Cadmium Substitution - garteur

GARTEUR LIMITED
The results of the bolt/block trials indicate that the Delta-tone coating is the most
promising since no rusting was detected. Rusting was observed on the other four
coatings examined when no passivation treatment was applied.
4.4 Effect of coatings on fatigue performance
The results of the constant amplitude fatigue tests conducted by SAAB, British
Aerospace and NLR are given in detail in Annex E.
The general effects of the coatings on fatigue strength are summarised in table 11. This
indicates that the greatest reduction in fatigue strength occurred with the
electrodeposited zinc-nickel coatings. Coatings such as SermeTel and Delta-tone were
found to be less damaging than electrodeposited cadmium.
4.5 Stress corrosion cracking and hydrogen embrittlement studies
Results of the stress corrosion cracking tests undertaken by SAAB are given in Annex F.
Failures were obtained for specimens protected with ED zinc-cobalt-iron and Delta-tone.
No evidence of stress corrosion cracking was obtained with the remaining coatings.
4.6 Resistance to aircraft chemicals
Tests to assess the resistance of the coatings to aircraft chemicals such as hydraulic
fluids, aviation fuels and aircraft cleaners were conducted by British Aerospace and
Shorts. The main findings of these tests are summarised in table 12. The results show
that several of the coatings including electrodeposited cadmium were susceptible to
corrosion attack by the aircraft cleaner Turco 5948. Skydrol also had a detrimental effect
on the electrodeposited zinc-cobalt-iron and cadmium coatings.
4.7 Paint adhesion
The results of the paint adhesion tests are presented in Annex H. The general
conclusion was that good paint adhesion can be achieved with metal coatings. An
important factor is the delay time between passivation and the application of a primer.
Data obtained by NLR indicate that if the passivated surfaces are exposed to the
atmosphere for too long, poor paint adhesion may be obtained.
4.8 Tribological properties
Studies of the tribological properties of the coatings concentrated on two aspects
• torque - tension measurements on coated nuts and fasteners
• coefficient of friction
Detailed results are presented in Annex I.
In the first series of torque-tension measurements the maximum preload, which could be
obtained was recorded together with the locking torque applied. Two sets of tests were
conducted the first using coated steel pins with coated steel collars and the second
involving coated titanium pins with coated steel collars. In each case it was possible to
obtain a preload in excess of 4kN which falls within the normal requirement of a preload
in the range 4 to 9kN.
In the second series of torque-tension measurements, tension loads developed for
coated bolts installed with lubricant were determined. Values were obtained for locking
GARTEUR SM/AG17 TP128 Page 13