of the heavy metals proscribed by regulations. In the right circumstances, they can offer environmental, operational and energy saving advantages.

The quality control of such pretreatments is very important. In particular, it is essential to control the concentration of the various solutions and to keep them free from contamination, in order to ensure a conversion layer with deposits in the correct film weight range. The rinses must also be kept free from contamination and the final rinse must remove any soluble residues. If not, general blistering of the film can occur through the onset of corrosion under the film. Poor control of the process will give insufficient corrosion resistance. Most resins transport moisture relatively freely and any soluble salts left under the film will soon become hydrated to form electrolytes, becoming centres for corrosion.

In our experience, the more simplistic the process and the fewer the number of stages – especially rinses – the more difficult the process will be to control and therefore the greater the likelihood that the final pretreatment will give rise to problems in service. Most of the good quality systems consist of 6 stages or more.

Aqueous treatments will inevitably involve waste treatment and disposal, which can be costly. However, we have recently been made aware of companies in Europe who manufacture equipment for low temperature evaporation to allow recycling of the water and concentration of the waste products – much like the solvent recovery plants. At first sight, one would expect that the cost of using electrical energy to recover water would be prohibitive. However, now that waste treatment and disposal costs are so high, we understand that it has become increasingly viable to recover through this process.

On the outer fringes and literally using ‘cutting edge technology’ we have laser ablation – irradiation of a surface with a laser beam to remove a layer of it. This is a closely controlled process, capable of dealing with glass or carbon fibre composites without damaging the base substrate. As an addendum to all of this, for the last 3 months, I have been working alongside staff and operators coating aircraft parts, where the chromates run wild and free in the surface preparation and the primers they use. And I think quite rightly so, until someone comes up with a replacement that is as good, since I prefer to fly in airframes that remain intact and corrosion free. (I can almost feel the environmental lobby shudder in horror at the very thought of it – wonderful!)

Whatever methods are used to prepare the substrate, there should be minimal delay and handling of components between those stages and the initial application of the coating. For the best results, all of those processes should be carried out in a continuous process. Finger marks can act as centres for corrosion. Some pretreatments may deteriorate with time, unless they are overcoated quickly. Others may need to be heated to reactivate them. At the very least, dust and fibres can settle on components left lying around and may then act as a conduit for moisture, salts and oxygen.

If you are interested in more in-depth information about some of the innovative options available, the Organics Group of Institute of Metal Finishing (IMF) is presenting a symposium entitled "Surface preparation and pre-treatment prior to finishing". It is scheduled for 14 October 2008 at the Birmingham Medical Institute and will include high energy systems such a plasma cleaning and surface etching.

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