How to decide what kind of sealing has been used

An earlier post explained about a couple of sealing methods. This post will give an idea of how to figure out what kind of sealing your product has been sealed with.

If chromate and dichromate sealed there will be a high concentration of chromate ions in the mouth of the pore.

Boiling water makes the anodic coating crystalline, addition of nickel salt (nickel acetate) used as a fix to prevent bleeding out of dyes. These are hydrolysable metal salts, absorbed into the coating where they are hydrolyzed and precipitated as hydroxides, plus if dyed a chemical reaction between nickel and the dye molecules will happen to form new metal complex.

Conversion of Al2O3 to boehmite only occurs at temperature of 80°C and above, pH lower than 4.5 leads to no conversion of aluminum oxide to boehmite. The formation of the crystalline boehmite depends upon coating thickness, pore diameter and requirements of sealing test specification. The three important parameters when hot water sealing are:

1. Water temperature
2. Sealing time
3. pH of water

The chemical composition of Al2O3 is a complex amorphous oxide with approx. 15% sulfur incorporated.

Al2O3 72%
H2O 15%
SO3 13%

When Hot Nickel Acetate (NA) sealing is used the chemical composition of this coating will be almost identical but with a small amount of nickel incorporated (around 1 – 2%).

The Cold Sealed Nickel Fluoride (NF) is a totally different process, an impregnation process (read more)

The chemical composition of the coating will be as follows:

AlOOH
Ni(OH)3
AlF6

Nickel uptake in the anodic coating is a function of pH and the presence of fluoride aids the precipitation of nickel salts in the film.

So to find out if the sealing is a NA or NF sealing you have to do some EDX measurements of the surface. You will find a small amount of Nickel in the oxide film if the parts are hot nickel acetate sealed and a higher amount of nickel, plus fluoride in the cold sealed parts.

Most alloys have no influence on the EDX results but if you have an alloy with high silicon content you will find silicon too in the coating.

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