Structural Steel Pretreatment

Question:

I am an industrial consultant. One of my new clients wants to fabricate small utility trailers. He has inquired about a process for cleaning the structural steel chassis and frame. He wants to know whether acids washing (pickling) should be done before assembly or after. What other technology is available for surface cleaning of such chassis and frames prior to painting? C. T.

Answer:

The choice of pretreating chassis and frame structures before or after assembly depends on the availability of the equipment and the size of the units. If the acid tanks are small and the assemblies large, the parts must be pickled before assembly.

It should be pointed out that pickling is required only when there is rust present on the surface of the structures. As to other available technology, the best pretreatment, with or without pickling, is a phosphate coating. Zinc phosphate or iron phosphate can be applied by immersion or by power spray machines. The zinc phosphate will require five to six stages while the iron phosphate will require three to five stages. Iron phosphate can also be applied by using a hand-held hot water or steam spray applicator. If it is not possible to apply phosphate coatings, the steel structures can be pretreated by abrasive blasting. Newer technology involves dried in place pretreatments. In all the aforementioned pretreatments, the surface must be clean, free from oily soils and corrosion products. Abrasive blasting is another pretreatment used when it is impractical to use chemicals. Instead various types of media are propelled centrifugally, by compressed air or low-pressure water. Typical blast media include sand, steel shot, glass beads and nut shells. High pressure is also used for blasting.