We often pre-treat certain elements of the home and in industrial cases, we pre-treat in medical equipment and aerospace equipment. We also pre-treat for paint before metals. A high-quality conversion coating is essential for the durability of painted metal goods. The process of applying an inorganic conversion coating to a metallic surface involves removing any surface contaminants, then chemically converting the clean surface into a non-conductive, inorganic conversion coating. Conversion coatings increase the overall surface area and promote adhesion of the subsequently applied organic film. In addition, conversion coatings change the chemical nature of the surface, which increases corrosion resistance.
It is these two functions, increasing surface area and changing the surface chemistry, that serve as a base for preparing the substrate material for paint finishes. There are a number of driving forces in the pretreatment industry today with quality, cost and the environment being the most predominant. While these aren’t new issues, the pretreatment industry has responded to the needs of finishers by creating technology to address each of these requirements. In understanding the complete manufacturing process, including paint formulations, application equipment and regulatory impacts, it’s possible to address each driver simultaneously. Companies like envirotech-europe.com supply solvents and cleaning solutions for metals. The conversion coating chemistries predominately used today are either zinc or iron phosphate. There is movement to replace these technologies with new types of phosphate-free or very-low-phosphate metal pretreatments. The new-generation technologies have been commercialised by many vendors over the past several years and are rapidly becoming industry standards. Regardless of the chemistry, conversion coatings are used to promote adhesion and improve corrosion resistance. Depending on the conversion coating and the desired performance, the conversion coating can be applied at a number of points in the process.
Before the first coat of powder ever gets applied, you have a decision to make: “How are you going to prepare your metal?”. To get the best powder coating results, the surface you will be coating must be clean. Depending on the quality and type of metal, there are different levels of cleaning and pretreatment to consider. You should also account for your customer’s requirements and how long the part should last in the field. First, determine what you’ll be coating the most often. If you are coating sheet steel, for example, you will usually only need light oil cleaning. Angle iron or castings often need sand/shot blasting to remove scale and surface rust. Aluminium is prepared differently than galvanised or regular steel; the oxidation layer of aluminium must be removed, which requires certain chemicals that provide a good etched layer for paint adhesion. Identifying which type of cleaning is right for your process is the first step in long-lasting, quality results.
The most common types of metal cleaning are:
Blasting. Blasting with sand or shot is a great way to clean up metal scale, laser scale, rough welds, or heavily rusted steel. Blasting is also used to strip off previously coated metal for refinishing. While blasting smooths out a lot of surface defects in raw metal, it does not fully clean the metal of oils or other contaminants. However, blasting does create a more adhesive surface for the powder coating after the part has been cleaned of residual soils.
Washing. Pressure washing, dipping, or automatic washing (with a soap specifically formulated for the soils specific to your fabricating process) are the most effective ways of cleaning the metal prior to the next finishing stage. Steam cleaning or hot water helps break down the oils and can reach difficult spots or gaps in the surface. Detergent is the best way of cleaning metal of oils, waxes, polishing compounds, or other substances that will prevent the powder from sticking to the metal.
Wiping. Solvent wiping is another way to clean up the metal of surface oils and contaminants, but it is an inaccurate way to clean. Since the part is manually wiped with rags, the rags can become saturated with the oil you are trying to remove.