Cadmium Substitution - garteur

GARTEUR LIMITED
D.3 Results and discussion
D.3.1
D.3.1.1
Accelerated corrosion tests
Daimler Benz Airbus Aerospace
The results of neutral salt tests are summarised in table D1. The blocks were inspected
on a weekly basis for evidence of rusting on the bolt heads. Data presented in table D1
indicate that the four substitute coatings examined were less prone to rusting under
these conditions than the electrodeposited cadmium control. The most promising coating
was the Delta-tone/Delta coat which showed no evidence of rusting during the exposure
period.
D.3.1.2
Fokker
Metallographic examination of the aluminium alloy blocks after testing showed that
corrosive attack had occurred in the countersink when cadmium plated bolts were used.
No evidence of corrosion was found in the countersinks when there was contact with
zinc-nickel and zinc-cobalt-iron plated bolts.
D.3.2
Natural exposure trials
During the 10 month exposure period at the Schiphol test site no evidence of corrosion
attack was observed.
D.3.3
Electrochemical measurements
In the first part of this study, the current flow between different coatings and two
aerospace aluminium alloys was monitored. In all cases, some galvanic current flow was
detected indicating that the coatings were interacting galvanically with the two alloys.
Curves showing the variation in galvanic corrosion current with time for each of the
coatings coupled to 2014-T6 and 7075-T6 aluminium alloys are reproduced in figures D4
and D5 respectively. In most cases, the current was found to flow from the coating to the
aluminium alloys. The exceptions were electrodeposited zinc-nickel alloy coating and
Delta-tone where current reversal effects were seen after coupling periods of over 100
hours. Tables D3 and D4 give the average current densities recorded for each of the
coatings. In general, higher galvanic current values were observed for the more active
zinc base coatings than the less active aluminium base coatings.
The coupling of a metal coating with an aluminium alloy can cause corrosion of the
aluminium alloy even if the coating is a sacrificial one. This occurs as a result of the build
up of alkaline conditions on the aluminium alloy surface, which unlike in the case of steel
which is stable at high pHs, can lead to damaging pit attack. In the second part of the
work, therefore, the effect of galvanic coupling on the corrosion rate of the aluminium
alloys was determined.
The galvanic corrosion rate of the aluminium alloy, R g was calculated from the following
expression,
R g = R a - R o --------------------- (D1)
where R o is the self corrosion rate and R a is the total corrosion rate of the aluminium
alloy.
The corrosion rates, R g, , of the two aluminium alloys were slightly increased by coupling
with cadmium plating as indicated in tables D3 and D4.
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GARTEUR SM/AG17 TP128