Cadmium Substitution - garteur

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GARTEUR LIMITED D.3 Results and discussion D.3.1 D.3.1.1 Accelerated corrosion tests Daimler Benz Airbus Aerospace The results of neutral salt tests are summarised in table D1. The blocks were inspected on a weekly basis for evidence of rusting on the bolt heads. Data presented in table D1 indicate that the four substitute coatings examined were less prone to rusting under these conditions than the electrodeposited cadmium control. The most promising coating was the Delta-tone/Delta coat which showed no evidence of rusting during the exposure period. D.3.1.2 Fokker Metallographic examination of the aluminium alloy blocks after testing showed that corrosive attack had occurred in the countersink when cadmium plated bolts were used. No evidence of corrosion was found in the countersinks when there was contact with zinc-nickel and zinc-cobalt-iron plated bolts. D.3.2 Natural exposure trials During the 10 month exposure period at the Schiphol test site no evidence of corrosion attack was observed. D.3.3 Electrochemical measurements In the first part of this study, the current flow between different coatings and two aerospace aluminium alloys was monitored. In all cases, some galvanic current flow was detected indicating that the coatings were interacting galvanically with the two alloys. Curves showing the variation in galvanic corrosion current with time for each of the coatings coupled to 2014-T6 and 7075-T6 aluminium alloys are reproduced in figures D4 and D5 respectively. In most cases, the current was found to flow from the coating to the aluminium alloys. The exceptions were electrodeposited zinc-nickel alloy coating and Delta-tone where current reversal effects were seen after coupling periods of over 100 hours. Tables D3 and D4 give the average current densities recorded for each of the coatings. In general, higher galvanic current values were observed for the more active zinc base coatings than the less active aluminium base coatings. The coupling of a metal coating with an aluminium alloy can cause corrosion of the aluminium alloy even if the coating is a sacrificial one. This occurs as a result of the build up of alkaline conditions on the aluminium alloy surface, which unlike in the case of steel which is stable at high pHs, can lead to damaging pit attack. In the second part of the work, therefore, the effect of galvanic coupling on the corrosion rate of the aluminium alloys was determined. The galvanic corrosion rate of the aluminium alloy, R g was calculated from the following expression, R g = R a - R o --------------------- (D1) where R o is the self corrosion rate and R a is the total corrosion rate of the aluminium alloy. The corrosion rates, R g, , of the two aluminium alloys were slightly increased by coupling with cadmium plating as indicated in tables D3 and D4. Page 72 GARTEUR SM/AG17 TP128
GARTEUR LIMITED Since cadmium plating is usually regarded as being galvanically compatible with aluminium alloys a slight increase in the corrosion rate was considered acceptable. The alternative coatings were found to fall into two categories:- (1) Coatings that increased the corrosion rate of aluminium alloys above that found for cadmium plating (2) Coatings that lowered the corrosion rate of aluminium alloys below that found for cadmium plating The coatings, which fell into the first category, were ED Zinc-cobalt-iron, Delta-tone, PVD Aluminium, UBMS Aluminium - magnesium and SermeTel 984. The coatings that lowered the rate of aluminium alloy corrosion were ED Zinc-nickel and ED Aluminium. D.4 Conclusions 1. Results of the neutral salt fog tests made on coated fasteners inserted into aluminium alloy blocks indicate that the Delta-tone coating is the most promising as no rusting was detected. 2. Galvanic current measurements have shown that several of coatings, including ED zinc-cobalt-iron, Delta-tone, PVD aluminium, UBMS Al-magnesium and SermeTel 984, increased the corrosion rate of the aluminium alloys above that found for cadmium plating. ED zinc-nickel and ED aluminium were found to lower the corrosion rate D.5 References [D1] Baldwin K.R. , Robinson M.J. and Smith C.J.E., “Galvanic corrosion behaviour of electrodeposited zinc and Zn-Ni coatings coupled with aluminium alloys” British Corrosion Journal 29 No.4 pp293-298 (1994) GARTEUR SM/AG17 TP128 Page 73
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Page 68 and 69: GARTEUR LIMITED B.7 Figures Hi - Lo
Page 70 and 71: GARTEUR LIMITED ED Cadmium IVD Alum
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Page 118 and 119: GARTEUR LIMITED Cadmium coated bolt
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