Cadmium Substitution - garteur

GARTEUR LIMITED
ANNEX J
Coating repair
J.1 Introduction
The use of brush plating for the repair of electrodeposited zinc-nickel, zinc-cobalt-iron
and PVD aluminium coatings was investigated. The approach adopted was as follows:
J.2 Experimental
• Establish the corrosion resistance of brush plated coatings in comparison with
bath applied coatings
• Examine the corrosion resistance of coated panels which had been damaged
and then repaired by brush plating
J.2.1
Brush plating of mild steel panels
A series of trials were conducted initially using commercially available cadmium and
alkaline zinc-nickel brush plating electrolytes and experimental acid zinc-cobalt and acid
zinc-nickel electrolytes. Mild steel 1mm thick 50mm x 50mm panels were brush plated to
produce coatings of various thicknesses in the range 5 to 25μm. Tape pull-off tests were
performed on several of the coated samples to check coating adhesion. Neutral salt fog
trials were also conducted and the time to red rust recorded.
J.2.2
Brush plating of damaged panels
J.3 Results
Coated panels prepared for the Garteur programme were first damaged by removing a
portion of the deposited layer in the centre of the test panel. They were then re-plated in
the damaged region itself and around the edge of the original coating with either zincnickel,
zinc-cobalt alloy or cadmium coatings (see figure J1) according to the matrix in
table J1.
The adhesion of electrodeposited coatings to aluminium substrates is frequently poor
because of the presence of the surface oxide film. Several samples of the panels coated
with PVD aluminium were treated prior to re-plating with cotton swabs containing zincate
solution. The zincate process employed was a relatively simple, cheap and reliable
technique that has often found use when there is a requirement to plate onto aluminium.
The corrosion resistance of the re-plated panels was established using neutral salt spray
tests.
J.3.1
Brush plated mild steel panels
Each of the electrolytes evaluated was found to provide suitable coatings in the required
thickness ranges. The zinc-nickel and cadmium coatings were found to be semi-bright at
best, whereas the zinc-cobalt coatings were bright in appearance. In the tape pull-off
tests no signs of adhesion failure were observed.
The results of the neutral salt fog tests made on coatings nominally 7-8μm thick are
summarised in table J2.
The data indicate that the brush plated zinc-cobalt coating provides the highest level of
corrosion protection. The two zinc-nickel coatings examined were less effective than
GARTEUR SM/AG17 TP128 Page 105