Cadmium Substitution - garteur

GARTEUR LIMITED
3.4 Effect of coatings on fatigue performance
Surface treatments such as pickling, anodising and plating may have a detrimental effect
on the fatigue life of aerospace components. Constant amplitude fatigue tests have
been made to determine the effects of the different coatings on the fatigue life of
specimens machined from AISI 4340 steel tempered to give a tensile strength of 1400
MPa. Both smooth specimens and notched specimens were used to give K t (stress
concentration) values of 1.0, 1.4, 2.5 and 4.0. Tests were conducted at a frequency of
185Hz and a stress ratio equal to 0.1. Details of the fatigue programme are given at
Annex E.
3.5 Stress corrosion cracking and hydrogen embrittlement studies
High strength steels are susceptible to hydrogen embrittlement. Electroplating
processes such as cadmium plating are less than 100% efficient and some hydrogen will
be evolved during deposition, which may diffuse into the steel substrate. To reduce the
risk of hydrogen embrittlement a post plating heat treatment is carried out on all steels
with a strength greater than 1400 MPa. In the case of a steel part manufactured from a
steel with a strength of 1850 MPa and electroplated with cadmium this would involve
baking at 200 o C for a minimum of 18 hours. In the current programme, tests are being
conducted on coated steel specimens to identify potential hydrogen embrittlement
problems. After coating the samples are de-embrittled in accordance with the plating
specification or the coating suppliers instructions.
Two aspects of hydrogen embrittlement have been investigated. In the case of coatings
prepared by electrodeposition, there is concern that the process itself may generate
sufficient hydrogen to cause cracking under tensile loading. To investigate this sustained
load testing and slow bend tests were carried out on coated samples. A second
consideration is the possible introduction of hydrogen into the steel substrate as a result
of corrosion occurring on the coating. This has been studied using notched tensile
specimens. These are loaded to 75% of the notched tensile strength after coating and
are then subjected to alternate immersion in 3.5% sodium chloride solution until failure
occurs. The performance of each of the coatings is being compared with cadmium
plating. Annex F describes the two test procedures employed and the results obtained.
3.6 Resistance to aircraft chemicals
Hydraulic fluids, aviation fuel, paint strippers and many of the other chemicals and liquids
used on aircraft and in the maintenance of aircraft are potential corrosion hazards to the
airframe structure. Immersion tests were carried out to establish the degree to which
coatings degrade when exposed to some of the more commonly used aircraft chemicals.
The range of chemicals includes
• ethylene glycol
• aviation fuel
• butyl phosphate type hydraulic fluid
• alkaline based general purpose cleaner
Two test procedures have been employed both based on the immersion of coated
panels in different aircraft fluids. The detailed test procedures are given in Annex G
together with the test results.
3.7 Paint adhesion
GARTEUR SM/AG17 TP128 Page 5