Cadmium Substitution - garteur

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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 Page 86 GARTEUR SM/AG17 TP128
GARTEUR LIMITED 1. The Delta-tone and electrodeposited zinc-cobalt-iron coatings were found to promote susceptibility to stress corrosion cracking when exposed to 3.5% sodium chloride solution 2. Additional testing, including the slow bend test, indicates that susceptibility to hydrogen embrittlement may be minimised by post plating heat treatments. F.5 References [F1] Hultgren E., CSM Materialteknik Technical report T60676MM.991 “Stress corrosion testing of AISI 4340 coated with cadmium, zinc alloys and aluminium.” (1996) [F2] Jaensson B., SAAB-SCANIA Investigation Report KFB-033 “Replacement of cadmium as a corrosion" [F3] (1996) Marchandise D, Hydrogen embrittlement data - Aerospatiale report [F4] PrEN 2831 Aerospace Series Hydrogen embrittlement of steels. Test by slow bending (1990) GARTEUR SM/AG17 TP128 Page 87
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GARTEUR LIMITED G.1 Introduction 89
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Cadmium Substitution - garteur

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