Pretreatments

Q. What is the chemical used to seal after a three-stage cleaning process and higher? Thanks. A.P.

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A. The three- or higher-stage cleaning process you are referring to is usually an iron or zinc phosphate pretreatment process. As mentioned in the previous question, the first stage of a three-stage process is usually a clean and phosphatize in one step. In a five-stage, the first stage is cleaning, then rinsing, followed by phosphate, rinse and seal.

The chemistry of the final stage seal isn’t necessarily common among all pretreatment chemical suppliers, although the function and purpose is the same. The phosphate coating will form on the surface as a crystalline or partially crystalline, partially amorphous structure through the chemical conversion process. The goal in that stage is to make the crystal structure as small as possible (microcrystalline) to effectively react with and “cover” as much of the surface as is possible. The seal is intended to both passivate any microscopic metal surfaces not effectively covered by the phosphate, and to minimize the electrochemical potential between the two that would be the driving force to initiate corrosion.

or many years, the primary chemical used in the final stage was fairly simple. A chromic acid rinse would provide hexavalent chromium to the surface, passivating the coating and “sealing” the surface. However, due to environmental laws and worker exposure concerns, there are now several alternatives to hexavalent chromium.

Alternatives that I am aware of initially included a trivalent chromium passivation, since it did not have as significant a problem with disposal and was not considered carcinogenic. However, newer chemistries that provide the same functionality have surfaced. Quite a few are based on inorganic salts of titanium, zirconium and/or hafnium (all in column IVB in the periodic table). Even newer seals involve the use of a water-soluble, low-viscosity organic polymer. I am less familiar with those because they are offered by fewer companies.

All of the above chemistries are usually delivered in a phosphoric acid containing aqueous solution. The phosphoric acid is relatively mild and is already contained in and compatible with the existing phosphate coating.(A stronger inorganic acid could start to strip the coating if present at a high enough concentration). It maintains the pH of the surface at a desirable level for subsequent painting. Most paints are compatible with a slightly acidic surface but can break down if exposed to an alkaline surface.