Major Contaminants
Oxides
The most common oxides found on metal surfaces include iron oxide (Fe2O3) and zinc oxide (ZnO). Various cleaning methods are used to remove these oxides. Among the most effective are mechanical sanding, which physically removes oxide layers, abrasive blasting with materials like sand or slag, useful for larger or severely corroded surfaces, and chemical deoxidizers that chemically dissolve oxide, preparing the surface for protective coating application.
Greases and Oils
Greases and oils are common contaminants on metal surfaces, especially in industrial or mechanical environments. These contaminants can originate from lubricants, hydraulic fluids, or industrial residues and can significantly hinder the adhesion and performance of protective coatings if not properly removed. Specific cleaning methods for surfaces contaminated with grease and oils include chemical degreasers formulated to dissolve and remove these greasy contaminants. Additionally, washing with high-pressure water or steam can be used to remove surface layers of oils and greases.
Dust and Dirt
Dust and dirt are common contaminants found on metal surfaces, especially in outdoor environments or construction and manufacturing sites. Methods to remove them include compressed air or brushes to remove loose dust and dirt particles. Additionally, washing with high-pressure water or specific cleaning solutions helps eliminate embedded dirt and surface residues. • Old and Loose Paint: Methods to remove old and loose paint include mechanical sanding to remove loose layers, abrasive blasting for extensive areas or irregular surfaces, chemical strippers for resistant paints, and high-pressure water washing.
Salts
Salts on metal surfaces can be a significant issue, especially in marine or industrial environments exposed to moisture and saltwater. Salts can cause corrosion under coatings and affect the adhesion of new paint systems. Specific cleaning methods to remove salts before applying protective coatings include washing with fresh water at high pressure to remove surface salts, using desalinating solvents that dissolve embedded salts, and abrasive blasting to effectively remove salts and tough contaminants on larger surfaces.
Welding Residues
Residues such as slag and spatter from welding can compromise the adhesion and quality of protective coatings if not properly removed. Specific methods for cleaning surfaces with welding residues include using mechanical tools such as wire brushes or grinders to physically remove slag and spatter. Additionally, chemical strippers designed to dissolve welding residues can be applied to harder-to-reach areas.
Chemical Contaminants
Chemical contaminants on metals before pretreatment and coating removal may include corrosive acids such as hydrochloric acid and sulfuric acid used in industrial processes and cleaning, as well as bases such as sodium hydroxide and potassium hydroxide, and volatile organic compounds (VOCs) derived from industrial solvents. Effective cleaning mechanisms to remove these contaminants include solvent cleaning to dissolve chemical residues, chemical stripping with controlled acids or bases to eliminate persistent contaminants, washing with high-pressure water to remove surface residues, and chemical neutralization to ensure complete removal of acids and bases.
Moisture
The presence of moisture can cause corrosion and oxidation of the underlying metal if not properly controlled. Effective drying methods such as using dehumidifiers, heaters, or fans to remove surface moisture are essential for ensuring that the surface is completely dry before applying any coating.
Implications of Contaminant Residue
Poor Adhesion
Contaminants cause poor adhesion of the coating primarily because they create a physical or chemical barrier between the metal and the applied coating. Surface oxides, such as iron oxide, form a hard and irregular layer that prevents the coating from firmly adhering to the underlying metal. Industrial greases and oils act as separating agents that prevent paint or coating from coming into direct contact with the metal surface, thus hindering its adhesion. Dust and dirt can physically interfere with the bonding of the coating to the metal by filling the necessary spaces and porosity required for a strong bond. Old paint residues may be poorly adhered or have an uneven texture that prevents the new coating from adhering uniformly. Finally, chemical contaminants such as acids, bases, and solvents can leave corrosive residues or residues incompatible with the applied coating, compromising its ability to adhere effectively to the metal.
Rust propagation
Allowing rust to remain active under the coating can continue spreading beneath the protective layer, compromising the integrity of the metal and eventually causing coating failures.
Bubble formation
The presence of contaminants like moisture, oils, or trapped air under the coating can cause bubble formation. This not only affects the aesthetic appearance but can also weaken the coating and increase the risk of premature failures. In addition to creating areas of low adhesion susceptible to delamination due to thermal expansion or vibration, bubbles act as stress concentration sites where mechanical and thermal stresses concentrate. This can lead to cracking or fracturing of the coating.
Aesthetic Appereance
Visible contaminants such as oil stains, dirt, or poorly prepared areas can affect the appearance of the final coating, compromising the aesthetic of the finished product.
Coating Contamination
The presence of chemical contaminants can affect the composition and integrity of the applied coating, reducing its effectiveness and durability. Trapped particles, especially if large or irregularly distributed, can create stress points and lead to cracking of the paint layer. Moreover, it also affects the flexibility that the paint must have to be able to flex and adapt to small movements and deformations of the metal substrate without breaking or peeling. Trapped particles can create rigid or inflexible areas within the paint, reducing its ability to withstand movements and vibrations without cracking.
Corrosion Induction
Contaminants such as acids or salts can accelerate the corrosion process under the coating, weakening the metal structure and compromising the functionality of the treated component.
