Introduction
Metal is the most widely used material in various industrial sectors worldwide; preserving its integrity is a priority to guarantee the useful life and functionality of assets. To achieve this objective, precise compliance is required in the application process of metal surface coatings. With a good application procedure on the surface to be protected, a protective barrier is obtained to protect the metal from environmental and operational adversities.
The efficiency of an effective coating lies in the precise application of the protective system. Precision during the process not only involves the correct selection of the coating itself, but also proper surface preparation, uniform application, and proper curing. Each of these stages dictates the quality and effectiveness of the system, and therefore the magnitude of surface protection achieved on the protected metal assets.
Importance of metal integrity
It refers to the ability of a metallic material to maintain its structural properties, even under adverse conditions, thus ensuring its functionality and durability. In the context of surface coatings, metallic integrity is critical to ensuring that industrial assets resist corrosion, wear, and other forms of degradation, thereby enabling safe operations.
What are surface coatings and what is their purpose?
Coating metals consists of a process where one or more layers of paint or coating are applied to the surface or substrate of the metallic element. Paint systems for steel structures have been developed over the years and are a fundamental process to maintain functional properties, and structural integrity and improve their aesthetic appearance.
This process is one of the first lines against attacks of corrosion, erosion, and wear of materials; which leads to the need to provide this protection, especially when the metals are going to be subjected to extreme conditions.
The main purpose of coating metals is protection. This action should not be underestimated, because when the surface of metals is bare it interacts with oxygen, carbon dioxide, and other contaminants, it oxidizes and is where corrosion is triggered, consuming the metal until it weakens and degrades over time. time. Likewise, it protects metals from impacts and underground and maritime environments.
Accuracy in the surface preparation process and application of coatings
Firstly, we must keep in mind that the performance of the coatings is directly related to the preparation of the surface. Implementing an effective protective system is a very important factor that has a direct impact on the final result of the procedure carried out. This not only improves the resistance of metals to multiple degradation factors but also optimizes their performance in the various industrial environments where the element operates.
The main steps to follow during a surface preparation and coating application process are described below:
Selection of the coating: It is carried out according to the characteristics of the surface to be protected, the environmental conditions to which it will be exposed (humidity, salinity, temperatures, changing environments); Likewise, the operational variables are evaluated. Anticorrosive tape coating systems (figure 1) are used on the outside of carbon steel pipes to transport water and liquid hydrocarbons, among others; with the purpose of providing underground or underwater anti-corrosive protection; to ambient conditions.
Factors such as adhesion capacity, corrosion resistance, desired durability, and aesthetics, among others, must also be considered. On the other hand, it is necessary to ensure that it is compatible with the metal surface and with any previous coating, if any.
Surface preparation: The performance of a coating is significantly influenced by the quality of the surface preparation; the optimum adhesion of the coating depends on this. These activities are standardized by international associations that categorize the different procedures to perform a correct surface cleaning before applying a coating.
Abrasive blasting surface preparation video, courtesy of
BCSATataSteel.
In general, the area should be clean and free of rust, rust, grease, dust, paint residues, and other contaminants that affect adhesion. Among the main surface preparation methods are manual, mechanical, and chemical cleaning, and abrasive blasting (shot blasting, sandblasting and hydroblasting); the latter being the methods that generate an anchor profile on the surface to obtain a better adhesion of the coating.
In this step, when mechanical or abrasive cleaning is applied, since the metal surface is exposed and vulnerable to humidity in the air, it must be ensured that the element or structure is kept in a dry environment after cleaning.
Coating application: Normally done under the manufacturer’s specifications. Here it is essential to respect the environmental temperature ranges (relative humidity, temperature, dew point). This procedure may include the application of a primer followed by the main coating, or several layers of paint, either of the same or several types depending on the system.
Application techniques could include brush, roller, dipping, low or high-pressure spraying, these vary depending on the type of coating and the characteristics of the element to be protected.
Drying and curing time: After application of the coating, the paints need time to dry and cure before being used. The time required varies depending on the type and environmental conditions. It is essential to respect these times, as indicated in the manufacturer’s technical sheet to ensure its efficiency.
Inspection and testing: During the application process and once completed, the necessary monitoring and testing are carried out to ensure correct application. This includes adhesion tests, visual inspection, measurement of thicknesses between layers, holliday testing, among others. It is recommended that these tasks be carried out by trained personnel, using state-of-the-art equipment and instruments.
Maintenance: As a strategy to extend useful life and efficiency, it is necessary to establish and follow a maintenance plan that guarantees that the coating continues to offer effective protection over time.
Compliance with these steps meticulously guarantees effective and long-lasting protection for steel surfaces, protected with some type of coating; contributing to the preservation of metallic integrity and the extension of the useful life of industrial assets.
Associated risks and case studies
Incorrect surface preparation exposes metals to failure mechanisms such as corrosion, compromising their performance and reducing their useful life. Corrosion is detrimental as it weakens the metal structure at a molecular level, ultimately resulting in structural failure. The degrees and times of deterioration due to corrosion vary depending on factors such as the environment (industrial, marine, humid, etc.), the exposure of the structure, and the design of the structure1.
Case study based on the failure analysis of the coating of a cargo container
The case study addresses a serious failure in the liner system of a cargo container after only 8 months of use, despite an expected useful life of 20 years. Inspection showed marks, cracks, and rust on the container, with testing revealing multiple discontinuities in the coating in various areas.
The tests also revealed areas of under- or over-coating, and at least two of the five adhesion tests failed to meet standards. SEM/EDS (Scanning Electron Microscopy/Energy Dispersive Spectroscopy characterization) analysis of one sample revealed details about the coating layers and the presence of elevated salt levels underneath the failed coating. Best engineering practices were suggested to improve adhesion and uniformity2.
Case study on the failure of polyurea (PU) elastomer lining in a seawater storage tank
The case exposes a failure of the lining system of a seawater storage tank after 14 months of service, contrary to the 15-year life expectancy of the tank. A team investigated PU elastomer (polyurea) failure on a project in the Arabian Gulf.
Inspections revealed rust, blistering, delamination, and thickness variations. Tests revealed interlayer flaws and pinholes near weld seams. FTIR (Fourier Transform Infrared) analysis showed similarity of the coating to polyurethane, while water resistance tests showed no significant deformation, illustrating the failure mechanisms of the coating in service3.
Main standards
Numerous protection systems have been created to mitigate and manage the protection of industrial assets, mostly based on corrosion management. Two of the leading organizations that have historically defined the norms and standards in this area have been: NACE (National Association of Corrosion Engineers) and SSPC (Steel Structures Painting Council), both merged in 2021 to emerge the AMPP (The Association for Materials Protection and Performance).
The AMPP aims to provide a unified voice for the corrosion control and protective coatings industries. It is a professional association focused on asset protection and materials performance, in order to safeguard both industry assets and the environment. This underscores the importance and impact that surface preparation and coating application has on preserving the integrity and useful life of industrial assets.
Innovations in preparation and coatings for metal surfaces
Surface preparation and coating application on metallic substrates is and will continue to be a challenging task for researchers due to the multiple needs they continually have to address. Today, sustainable solutions are required, maintaining a high level of quality and efficiency in products and processes.
These innovations not only contribute to the protection and improvement of metal assets but facilitate adaptation to the changing demands of modern industries. Here are some trends in coatings and metal surface preparation equipment:
Innovative coatings: High-performance coatings are mentioned, which are based on epoxy polymers, polyurethane, and modified phenolic resins, they have greater adhesion and offer exceptional protection against corrosion. Nanostructures have improved protection through nanometric particles that penetrate the substrate surfaces and provide better protection. While sustainable coatings focus on water-based solutions with low emissions of volatile organic compounds (Low VOC).
Robotic surface treatment: Robotic shot blasting provides precise and safe cleaning of metal surfaces, removing the operator from dangerous areas. This technique adapts the process to different sizes of structures, reducing costs and errors. It stands out for improving surface quality, increasing productivity, and providing a safer work environment, being relevant in the preparation of surfaces in industries such as oil, shipping, and construction.
Coating application with robotic systems: The integration of robots ensures uniformity and precision in the process, improving the quality of the final result. They control the amount of coating, speed and angle of application, even in hard-to-reach areas, optimize the material and reduce waste. An example of this application is the Oil&Gas industry, where large storage tanks are painted, with quality and safety in less execution time.
Conclusions
The importance of careful preparation and application of coatings on metal surfaces prevents their degradation and maintains the integrity of industrial assets, protecting them from corrosion and other wear. Precision at every stage, from coating selection, and surface preparation, to application and final testing, is critical to ensuring effective protection.
It is important to comply with the standards established by recognized organizations in the field of materials protection and corrosion control. Currently, innovations in coatings and automation using robots for surface preparation and coating application improve the quality, efficiency, and safety of the process.
Preventing metal degradation not only preserves the mechanical integrity, but also maintains its visual appeal, thus ensuring that the metal continues to serve its purpose effectively and efficiently.
References
- https://www.mdpi.com/journal/metals/sections/structural_integrity_metals#:~:text=Section%20Information,the%20art%20on%20this%20topic
- Bukhari, Syed Umair Niaz. “Case study based on the failure analysis of a cargo container liner.” Paper presented at the AMPP Annual Conference + Expo, San Antonio, Texas, USA. USA, March 2022.
- Rana, Ahmed Raza Khan, Jarjoura, George and Syed Umair Niaz Bukhari. “Case study on the failure of polyurea (PU) elastomer liner in a seawater storage tank.” Paper presented at the AMPP Annual Conference + Expo, Denver, Colorado, USA. USA, March 2023.
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