Cold sealing (Topic: 14125)

Cold sealing is a process to obtain sealing of the anodic oxide coating at a much lower temperature than conventional hot water sealing. The increasing cost of energy has made many companies seek ways of reducing the cost of the sealing operation. Cold seal systems are often based on solutions which contain fluoride or silica compounds in the presence of nickel salts, and often in a water-alcohol mixture. The water-alcohol solvent apparently lowers the solubility of the salts and facilitates precipitation within the pores of the anodic film.

Conventional hydrothermal sealing is not involved in these processes and pore plugging by nickel salts, fluorides or silicates is more the mechanism of cold sealing. For this reason there have been some objections to the term "cold sealing", and "impregnation" has been suggested instead.

A typical cold sealing solution may consist of a nickel compound, a fluorine compound, and of 2ñbutanol. In some formulations part of the nickel is replaced by cobalt, which masks the green coloration which can develop if the nickel concentration is high. The optimum sealing time appears to be 1 minute/micron at a temperature between 24 and 32ƒC is recommended. Short sealing times are ineffective and can give rise to a white powder developing later on the surface. The pH is between 5,5 and 6,5. Work sealed in this way has been shown to have good corrosion resistance and good seal quality, and is by many considered equivalent in performance to that produced by conventional sealing. The cold sealing system is becoming widely used in many countries, and has recently been approved by Euras (Qualanod) for outdoor use. But there is still concern, particularly in external architectural applications, about long term properties. To overcome this sealing can be carried out first in the cold seal, and then giving a short treatment in a conventional seal.

As the sealing mechanism does not depend on normal boehmite formation, there is also controversy over whether the normal corrosion and seal quality tests can be applied to work sealed in this way. Certainly the mechanism is very different, as work tested soon after sealing will usually give very high weight loss values, and it is only after 24 hours or more that they fall within normally accepted limits. This creates problems in terms of on-line production evaluation of seal quality. To overcome this sealing can be carried out as mention above.