Thoroughly clean the hot rolled steel parts, removing all dirt, mill scale, rust and any surface imperfections or defects. Use grinders, scrapers, sanders, pressure blasting equipment or abrasive tumblers. Choose from abrasives such as silica sand, steel grit or shot, garnet or aluminum oxide.
To remove oil or grease, immerse the parts in an organic solvent with emulsifying agents or spray the parts thoroughly. Rinse with water and dry.
To chemically remove light rust and mill scale by a process known as "pickling," soak the parts in warm solutions either of hydrochloric acid, phosphoric acid or sulphuric acid with rust inhibitors for at least half an hour. Thoroughly rinse with water.
Bathe the cleaned parts in a mild alkaline solution that contains an accelerator such as hydroxylamine to prepare the parts for the conversion coating, which serves as an undercoat for powder coating.
Choose an iron phosphate, zinc phosphate, or zirconium-based coating. Zinc phosphate has the best performance rating. It adheres rapidly to the cleaned hot rolled steel parts, protecting them and providing a surface that works well with powder coating. The crystals of zinc phosphate conversion coating are small and uniform, preventing air from reaching the parts to which it is applied. After you apply the coating, perform a final rinse.
A type of organic finishing called "plaforization" provides a pretreatment method that cleans, removes grease and applies an organic polyphosphate conversion coating all in one step, without the need for rinsing. Solid particles of debris are filtered out of the solution. You add more product as needed during the process. Organic fluids absorb oily contaminants, which then become part of the product and act as plasticizers in the resin. As the pretreatment process is completed, the organic polymeric resin forms a uniform conversion coating that interacts well with the top coat or powder coat.
Rinsing with water predisposes hot rolled steel to flash rusting, rust that starts to form immediately after old rust has been removed. Water that contains salt makes this problem severe.
Oven drying immediately after each salt-free water rinse helps prevent flash rusting.
Traditional pretreatment methods create sludge and may pollute water. These methods use volatile organic compounds, or VOCs. Some are flammable.
When you install a system to pretreat hot rolled steel for powder coating, you must consider air quality, fire protection and worker safety as well as cost. You also will need to check local, state and federal regulations.
Pretreatment by plaforization is more environmentally friendly and less hazardous to workers, but the end result is not quite as good as when you pretreat by traditional methods and use zinc phosphate to form your conversion coating.