Zinc-phosphate rinsewater is clear but it's tank is green

Hi,

We have an immersion E-coat process with a Zinc Phosphate pre-treatment. The sides of the city water rinse after the zinc stage are green, even though the metal is stainless steel. What is causing this? and what can I do about it besides a complete boil out?

Thanks,
Ben, the curious E-coater

Probably algae. Maybe work in complete darkness (robotics?) or add algae-eating freshwater snails or fish.

Seriously, ask the E-coat supplier to recommend a compatible biocide. Perhaps a isothiazolinone type such as Acticide.

Beside boiling, some periodically shock treat with hypochlorite or scrub down with hydrogen peroxide solution (using a plastic bristle broom). Clean residue thoroughly before refilling. Try to purge all contaminated tanks at the same time.