How to Prevent Coating Failures & Costly Structural Repairs
To prevent coating failures requires an integrated approach. It starts with proper selection of the coating depending on the material’s end usage and external environments. It’s also crucial that specifications are being strictly met (quality control and inspection) to ensure proper bonding and longevity of the coating.
The coating’s purpose is to limit the exposure of the metal underneath to the corrosive environment. For instance, for iron in structural steel to corrode, there MUST be the combination of these four things, water, oxygen an anode and cathode. As a result, the iron from the steel “corrodes” which compromises the structure of the steel.
The goal then is to limit the exposure of iron to an electrolyte and oxygen. This means the protective coating should be relatively inert against water and oxygen (and other substances that can induce or accelerate corrosion). In some circumstances such as internal linings, the coating should also resist physical abrasion so the protection can remain intact. This is important in pipes that transport water, fuel and chemicals. The flow of these liquids often gradually erodes the interior coatings.
Coating failures are expensive
Huge financial losses could result from coating failures. For example, the corrosion of steel storage tanks could lead to expensive repairs and long downtimes (which further adds to expenses and lost income) and can have huge environmental ramifications on the surrounding environment. This could also affect many production and operational tasks which heavily rely on water or other liquids stored in the tank.
This failure might have resulted from:
- Coating didn’t adhere well with the metal
- The coating didn’t remain intact (physical abrasion & chemical reactions)
- Inadequate surface preparation
- Inappropriate choice & application of coating
- Lack of maintenance (recoating & repairs were not done promptly)
Whether it’s for storage or transport of liquids, metals should be adequately protected from physical abrasion and chemical reactions. This is to maintain the structural integrity of the pipes and storage tanks (and protect the contents). This is also to ensure minimal interruptions and downtime in key operational tasks of a facility.
To prevent coating failures, the most effective strategy then is to analyse the causes of those failures as mentioned above (e.g. coating didn’t adhere well with the metal or substrate). Then, the specifications of the selected coating should address those failures for effective and long protection by specifying certain parameters around coating application to ensure the intended life span of the asset it is protecting (years may vary depending on application and substrate).
Challenges in attaining coating success
The purpose of coating is straightforward which is to protect the underlying metal or surface. However, a variety of things can make it a challenge to make coating system a success. There are various substrates, application methods and environments to consider.
For instance, there are environments with varying salinity levels which can affect corrosion rate (especially if oxygen is being released or dissolved in large amounts). Both macro- and microclimates (overall climate and the temperature and humidity levels in a particular site) can also affect the effectiveness of a coating. Pipes and tanks are also used in a wide variety of applications to store or transport water, fuel and other liquids with varying chemical compositions.
Although all coatings will eventually wear off or show signs of deterioration given enough time, they should still withstand internal and external reactions in the number of years specified. These are just estimates but it’s always recommended that the estimates are not too far off. This way, you and your team can accordingly plan and schedule for a recoating or maintenance. This also makes it easier to plan for future expenses related to repairs and maintenance of pipes, tanks and other structures.
The years a coating system should last vary depending on the quality of the application, the coating system specified and the environment it is in. It’s especially the case in offshore areas with extreme humidity levels and splash zone environments occur. There might be a need for more frequent recoating to ensure integrity of metal tanks and pipes. In contrast, rural areas might have milder environments wherein corrosion and aggressive conditions might not be too much of an issue.
Bridges, pipelines and aircrafts often require regular monitoring of their surfaces. Small Localised corrosion can compromise the whole structure especially when it occurs in welds and other critical areas.
There are many challenges in ensuring coating success (adequate and cost-effective protection in a reasonable amount of time). That’s why engineers address those challenges one by one through various viable and cost-effective methods.
Effective ways to prevent coating failures
One method of maximising the chances of coating success is by intelligently designing the structure itself. The goal here is to avoid creating dirt and water traps, and “hard to get” areas for the contractor applying the coting. Things such as crevices and small voids should be avoided. This is to successfully allow for a uniformly apply the coating. To accomplish this, many structures now are being designed with the minimal number of small structural components. The preference is to having large flat surfaces as often as possible so the coating will be more uniformly applied (and no traps that will be thriving corrosion sites).
Another way of using the structure itself to guarantee coating success is by minimising the exposure of the structure to water. Take note that water and oxygen are essential for corrosion. Remove either of those and corrosion won’t occur. So if we create a ventilation and drainage system so the structure (e.g. bridges) will dry out more quickly, there will be lower occurrences of corrosion. The coating will also likely to remain more intact in those scenarios.
When it comes to coating selection, this requires technical expertise about the coating, end usage, exterior and interior environments and substrate material. The coating application itself should also be done properly by meticulously documenting and following each step in the specification and suppliers product data sheets (including the method of surface preparation, overcoating intervals in the application of subsequent coats of coating, specific instructions such as stripe coating application using brush and roller to all areas that cannot be reached by conventional spray methods).
For instance, surface preparation is crucial to the excellent performance of a coating. After all, it also depends on how well the coating can adhere with the substrate material. Therefore, surface contaminants such as oil, grease, mill scale should be removed first. In the case of steel surfaces, this can be done through high pressure water wash and abrasive blast cleaning (mechanical cleaning using abrasives at high velocities).
After surface preparation, the steel surface should be maintained in a dry and clean condition right before the application of coating. This is to prevent water from causing immediate corrosion which could make the coating less adherent and the underlying structure compromised, and also prevent dust from causing poor adhesion between the coating and the substrate (e.g. introduction of traps and uneven surfaces).
There are also special considerations when dealing with welded and bolted connections. Many construction activities such as welding and site cleaning can introduce contaminants to the surface. This makes the adherence of the coating less effective and far from being uniform and consistent.
Aside from intelligent design of the structure (minimal number of small structural components) and proper coating application (including surface preparation), it’s also recommended to have ongoing procedures in place. This includes regular inspection and maintenance of coated structures and verifying whether the coating is still intact and performing as specified.
Visual (for aesthetics and looking for obvious signs of deterioration) and mechanical inspection (is the structural integrity compromised?) are often regularly performed by engineers and the staff. It’s especially the case when the coating was last applied a few years ago (or there’s a recent weather disturbance).
These are all important in preventing the breakdown of your assets which could result to costly repairs and replacements. It’s an ongoing requirement to protect your assets especially from the gradual and often subtle effects of corrosion.
More importantly, the application and treatment of coating should always be done correctly the first time. This is to prevent costly rework (financial losses and expensive downtimes) and also avoid introduction of corrosion sites in the first place.
To accomplish this, many engineers contact us here at Corrims to obtain specifications and then to ensure proper coating application according to the specification and strict standards. Our highly certified professionals who use the most up to date testing equipment are committed to avoiding large failures at the end of the project. As a third-party inspection company, you can rest assured that your contractors will perform as expected.