Having been engaged in the steel structure industry for over 20 years, we often encounter peers asking, "We use the best anti-corrosion coatings, but why does rust begin to appear after just three to five years?" Some peers also ask us, "You repeatedly polish corners and hard-to-reach areas, wasting manpower and resources. What's the point?" Now let's explore the relationship between "node" polishing and anti-corrosion.
When many people inspect and accept anti-corrosion projects, their first concern is whether the large surfaces are evenly colored and the paint film is continuous. If they look beautiful, they assume the quality is good. However, the real corrosion hazards never appear in the most conspicuous places first. They always hide in cutting edges, bolt holes, weld edges, and areas near lap joints. These "nodes" may seem unremarkable, but they are the most vulnerable links in the entire steel structure anti-corrosion system, and the cost of repair in the later stage is also the highest. The truly difficult part of steel structure anti-corrosion management lies precisely in these easily overlooked nodes.
Why are nodes most prone to problems?
Through our experience in numerous projects, we have identified that nodes prone to issues generally share three common characteristics:
Firstly, the shape is irregular, making it difficult to control the spraying quality.
When spraying on large surfaces, the spray gun distance, spraying speed, and overlap width are relatively easy to control, resulting in a uniform paint film thickness. However, when it comes to corners, bolts, weld seams, and areas near holes, the complex shapes make it difficult to maintain consistent coverage of paint: some areas may have insufficient film thickness, some may have paint buildup and sagging, and some positions are prone to forming blind spots or even leaving pinholes.
We have conducted tests and found that the paint film thickness at sharp right angles is often only 30%-50% of that on large flat surfaces. After the paint cures and shrinks, gaps that are invisible to the naked eye may even appear at the edges, making it easy for corrosive media to penetrate from these locations.
Secondly, surface treatment is more difficult to achieve complete cleanliness.
Residual welding spatter, slag, and oxide scale are unavoidable near the weld seam, and burrs and sharp edges often occur at the trimming position. Dust, oil, and even residual salt tend to adhere to the area around bolt holes. These impurities may seem unremarkable, but they can directly affect the coating adhesion.
Many projects may not show any problems when the anti-corrosion measures are just completed. However, after several rounds of rainwater washing, alternating temperature differences between cold and heat, and ultraviolet radiation, the underlying defects will gradually magnify, evolving from spot rust to extensive corrosion.
Thirdly, nodes are more prone to accumulate corrosive media.
For steel structures used outdoors, rainwater flows along the surface of the components. At nodes with poor drainage, water vapor stays for several times longer than on large flat surfaces. If located in coastal areas, chemical parks, bridges, or industrial plants, they will also be subject to the corrosive effects of chloride salts, acid-base atmospheres, and dust deposition.
The greatest threat to corrosion is "long-term dampness, long-term dirtiness, and long-term neglect of inspection". If a node simultaneously meets these three conditions, it naturally becomes a "breach" for corrosion.
Many cases of corrosion failure begin with "uneven" film thickness.
Most current anti-corrosion schemes specify the total film thickness requirements, such as the thickness of primer, intermediate coat, and topcoat in micrometers. These are, of course, basic indicators. However, we have always emphasized in project management and control that on-site acceptance inspection should not only focus on the average film thickness, but also pay close attention to the edge film thickness and local film thickness.
Sharp edges are the most typical risk points. It is difficult for paint to form a sufficient thickness on sharp edges, and it will further shrink after curing. The sharper the edge, the more likely the coating film becomes thinner. Therefore, experienced engineers will require chamfering and polishing of the sharp edges of components, and even pre-coating of corners, welds, and bolt areas in advance.
The pre-coating process is labor-intensive and is most easily omitted. However, the quality difference in many projects lies precisely in this step: large-area spraying may seem efficient, but if the nodes are not handled in advance, no matter how thick the topcoat is applied later, it cannot compensate for the defects in the underlying layer.
Weld seam corrosion prevention is never as simple as "just spray it on".
Weld joints are the most complex type among all nodes. The weld itself has uneven heights, and there is also a welding heat-affected zone nearby. If the surface treatment is not adequate, the coating can easily form weak areas near the weld toe, undercut, and splash points.
Some welds may appear to have been painted, but in reality, the paint has not reached the recessed areas. Once moisture enters, corrosion will gradually spread along the weld. By the time rust is visible on the surface, the internal corrosion is already severe. There is another more subtle situation: small areas of blistering first appear near the weld. During on-site repairs, it is assumed to be a problem with the topcoat. However, after sanding and repainting, the issue soon reappears. In this case, the problem is often not with the topcoat, but rather a combination of inadequate surface preparation, poor primer adhesion, weld appearance defects, and localized corrosion media. Simply repairing the surface does not address the root cause.
The bolt connection area is the most easily underestimated anti-corrosion blind spot.
The bolt connection area poses another significant and longstanding issue in terms of corrosion prevention. This area inherently features gaps, edges, and obstructions, and is prone to bruising during installation. After tightening, numerous tiny gaps emerge between the gasket, nut, and connecting plate, making it challenging to dry out once water infiltrates these gaps.
If the connection sequence and touch-up windows are not properly planned before construction, and are dealt with only after the entire structure is installed, many locations will become difficult to grind and spray, ultimately becoming "dead ends" in terms of corrosion prevention.
This is also what we always emphasize to our customers: when it comes to steel structure corrosion prevention, it's not just about the quality of the coating itself. Every aspect, including design, processing, installation, surface treatment, and coating sequence, can affect the final corrosion resistance lifespan. The coating is only the last step in film formation and protection. If the preceding steps are not handled properly, even the best coating will have to bear the blame for the previous processes.
The cost of node rework is several times higher than that of large-scale production.
When a node encounters a problem, the repair cost is often much higher than that of a large flat surface. At least for large flat surfaces, the repair can be clearly seen, easily accessible, and convenient for grinding. However, node locations are often in high places, at angles, on the back of equipment, at the bottom of supports, or in areas with dense pipelines. During repair, scaffolding needs to be erected, work needs to be coordinated for downtime, local rust removal needs to be carried out, and the compatibility of new and old coatings needs to be considered. Finally, the film thickness and appearance need to be re-inspected and accepted.
Many times, the most expensive part of repairing is not the paint, but the downtime costs, labor costs, lifting fees, safety measures investment, and even compensation for project delays. Therefore, when we do anti-corrosion projects, we never just calculate "how much is a barrel of paint", but first ask about the key issues:
How is this anti-corrosion system handled at node locations? Are there any requirements for pre-coating of welds and sharp edges? How is the coating reapplied after installation in the bolt area? How is the local film thickness inspected on site? Which locations require special inspection during later maintenance? By clarifying these issues in advance, the anti-corrosion risk of the project can be reduced by at least 70%.
A good anti-corrosion plan must include "node requirements"
A reliable anti-corrosion plan for steel structures should not only outline the matching of primer, intermediate coat, and topcoat, but also clearly state the requirements for node treatment: for example, sharp edges should be chamfered, burrs should be polished, weld spatter and slag must be cleaned up, and pre-coating must be applied around edges, corners, welds, and bolt holes. After spraying, local film thickness should be checked instead of just looking at the average value. There should be clear repair methods for coating damage caused by transportation and installation. The humidity, dew point, surface salt content, and roughness during on-site construction should also be recorded.
These requirements may seem trivial, but they directly determine whether the coating can achieve the designed service life. Anti-corrosion is actually not that mysterious. Many failures are not due to the material itself, but rather to loopholes in node treatment, construction control, and acceptance standards, which gradually amplify the risks.
Nowadays, there are many high-performance anti-corrosion materials in the industry, such as polysiloxane topcoat, zinc-rich primer, and epoxy intermediate coat, which excel in weather resistance, gloss retention, color retention, water resistance, and adhesion. However, we always believe that no matter how good the material is, it needs to be matched with proper surface treatment and standardized construction procedures. If the coating thickness at the node position is insufficient, the substrate is not cleaned thoroughly, and the edges and corners are not pre-coated, even if the topcoat is expensive, it is difficult to compensate for the defects of the underlying layer.
As a company with over two decades of experience in the steel structure sector, we have always believed that the principles of steel structure engineering and corrosion prevention are intertwined: stability depends on the nodes, and quality hinges on the details. It is only by securing the most vulnerable nodes first, and then discussing system upgrades and material optimization, that we can truly be responsible for the project and our customers.
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