Cadmium Substitution - garteur

GARTEUR LIMITED
compared with cadmium plating. Specimens with a configuration shown in figure F1 were
stress corrosion tested to ASTM G44-75. In this test, the specimens were subjected to
alternate immersion in 3.5% sodium chloride solution. One cycle in this standard
includes 10 minutes in salt solution and fifty minutes drying in air. The total exposure
time was 30 days.
The notched tensile strength was determined on three specimens. Values of 2700, 2705
and 2690 M Pa were achieved. For one set of stress corrosion specimens the notch was
covered to 180 o by an aluminium casing. The reason for using this casing is to initiate
galvanic corrosion and crevice corrosion. The specimens were then loaded to and held
at 75% of the notched tensile strength. Each treatment was represented by three
individual specimens The treatments tested were:-
F.3 Results and discussion
• Reference without protective coating
• Porous cadmium plating
• Zinc-nickel plating
• Zinc-cobalt-iron plating
• Sermetel
• Electrodeposited aluminium
• Delta-tone
F.3.1
Susceptibility to hydrogen embrittlement
The results of slow bend tests conducted on a 35 NCD 16 steel (1800 MPa) are
presented in figure F1. The results indicate that for the SermeTel coatings, there is
minimal embrittlement due to plating. In the case of the electrodeposited aluminium
some reduction is observed but this may be recovered by heat treating at 190 o C.
One of the three electroplated zinc-nickel notched tensile specimens tested, failed within
the 200 hours test period. In the second series of specimens tested, no failures occurred
and it is considered that the electrodeposited zinc-nickel coating is non-embrittling.
F.3.2
Susceptibility to stress corrosion cracking
In the sustained load tests, two specimens of three of those protected with zinc-cobaltiron
and those protected with Delta-tone coating in the test set using the aluminium
casings, failed within two days. Sacrificial coatings may add to the risk of post plating
embrittlement. SAAB uses ZnCoFe plating to 1250 M Pa, a strength level considered to
be safe. The specimens used in this investigation were hardened to 1400 M Pa.
In the second test set, where no casing was used , two specimens of three with Deltatone
coating failed within two days. All other specimens remained unbroken after 30
days.
Previous research undertaken by SAAB [F2] compared the performance of IVD
aluminium and electrodeposited cadmum coatings applied to notched tensile specimens
machined from either a 1366 Ni-Cr-Mo steel or a 1745 precipitation hardening stainless
steel. As in the current research programme, the specimens were loaded to 75% of the
notched tensile strength. From the tests undertaken, there was no indication that
aluminium-coated specimens have a higher tendency to delayed fracture in a corrosive
environment than cadmium coated ones.
F.4 Conclusions
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GARTEUR SM/AG17 TP128