Conversion coat and powder coat aluminum sand casting

Hooray for China. Quite correct - air is bad especially given that in US OSHA wants the hexavalent PEL (exposure limit) dropped from 52 ug/m3 to 1! At that level, we won't be able to take the lid off the tank much less stir it with air or mechanical! That's where all this talk about trivalent chrome processes (i.e. TCP) is coming from. Pushed by OSHA in the US and RoHS, WEEE, and ELV (the 3-headed ugly monster from Europe), don't expect much hexavalent chrome to be used in another decade. Where will it ever end?

A. Thank you Jia and Milt. I appreciate my new knowledge that mechanical agitation is better! But experience has taught me that agitation is necessary, and that air agitation does work. But yes it would be interesting to learn if air agitation in chromating raises chrome evolution/exposure; I don't know.

Regards,

A. Part of the difference in color is differences in the parent metal. Surface finish will play into this also, with an over etched part reflecting less light so it will look a bit darker. More probable is a difference in the tank chemistry or temperature or time in the tank which are controllable variables but are not carefully maintained by some finishing shops. The chemical used, makes a difference with the brand Iridite being slightly redder in color than the brand Alodine. Both are good products despite some color differences.
A white spot is very likely a spot that did not chem film properly. This is a bit more common than you would think with cast and extruded aluminum as the cheaper products have pits that will retain the prep chemicals and then bleed out during the drying. Small spots, about the size of a pencil eraser are nearly always caused by the parent metal. Larger spots are nearly always caused by the line operator.

A. Yellow chromate has many variables, all of which may affect the colour of coating you obtain. Probably the most common is differences in immersion time. The longer a part is immersed, the darker the colour. Concentration of the chromate salts will also affect your colour, as will pH and temperature. What you need to do is standardise the process, fixed concentrations, temperatures, times of immersion etc. This should take out some of the colour variability, but you will never cure all colour variability.

The white spots are probably due to leaching of the pre-treatments from pores in the casting. You may want to consider impregnating the casting if the problem is particularly prevalent. Very good rinsing, warm and cold sequentially until there is no leaching from the casting after treatment may well help as well.

You may wish to talk to your chromate salt supplier for advice about process variabilities, but I doubt that they will be able to provide you with standards. You could always make your own standards, with what is considered acceptable colour and what is considered unacceptable.

A. You anodize only aluminum. You do not anodize the silicon and copper that is on the surface. Anodic treated silicon is gray in color. Anodic treated magnesium in an aluminum anodizing solution is "white". Anodic treated copper is black. You must skip the alkaline etch which decreases the area of aluminum and increases the exposed area of silicon, then you cannot use a wrought metal deoxidizer, you must use a deoxidizer designed for removing silicon and limiting the attack on aluminum. Such a formula is universally understood. On most die cast (and some is impossible) you use 50% NItric Acid and one pound per gallon ammonium bifluoride.

Hi Max!

Ideally, there should be almost no wait between blast and conversion coating. The faster you can get it on there, the better the reaction will be, and the better coverage you will have, unbothered by oxidation.

After that, the parts will be protected from degradation. In fact, you are best to let them rest a couple days to 'cure' before applying the next coating to them. They can sit quite a while in this state!

The time after sand blasting is not important. What is important is the surface preparation for chromating castings, then you get in a hurry, like immediately. Also important is whether the "powder primer" is compatible with the chromate, ask your powder primer supplier. Some "primers" destroy the chromate, so the chromate should not be applied ahead of certain primers, ask the supplier.