Stainless Pretreatment for Painting

Q. I am doing a project to develop a new chemical for stainless steel pre-treatment prior to painting or powder coating. Any suggestions regarding methods or processes for pretreatment? Is it possible to treat by acid pickling solution before coating? Y.L.

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A. As with any coating process, the initial cleaning step is one of the most important. This will be used to remove any process lubricants, cutting fluids, and oils from handling, forming, stamping, etc. The type of cleaning will be based on the soil present, size of parts and equipment you may have available to you. Many typical industrial process lubricants can be removed sufficiently with an aqueous cleaning line consisting of cleaning and rinsing steps at a minimum. Agitation through spray wash or ultrasonics could shorten the cleaning time. That first step is important since any subsequent steps will not be effective if that is not done correctly.

Following the initial cleaning, there are several potential options available to you for further evaluation. I would suggest development, testing and evaluation to determine which works best in your application. Several factors to keep in mind are the surface cleanliness and final paint or powder performance, potential capital expenses, the throughput or unit cost of each process and the environmental, health and safety impact of each potential process.

As you mention, stainless steel can be treated through an acid pickling of the surface. Because stainless steel is relatively inert (which is why it is specified to begin with), it can be difficult to find a good chemical treatment that alone will suffice as a pretreatment. Stainless steel can be acid etched in a combination nitric (~10%) and hydrofluoric acid (~1%) bath at room temperature for a few minutes. However, the hydrofluoric acid can be extremely dangerous to handle and would be a hazardous waste when that bath is finished. Make sure to review any material safety data sheets prior to pursuing this option.

An alternate treatment may be to sandblast or mass finish the surface to roughen it slightly to improve paint adhesion and performance. Sandblasting is relatively straightforward and can be performed in a cabinet if parts are small enough. Mass finishing would provide even more options for featuring the surface using the many types of media available (such as stone, plastic, glass bead, etc.).

Both the above pretreatments may be effective, but depending on the paint requirements, I believe you may also need the use of a standard or wash primer to effectively prepare the surface to receive the final top coat. A wash primer is a two-step mixture that utilizes a resin mixed with inorganic fillers and a carrier that is then mixed with a phosphoric acid and carrier mixture. The phosphoric acid will effectively etch the surface of the metal and will provide a more compatible substrate for the final top coat to take hold.