Q. I own a trailer manufacturing company in South Africa. What advice could you give me on steel prior to primer and painting of our trailers. I think ideally I would like to “treat” the steel as it arrives from the supplier, before we work it. Maybe with some kind of acid dip to remove mill scale and oils? Thanks. L.S.
A. The pretreatment all depends on the level and nature of contamination on the steel as received, what additional contamination you add in manufacturing and the level of cleanliness required on the final product. Purchased steel can arrive in a wide variety of conditions, depending on your source and specifications. From your description of the condition of the material, you are likely receiving hot rolled, pickled and oiled steel. While that may be one of the least expensive ways to purchase it, the overall cost may be higher if you need to install dedicated pretreatment and associated wastewater treatment to remove mill scale.
I suggest leaving the mill scale until the end of the process prior to removal, unless 1) it is causing difficulty in your equipment, or 2) the final product is too large to effectively treat in an immersion or spray washer. In this case, you first need to remove both the mill lubricants and any process lubricants you may have added to the part in its fabrication. To accomplish this, you will likely need an alkaline cleaning or solvent cleaning step to remove that oil. The next step is the removal of mill scale and smut on the surface of the steel. This could be accomplished with an acid cleaning, possibly an inhibited sulfuric or phosphoric acid. I suggest speaking with companies in your region that produce process chemicals for such purposes and seek their recommendations.
Although more expensive, I suggest exploring the overall costs if you were to go with a cold rolled steel product. Assuming you do not allow it to rust in the manufacturing process, you could eliminate the need for the acid cleaning portion of the proposed process. If you are looking at setting up a new system, this will save you the cost of at least two stages (acid clean and rinse) along with the associated waste treatment capital and operating expense required for the acid waste generated. The cold rolled product can be specified with a number of different surface finishes, all of which are better than that of the hot rolled product. The total cost of ownership calculation will point you to the direction you should go.