In one of your previous columns regarding duplex and dichromate sealing, you stated that small amounts of phosphates and sulfates could affect the quality of the seal. Can you tell me what these levels of contaminants are? D.H.
Dichromate seals are used where enhanced corrosion resistance is required and where the imparted yellowish color to the coating is not important. Military and industrial applications requiring exceptional corrosion resistance come to mind. Furthermore, these types of seals are often one part of what is referred to as a “duplex” sealing process. Dichromate seals are often used in conjunction with either nickel acetate (NiAc) or near boiling deionized water sealing processes. The order of sealing may be dichromate followed by water or NiAc or the dichromate may come after either of those processes. Potassium dichromate is usually the preferred chemistry to use for this type of sealing.
It is generally agreed that the following minimum concentrations of contaminants can be detrimental to the sealing process whether it is dichromate, NiAc or hot water:
- Fluoride 30 ppm
- Silicates 10 ppm
- Phosphates 5 ppm
Additionally, detectable levels of chloride may reduce the corrosion resistance of the seal and caution should be used when the level of sulfates is greater than 100 ppm.
Another interesting fact about dichromate sealing is that it can largely overcome the loss of fatigue properties caused by the anodic coating, especially hardcoat.
I would like to add that in that previous article on duplex sealing (May, 2002) there was a mistake in printing. Referring to the concentrations of nickel acetate sealing baths in that article, they should read 1 gram/liter (1g/l), not 1 gal/liter, and 3-5 grams/liter, not gal/liter.