Key factors in aqueous metal surface preparationAnna Lee
Metal products or components will often require finishing steps or added processing. Some of the more common finishing steps include painting, plating, welding, soldering, brazing and mechanical assembly. Most metals in their “pure virgin” state are very receptive to any of the processes mentioned, however given time and environmental stresses the metal surface becomes contaminated. This contamination will prevent or inhibit subsequent processing of the metal. Thus the need for surface preparation. Good surface preparation usually involves two steps.
- Surface Cleaning: Removing existing contaminants
- Surface Coating: Applying a material that prevents or retards further contamination.
There are many types of containments on a surface including rust, dirt, grime, oils, chemicals even biological growth if subjected to the right environment. A proper cleaning process will remove all containments. Different contaminants will require different chemistries to clean. For example rust is best removed with an acid that will react with it and break it up. Dirt can be removed by a chemical that can break it apart and remove it by changing the surface tension of the material it is adhered to.
Cleaning is defined as the removal of soil or unwanted matter from a surface to which it clings. The cleaning process can be accomplished by one or more of the following cleaning mechanisms:
Wetting: Through the use of surface active agents (surfactants), the cleaner penetrates and loosens the substrate-soil bond by lowering the surface and interfacial tension of the water.
Emulsification: Once wetting occurs, two mutually-immiscible liquids are dispersed. Oil droplets are coated with a thin film of surfactant therefore prevented from recombining and floating to the surface.
Solubilization: The process by which the solubility of a substance is increased in a certain medium. Certain solvents for example solubilize mineral oils.
Saponification: The reaction between any organic oil containing reactive fatty acids with free alkalis to form soaps.
Dispersion: The process of breaking the soil into very fine particles and dispersing them into the cleaning media. The soil is then maintained as dispersion and prevented from agglomerating and re-depositing on the clean surface.
Sequestration: Undesirable ions such as calcium, magnesium or heavy metals are deactivated, thus preventing them from reacting with material that would form insoluble products.
Cleaning methods can include emersion, spraying or foaming. The cleaning process can sometimes be accelerated with heat, radiation, mechanical abrading or even ultrasonic’s. The size, volume and configuration of the product are often key in determining the type of technology to use. Small parts can be placed in a basket and submerged in an ultrasonic cleaner. A large vat or tank will require manually spraying or foaming. In some cases the surface is badly rusted or corroded and aqueous cleaners may not be viable. In such cases sanding, brushing, sand blasting or high pressure sprays are needed.
Effective cleaning solutions also need to be safe for people, the environment and the metal. Aggressive cleaners can damage metal surfaces making them more suseptable to oxidation.
Once a metal is clean, it may need to be coated with a material that prevents corrosion or other surface contamination. If the processing is immediately after cleaning, then the cleaning material can also act as the coating material. In other cases there may be significant time elapsed before the next fabrication step is performed. In other cases the coating may need to remain on indefinitely. The type of coating material used will depend on length of time as well as expected environmental conditions.
In summary, choosing the most effective surface preparation will require a thorough understanding of the following:
- The type of metal
- The type of contamination
- The processing that the metal sees before, during and after finishing
- Equipment available (often determined by size of part)
- Ecological considerations
- The process/life cycle of the metal (what conditions does the product see when in use)
Packnet offers a large selection of Cortec materials that will meet just about any kind of surface preparation challenge. Our engineers start by analyzing all the critical factors relating to the product and processes before making recommendations. After the solution is recommended, Packet will follow through to ensure that the desired results were achieved. If you are looking for effective cleaning and preparation solutions that are environmentally friendly, then contact Packnet to speak to one of our technical experts.