Cadmium Substitution - garteur

GARTEUR LIMITED
E.3 Results and discussion
S-N curves were obtained for each of the coatings evaluated. Figure E2 gives examples
of curves obtained for specimens electroplated with zinc-cobalt-iron and zinc-nickel. In
these tests the specimens had a stress concentration factor (K t ) of 1.4.
All the S-N curves obtained are given in reference E1 together with the individual fatigue
specimen results.
Data obtained from the S-N curves were used to calculate the relative effects of the
different coatings on the fatigue strength of the steel. These are summarised in table E1.
The fatigue strength for each of the coatings is compared with the fatigue strength for
specimens, which have been abrasive blasted with alumina grit.
The results obtained show that the coating causing the greatest reduction in fatigue
strength is ED zinc - nickel. ED zinc-cobalt-iron was similar to ED cadmium giving a
reduction of ~10% whilst the other coatings investigated resulted in a lowering in fatigue
strength of about 5%.
Several factors will influence the initiation of fatigue cracks on coated specimens. These
include,
• coating thickness
• surface roughness due to grit blasting
• peening effect due to grit blasting
• strength of the base material
• hydrogen embrittlement
A detailed study has been made at NLR [E1] in order to identify the impact of these
factors on the fatigue life. Metallographic sectioning has shown that there are
considerable variations in the thickness of the coating within the notched region for each
of the coatings. There were also found to be large differences in the surface roughness
as a result of the pre-coating abrasive blasting treatment. This arises from the fact that
the abrasive blasting was carried out by each of the organisations responsible for the
different coating processes. Although surface roughness will influence the fatigue crack
initiation, NLR's study suggests that the embedded aluminium oxide particles may act as
preferred crack initiation sites.
E.4 Conclusions
1. Based on constant amplitude tests the following conclusions can be drawn
concerning the effects of coatings on the fatigue strength on steel.
2. By the aluminium oxide blasting treatment before the coating process the surface is
roughened but also aluminium oxide particles are trapped in the surface metal.
3. Entrapped oxide particles are potential fatigue crack initiation sites.
4. The ED Zinc - nickel coating resulted in the largest reduction in fatigue strength,
25%. This was due to the hard brittle coating and the relatively large coating
thickness.
5. ED zinc - cobalt - iron gave no larger reduction in fatigue strength than cadmium,
~10%. The negative effect of cadmium on fatigue strength is unexplained.
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GARTEUR SM/AG17 TP128