Cadmium Substitution - garteur

GARTEUR LIMITED
cadmium although it can be seen that as the nickel content was increased so the time to
red rust was extended.
J.3.2
Neutral salt fog tests on repaired panels
The re-plated panels as described by the matrix in table J1 were subjected to neutral salt
fog for a period of 336 hours and then inspected for signs of corrosion. It was found that
the Garteur cadmium coatings were satisfactorily re-plated with cadmium, zinc-nickel or
zinc-cobalt alloy. Adhesion tests carried out on re-plated coatings showed no signs of
failure. On exposure to neutral salt fog, the repair coatings of zinc-nickel and zinc-cobalt
failed within 336 hours, as anticipated from the data shown in table J2, where in each
case red-rust occurred after 110 and 310 hours respectively. However no red-rust was
observed on the Garteur coatings.
When re-plating a metallic layer with a coating of a dissimilar metal there is always the
risk that galvanic corrosion may occur at the interface between the two coatings. In the
present study, no evidence of galvanic corrosion was observed where the coatings
overlapped for the cadmium - cadmium or cadmium - zinc-nickel combinations, indicating
that they were fully compatible. In contrast, when the zinc-cobalt alloy was employed in
re-plating, the cadmium coating was badly pitted around the interface, suggesting that
galvanic corrosion had occurred leading to attack on the cadmium plating. For cadmium,
which had been repaired with zinc-cobalt, it was found that after 380 hours, red rust
started to bleed from the interface showing that the patch had broken down. Table J3
summarises the occurrence of galvanic corrosion between the original coatings and the
repair coatings.
The repair of PVD aluminium coatings presented particular difficulties due to adhesion
problems and the zincate treatment was applied in each case. For cadmium, it was
found that the coatings could not be brush plated onto aluminium despite the use of the
zincate treatment. However it was found that the zincate treatment could be successfully
employed in the brush plating of alkaline zinc-nickel coatings onto PVD aluminium. In the
absence of the zincate treatment adhesion failures occurred. In contrast, for the acidic
zinc-nickel and zinc-cobalt electrolytes no adhesion problems were observed even in the
absence of the zincate treatment. In the neutral salt fog tests the zinc-nickel and zinccobalt
coatings were found to afford good protection with no red rust on the re-plated
areas. In the absence of the zincate treatment, the more electrochemically active zinccobalt
coating caused some sever galvanic corrosion on the aluminium coating,
however, when the zincate treatment was applied this effect was considerably reduced.
J.4 Conclusions
1. In neutral salt fog trials, the zinc-cobalt alloy was found to provide the highest levels
of corrosion protection to a mild steel substrate giving greater protection than either
cadmium or zinc-nickel alloy plating.
2. Simulated corrosion damage and re-plating tests showed that brush plated zinccobalt
and zinc-nickel coatings could be used to re-protect a range of Garteur
coatings including zinc-nickel, zinc-cobalt-iron, PVD aluminium and cadmium.
3. Some evidence of galvanic interactions were observed when using the brush plated
zinc-cobalt alloy coating for repair. These effects were minimised when a zincate
chemical treatment was used.
J.5 Tables
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GARTEUR SM/AG17 TP128