4 Quick Chromate Tips for the Winter

As we head into the fall and winter months, no doubt the service phone will start ringing about chromate rub off on zinc plated parts due to ambient going from temperatures in the 80s and 90s to temperatures in the 60s (and in some plants saving on heating bills, chromates may be trying to work as low as temperatures in the 50s).  But as we see a rise in the trivalent chemistries and less of the “ambient” running hexavalent chromates, the simple fix of raising the temperature may not be the solution to the rub-off headache during these cold days of winter.  Here are a few other quick tips to help with your chromate issues:

  1. Temperature:  As the rinse water temperature is colder during the winter months, particularly in northern climates, the rinsability of many of the processes, particularly alkaline zinc is diminished. Therefore, longer rinse times increased flow rates, and if possible, warming of the rinse waters is highly recommended. Although not as big an issue from the days of ambient running chromates, as mentioned above, make sure the temperature of the chromate is within the manufacturer’s recommended ranges.  Within Asterion’s own chromate product line, operating temperatures range from “ambient” to 110 degrees F.  Some manufacturers consider ambient 70-75 degrees F, not the surrounding air temperatures in a plant in the middle of a Minnesota winter, so be sure you understand their definition of ambient.  Although we’ve seen instances where chromates work well enough in the winter months around 60 degrees, a more consistent chromate film can be achieved if the chill is removed from the bath via an over the side electric heater.  Temperature is still a critical parameter of the chromate film reaction and shouldn’t be overlooked.
  2. pH and product concentration:  Typically pH is controlled by the addition of product; however over time, too much product can be added to maintain the pH.  When the pH is checked, the range may call for the pH to be lowered, however, the concentration of the product is already out of range or near the top control parameter.  In this case, consult your vendor on the specific acid to use to lower the pH without increasing the product concentration.  Be careful, using the wrong acid could significantly affect the reaction and color of the chromate film.
  3. Zinc thickness:  Sometimes it isn’t the chromate at all.  The chromate may have a few parameters nearing the upper or lower control limits, and the first thought is to start adjusting the chromate in order to achieve the desired finish.  Before reacting,  assess the entire process.  Many times it could simply be that a change the part configuration (increase in surface area) lead to a thinner Zn deposit in the low current density areas.  The chromate film needs a particular thickness of Zn deposit to properly form.  In the LCD areas of the work it may look as if the chromate film is not working, it isn’t that it is not working, it is a matter of there isn’t enough Zn present for it to work on.  The Zn bath may simply need an increase in amperage or in some cases, an increase in amperage and adjustment of chemistry to help with distribution and burning.
  4. Streaks and patterns not detected until chromate is dry:  Many times streaks, patterns, and stains are seen suddenly after the chromate is dried.  Check for clean rinsing, Zn brigthener levels, nitric sour dip concentrations and part cleanliness prior to plating.  All of these things could be sources of patterns magically appearing after the chromate is dry.  The chromate is just highlighting the issue, not causing it.