Cadmium Substitution - garteur

GARTEUR LIMITED
c) Exposure to 100% humidity
The exposure of panels plated with cadmium, zinc alloys and the two metallic ceramic
coatings Delta-tone and SermeTel to 100% humidity was investigated by Fokker. The
tests were terminated after 2000 hours and the results obtained are presented in table
C7. Rust spots were only found on the Delta-tone sample, all the remaining test panels
survived the 2000 hour exposure period.
C.3.1.2
Outdoor exposure
Results of the outdoor exposure trials on non-scribed panels are given in table C5. Trials
and Schiphol were terminated after 462 days. During that time only the electrodeposited
zinc-nickel coating showed evidence of red rust. Tests conducted at the DERA Fraser
marine exposure site were continued for two years. During that period evidence of
rusting was found on all the aluminium coatings and the non-passivated zinc-nickel
coating. The two metallic-ceramic coatings, Delta-tone and SermeTel, and the
electrodeposited zinc-cobalt-iron coatings showed no evidence of rusting.
C.3.1.3
Electrochemical measurements
The polarisation studies were used to estimate the corrosion current density for each of
the coatings in sodium chloride solution. The results obtained are presented in table C6.
As found in the accelerated tests, passivation treatments generally decreased the
corrosion rate of the coating. The aluminium based coatings had lower corrosion rates
than the zinc rich deposits suggesting they possessed better barrier properties.
C.3.2
Sacrificial properties
Accelerated corrosion tests and outdoor exposure trials on scribed panels were used to
compare the relative sacrificial properties of the coatings. The time to the appearance of
red rust in the scribe was used as a measure of the coatings ability to cathodically
protect the substrate at areas where the coating is damaged.
C.3.2.1
Accelerated corrosion tests
Results of the neutral salt fog, MASTMAASIS and 100% humidity tests are given in
tables C7, C8 and C9 respectively. The data refer to the occurrence of red rust within
thescratch region. In the case of the zinc rich coatings, the time to red rust was
comparable with cadmium plating. However the aluminium coatings generally showed
relatively poor performance, with red rust being detected in the salt spray test after
relatively short exposure periods. Variable results were obtained with the SermeTel
coatings. These were considerably thicker than the zinc alloy and aluminium alloy
coatings and may have had an influence on the results.
C.3.2.2
Outdoor exposure trials
Outdoor exposure data obtained from four test sites are summarised in table C10. The
results indicate that electrodeposited zinc-nickel coatings are less effective than
cadmium plating in protecting the scribed area. The zinc-cobalt-iron system appears very
similar to cadmium in performance in both marine and relatively rural environments The
aluminium based schemes including SermeTel 984 were the least efficient coatings and
in the most favourable case did not delay the onset of rusting for more than six months.
This suggests that in a marine environment, sacrificial coatings are more effective than
simple barrier coatings.
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GARTEUR SM/AG17 TP128