Electrocoating Cast Iron and Steel Castings

Question:

How well does electrocoat work on cast iron and steel castings? Most of our parts are machined before painting and require some masking. Parts range from 5-200 lb each. Cost and corrosion protection are important. R.R.

Answer:

Electrocoating castings is usually not a problem. Nonferrous metals pose more problems than ferrous castings because of their temperature limitations. Iron and steel castings coat well. Those who are familiar with electrocoating are aware that the coating does not hide any surface defects; it usually accentuates any imperfections. Therefore, if you are interested in using electrocoat as an alternative to liquid or powder coating, take this “electrocoat is an excellent inspector” into consideration.

The challenges in coating castings usually are dependent on porosity, maximum metal thickness, part weight and ability to support the part without having air pockets or “puddles.” Porosity of ferrous castings seems to vary by the casting process. But, if “gassing” after coating could be a problem, pre-baking the casting to a temperature above that for electrocoat curing can improve or eliminate the gassing. Many times the gassing is so fine that it is not visible on a rough casting; however, gassing can show up when corrosion testing (salt spray, humidity, etc.) is completed.

Maximum metal thickness and part weight is critical in determining energy cost for curing the electrocoat and making sure the coating is completely cured. Determining pounds/hour of parts and tooling and conveyor weight is important in sizing the burner for the oven. Actual metal temperature (versus oven set point) is also important in determining the curing of the part, especially on heavy or thick parts. Checking part temperature on the maximum part thickness using a travelling thermocouple type recorder will provide valuable information to your equipment and coating suppliers. They usually have a recording unit available that they can loan to you or use with your technician.

Many times castings can be coated before machining. This eliminates air pockets or “puddles” in blind holes and the need for masking the machined surfaces. Coating before machining also can create problems, such as other air pockets or “puddles” or no hole(s) to hang the part. Sometimes the answer is to partially machine the casting, coat it and then finish machining. This scenario also can provide corrosion protection on machined surfaces with the electrocoat. With the uniform coating thickness of electrocoat, many times masking is not required.

Due to the transfer efficiency and uniform thin film thickness of electrocoat, it is usually the most cost-effective of organic coatings. Energy cost can be a significant factor if you are comparing electrocoat to air-dry or catalyzed liquid coatings. Electrocoat will be less costly than powder coating, but capital equipment is considerably higher if you are starting from scratch.