Maintenance of Protective Coating Systems
One way to maintain protective coating systems – although expensive – is to apply the coating system during new construction and allow it to deteriorate with age until the coating can be totally removed by abrasive blasting and completely replaced.
A second way is to periodically maintain the system whenever small areas begin to deteriorate but before corrosion becomes extensive. Using the least invasive surface prep tool, the intent is to remove loose coatings and rust without destroying the existing surface profile. A spot prime and a full topcoat of the existing system is then applied. This repair procedure can be done several times at five-year intervals before excessive coatings thickness causes the system to break down.
Typically, large structures are fabricated with either inorganic or organic pre-construction zinc-rich primers applied over centrifugally blasted steel to a total film thickness (DFT) of 15 to 18 μm. With exception of structure areas that are intended for immersion service, the preconstruction primer remains intact prior to the application of a full coat of 50 to 74 um, zinc-rich primer. A full coat of high-solids, solvent-borne epoxy is applied at a DFT of 125 to 175 μm either in block stages or at completed fabrication stages. The finish coat of solvent-borne aliphatic polyurethane (PUR) is applied at a DFT of 50 to 75 μm before structure delivery.
For rough service areas, abrasion-resistant epoxies applied at total DFTs of 500 to 750 μm are more commonly applied over the zinc-rich prier. Finish coats of 50 to 75 μm of aliphatic PUR may or may not be used depending on preference.
The corrosion resistance is provided by the zinc-rich primer while the intermediate epoxy coat provides a barrier against penetration of corrosive liquids. The finish coat of PUR adds another layer of barrier protection plus resistance to ultraviolet (UV) rays of sunlight –the real culprit in the breakdown of a typical system.
Newer Resins Provide Increased Service Life
Advances in coatings technology has led to increased use of solvent-free epoxy and PUR systems. While the increased DFT of these systems has made it easier to go beyond the typical 10-to 15-year service life, it doesn’t mean a 20 to 25-year service life will be achieved.
For a 25-year service life, it is important that the coating system chosen is appropriate for the environment in which the structure will serve. Out of competitive necessity, manufacturers make and sell epoxies and PURs at various price points, with cheaper resins used in those with a lower price. As resins are vital to service life, it is critical to opt for the higher priced coating.
Newer resin/curing agent combinations can increase the flexibility of the epoxy coating without compromising the coating’s barrier properties. Newer combinations of epoxy resins and polysiloxane resins, along with selective curing agents, can also make 25-year coating system life achievable. And while the initial cost to apply a newer coating system may be higher, over a 25-year service life, that cost is usually offset by the savings from not having to remove the old system completely.
Predicting Coating Life
A different side of the same coin is estimating coating life. How coating failure is to be judged and at what point is repair or repainting needed must first be established. For coatings that have been properly selected and applied, failure can usually be based on the amount of primer exposed. At 20% primer exposure, for example, the structure can be cleaned, spot-primed where necessary, and its topcoat replaced. Economical, this approach will depend on the appearance one is willing to accept. A similar approach can be taken if the failure is caused by rusting of the substrate.
The estimation of coating life is simplified when the following criteria are met:
• Good specifications
• Qualified contractors
• Good inspection
• Quality coatings
When these criteria are met, one can expect excellent performance. When they aren’t, the estimation of coating life is more difficult and requires knowledge about the following:
• Coating adhesion
• Dry film thickness
• Chalking rate
• Severity of the environment
• Coating type and system used
Estimating coating life on these parameters alone will not provide reliable estimates. For example, if the coating was applied at the specified DFT, but was applied over a layer of rust and dirt, the life would undoubtedly be shortened.
Measurement of the criteria does not generally require great accuracy (with the exception of DFT); rating adhesion can be adequately determined by using a simple pocket knife, chalking rate can be either slow or fast, and a corrosive environment can be flagged as “mild, moderate, or severe.” A major oil company used these simple procedures and was successful in predicting coating maintenance requirements many years into the future.
Of course, there are always exceptions: a knock-out drum in a refinery had been painted with alkyd enamel. These drums receive product from units when there is an upset and generally, they are too hot for an alkyd. With coating on the drum for some 20 years, it was decided that repainting it, using the same system, would be done. Within 30 days of repainting, the unit had an upset and dumped hot product into the drum destroying the alkyd.
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Source: “Service Life Extension for Protective Coatings” by Louis D. Vincent, President of L.D. “Lou” Vincent PhD LLC and technical editor of CoatingsPro Magazine. “Predicting Coating Life,” by Stanley P. Thompson, Phillips Petroleum Co., S.G. Pinney & Associates, Saudi Arabian Oil Co. (retired) and Certified NACE Coatings Inspector. Both originally appeared on materialsperformance.com