We have a zinc/phosphate immersion tank followed by hex-chromate seal. Control of this Zn Phosphate tank is maintained by analyzing the free acid/total acids. In the last two weeks, iron parts exposed to only 12 hours slack time prior to painting are developing rust. Assume that adjacent production lines have been the same consistent mix for the last three months and that the tank is only operated when the daily temps, F.A.(lowest allowable), and T.A. are in spec.
Q1) Should this tank be monitored for any other mitigating cations.-two suspects are Al or Cr.
Q2) Does the sealer need total dissolved solids spec.
