Adding a Pickel Stage

Q. We are looking at a new e-coat system and are considering adding a pickle stage to the pretreatment to remove rust and laser scale before coating. Is this a good idea or what do we need to consider? J.C.

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A. The addition of a pickle stage to an e-coat system must take into consideration what is going to be removed by the pickle and what type of system is being designed. If an acid pickle were to be added to a standard e-coat pretreatment system, it would usually be a phosphoric acid pickle. Phosphoric acid is used to reduce chemical problems with an iron or zinc phosphate pretreatment prior to e-coat. Phosphoric acid is not the most aggressive acid for pickling but is the only acid that is compatible with phosphate pretreatments. Hydrochloric (muriatic) or sulfuric acids are more aggressive than phosphoric, but are more likely to result in contamination.

Besides the type of acid to be used, pickling time, acid concentration and temperature are also variables that need to be considered depending on the material to be removed. The four most common reasons to pickle parts prior to e-coat are: 1) “flash rust” on parts that get delayed between fabrication and coating, 2) hot-rolled scale on the base substrate, 3) laser scale from laser fabrication [Also addressed in the September 2006 Clinic] and 4) slag from wire welding. (Pickling to remove weld slag was addressed in the previous question; see above)

Unless a large percentage of your parts require pickling for one of the above reasons, I would recommend having the pickling (followed by application of water-soluble oil as described above) done off-line or outsourced prior to e-coat pretreatment.

If most of your parts do not require pickling, the other parts might actually rust if exposed to the fumes from the pickle stage. Since the amount and degree of rust or scale identified as “flash rust” or hot-rolled scale probably varies significantly, I would also recommend having pickling of those parts done off-line or outsourced before e-coat pretreatment.

Based on these recommendations, the only in-line pickling I would recommend is for coaters handling a large percentage of parts with laser scale and/or weld slag that must be removed prior to e-coat. These two conditions can be handled with phosphoric acid with a relatively short pickling time.

The pickle process usually consists of 2–3 stages (pickle plus 1–2 rinses) between the cleaning stages and the phosphate stages. This will add considerable length (floor space) to a monorail system. A square transfer system would require less floor space, but fume containment becomes more of a challenge.

The best type of e-coat system for processing a variable amount of parts requiring pickling with a variable amount of rust, scale or slag is a programmable hoist system. This type of system allows for varying “soak times,” better fume containment and the possible use of more aggressive acids at higher concentrations and temperatures. The biggest disadvantage of a programmable hoist system is its limited throughput.