Soluble salts pose a significant challenge in the protection of metallic surfaces, as they can accelerate corrosion and cause issues such as coating detachment due to osmotic blistering. This phenomenon occurs because soluble salts attract moisture from the environment, creating conditions that favor electrochemical corrosion of the metal and compromise the integrity of the coating. Corrosion is facilitated by the presence of chlorides, sulfates, and other ions, which form electrochemical cells on the metal surface, accelerating material degradation.
To mitigate these effects, it is essential to implement preventive practices in coating project specifications. Testing for soluble salts on metallic substrates is crucial to assess and control contamination. Techniques for detecting specific ions allow for the identification and quantification of ions such as chlorides, sulfates, and nitrates. Conductivity measurement, while not identifying the type of salt, is useful for determining the general presence of soluble salts.
Methods for the retrieval and analysis of soluble salts are varied and well-documented in SSPC-Guide 15 and ISO 8502. These methods are classified into three general categories, with the Class A method being the most common. This method involves automatic or manual techniques for containing a liquid in contact with a predetermined surface area, facilitating the dissolution of salts into the solution. The conductivity of this solution is measured to determine the total concentration of dissolved salts.
Automated conductivity extraction techniques are advanced and efficient. Specialized devices adhere to metallic surfaces using magnets or residue-free adhesive tape. These devices automatically dispense a fixed volume of extraction water, which is agitated against the surface to retrieve soluble salts. Conductance measurements are taken in real-time, and the data is digitally stored, providing a precise assessment of salt contamination.
In addition, manual extraction methods are equally effective. The patch cell method uses an adhesive patch covered with a latex film that adheres to the surface, forming a cavity. An extraction liquid is injected into the cavity and massaged against the surface to dissolve the salts. The liquid is then retrieved and analyzed. The sleeve method uses a flexible latex sleeve that adheres to the surface, creating a cavity where an extraction solution is dosed. This solution is massaged against the surface and then retrieved for analysis.
In addition to soluble salt extraction, the preparation of metallic surfaces includes various cleaning techniques to ensure coating adhesion. High-pressure water jetting, with or without abrasives, is a standard practice for removing contaminants, including soluble salts, oxides, and oil residues. This technique is especially effective for preparing rough or corroded surfaces where salts may be trapped in deep pits or craters.
Chemical treatment of surfaces is also a key strategy for improving coating adhesion. Corrosion inhibitors and neutralizing solutions are applied to passivate the surface, reducing its chemical reactivity and prolonging the coating's lifespan. The choice of appropriate inhibitor or neutralizer depends on the type of salt present and the specific operational conditions of the substrate.
Innovations in protective coatings have led to the development of smart coatings that incorporate encapsulated corrosion inhibitors. These coatings release inhibitors in response to incipient corrosion, providing additional protection and reducing the need for costly maintenance and repairs in the long term. This advanced technology ensures more durable and effective protection against corrosion.
In summary, managing soluble salts is fundamental to the preparation of metallic surfaces and the application of protective coatings. The combination of cleaning techniques, precise extraction methods, and advanced coating technologies ensures robust and long-lasting protection against the adverse effects of soluble salts, preserving the integrity and functionality of the underlying metal.
