Cadmium Substitution - garteur

GARTEUR LIMITED
The barrier properties of the coatings were determined from electrochemical
measurements. These established that the zinc alloy coatings and zinc based metallicceramic
coatings were similar in performance to electroplated cadmium. The aluminium
based coatings all gave much lower corrosion currents implying that they were more
effective barrier coatings. The sacrificial properties of the coatings were assessed from
open circuit potential experiments and from the use of scratch model specimens and
protection distance measurements. The main conclusions were that pure aluminium
coatings and an aluminium based metallic-ceramic coating were less effective than
cadmium plating. Overall the zinc alloy coatings were more effective than the aluminium
coatings.
Two methods were employed to study the galvanic compatibility between coatings and
aerospace aluminium alloys. In the first coated bolts inserted into aluminium alloy blocks
were exposed to neutral salt fog and at an outdoor exposure site. The results obtained
indicated that the zinc based metallic-ceramic coatings were the most promising as no
rusting was detected. The second approach used was to measure the galvanic current
developed between coated panels and aluminium alloy. The results obtained indicate
that ED aluminium and ED zinc-nickel coatings lower the corrosion rate of the aluminium
alloy. Other coatings studied were found to accelerate the rate of corrosion above that
found for cadmium plating.
The effects of coating on fatigue performance were assessed using notched specimens
tested under constant amplitude tests. It was established that for the ED aluminium, the
two metallic-ceramic coatings and UBMS aluminium - magnesium coatings the reduction
in fatigue strength was less than 5%. ED zinc-cobalt-iron and cadmium gave similar
reductions of ~10% whilst the ED zinc-nickel coatings had the most detrimental effect
being ~25%.
Sustained load tests conducted on coated notch specimens, exposed to sodium chloride
solution, indicated that the zinc based metallic-ceramic and ED zinc-cobalt -iron coatings
may promote stress corrosion cracking in high strength steels. Additional tests carried
out including the slow bend test suggest that any susceptibility to hydrogen
embrittlement may be minimised by heat treatment after electroplating.
Most of the replacement coatings examined failed to show any significant degradation on
exposure to a range of chemicals used on aircraft. Exceptions were ED zinc-nickel
coatings in contact with Turco 5948 and ED zinc-cobalt coatings immersed in Skydrol
hydraulic fluid. Cadmium plating was also found to be degraded by these fluids.
Cross-cut tests show that good paint adhesion may be achieved with metal coatings. An
important factor is the time delay between passivation and the application of a primer.
Data obtained indicate that if the passivated surfaces are exposed to the atmosphere for
too long, poor paint adhesion may be obtained.
Tribological studies were conducted to allow the suitability of the different coatings for
use on fasteners to be established. The coefficient of friction of several of the coatings
has determined and the torque-tension characteristics of coated fasteners after repeated
tightening and untightening has been compared. It is concluded that only the zinc based
metallic-ceramic coatings have a coefficient of friction comparable to cadmium plating.
All the coatings examined resulted in pre-loads on Hi-Lok fasteners greater than the
minimum 4kN required. With the ED cadmium, ED aluminium, ED Zn-Co-Fe and ED Zn-
Ni coated steel fasteners the maximum preload of 10kN was exceeded.
The use of brush plating to repair several of the coatings was investigated. Simulated
corrosion damage and re-plating tests showed that brush plated zinc-cobalt and zinc-
GARTEUR SM/AG17 TP128
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