Page 167 - Corrosion Engineering Principles and Practice

142 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 143

plugged and the coating becomes resistant to further staining and
corrosion. Nickel containing seals will, in most cases, prevent leaching
of dyes during the sealing operation.
Due to the higher energy costs inherent in hydrothermal sealing,
chemical manufacturers have developed mid-temperature seals (70
to 90°C). These seals, which contain metal salts such as nickel,
magnesium, lithium, and others, have become very popular due to
the lower energy costs and their ease of operation. One disadvantage
of the lower temperature is the tendency of organically dyed parts to
leach during sealing. This can be compensated for by a slight increase
in the bath concentration and by operating the solution at the upper
temperature limits (90°C).
Nickel-free environment-friendly seals are fast becoming the seal
of choice where clear or electrolytically colored parts are concerned.
Because there is nothing to leach, these mid-temperature seals
accomplish hydration of the oxide without the use of the heavy metal
ions. When the seals become contaminated or are no longer effective,
they can be discharged to the sewer without subsequent treatment
(except possible pH adjustment). This offers the finisher a safer
alternative to the effluent treating necessary with heavy metal
containing seals.
A significant modification in the sealing of anodized aluminum
was also developed for room temperature sealing (20 to 35°C). Unlike
the high temperature and mid-temperature seals, which depend on
hydration for sealing, the cold seals rely on a chemical reaction
between the aluminum oxide and the nickel fluoride contained in the
seal solution. Unfortunately, this reaction is slow at ambient
temperatures and the sealing process can require up to 24 hours.
However, it has been found that a warm-water rinse (70°C) after the
cold seal immersion will accelerate the sealing process, allowing for
handling and packing of the sealed parts. The sealing of organically
dyed parts in cold seals has been found to be advantageous. Light
stability testing (fade resistance) has shown that parts sealed in cold
seals gain additional lightfastness.

5.6.6 Chloride Extraction
Electrochemical chloride extraction is a method particularly
appreciated by civil engineers who want to remediate existing
concrete structures contaminated with chlorides from deicing salts or
marine mist. The hardware involved is similar to that involved in
cathodic protection. Electrochemical extraction of chloride ions is
achieved by positioning an anode in contact with a caustic electrolyte
spread on the external concrete surface, and impressing a direct
current between the anode and the reinforcing steel acting as the
cathode (Fig. 5.45). Under the application of this electrical field,
chloride ions migrate away from the negatively charged steel and
toward the positively charged external anode.