WRITES Iqbal Johal, marketing manager at Galvanisers Association: “Traditionally, we think of the two anti-corrosion systems, paint and galvanizing, as competitive processes or products.
“Galvanizing is a single specification, single process, whereby zinc is metallurgically bonded to the surface of steel. It is cost-effective, highly predictable and its lifecycle accurately measurable.
“Paint, on the other hand, is a plethora of processes and specifications and chemical compositions whereby lifecycle predictions become less easy to manage.
“In truth, all paint fails – eventually. Yet that is not to say that paint products are without advantages.
“In oil and gas environments, and especially in the North Sea, paint plays a valuable role in anti-corrosion, branding and asset identity.
“But there is a way to combine the add-on functionality of paint, with the reliability of the known and highly-predictable process of galvanizing.
“As a result, time to failure is greatly reduced and significant cost savings are made over the lifetime of the coating.
“For many years, duplex coatings have been used in the construction industry.
“But as take up of duplex coatings are less prevalent in the oil and gas sector, Galvanizers Association is keen to share the added benefits that others have long enjoyed, and how the two systems together play to the on-going economic conditions within the sector.
“As a combination of two different coating systems designed to complement one another, a duplex coating is typically defined by a metallic coating over-coated by an organic system.
“Whilst a galvanized coating on its own will provide more than adequate corrosion protection, an additional organic coating, although not necessarily needed from an anti-corrosion perspective, can be applied over the galvanized finish.
“An example of this would be to add colour to try and blend the structure into its surroundings or make it stand out from them. Additional paint layers also provide additional corrosion protection in very severe environments such as the offshore oil and gas industry.
“The reasons for this are twofold.
“Firstly by applying a paint product over a galvanized coating, a high level of underlying corrosion protection is provided, with the result that if the organic system becomes damaged there is no urgency to conduct immediate remedial work.
“The galvanized coating prevents the formation of red rust, which would otherwise undercut the organic system had the coating been applied directly on to a steel substrate.
“Secondly, duplex systems are synergistic, in as much as they are greater than the sum of their parts. The galvanized coating causes the organic system to perform better than it would do if applied directly on to a steel surface as a single coating.
“When paint is applied on to a steel surface, over a period of time moisture permeates through the coating’s natural porosity leading to red rust and coating failure.
“Conversely, when applied on to a galvanized coating, moisture may penetrate through to the galvanized substrate leading to the formation of zinc corrosion products.
“However, as zinc corrosion products are smaller in volume than the rust from steel, much lower stresses are created within the organic coating. Residues may even backfill any porosity within the paint system, preventing further moisture penetration.
“For those new to the combined approach, standards are available to allow specification of the different components of a duplex system and information can be accessed easily through Galvanizers Association.
“Correct preparation of the galvanized surface is required for duplex coatings and special attention must be paid to facilitate good adhesion.
“Removal of any oil, grease and other surface contaminants is required prior to further pretreatment.
“Adhesion is enhanced by either chemical treatment, with an application of a mordant solution, or with a mechanical treatment such as light sweep blasting. In addition to the above pretreatments, there are now a number of direct application paints, which may be applied without the need for preparation, increasing options further.
“Change is often difficult to implement, and for those who have a proven history with paint applications it may seem easier to continue down a well-trodden path.
“But if a duplex coating could increase asset life while reducing maintenance costs, why would you not?”
Iqbal Johal is marketing manager at Galvanizers Association, the industry body providing free information and advice on design, specification and performance of hot dip galvanizing.
For further information, visit http://www.galvanizing.org.uk/ or call 0121 355 8838.
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