Cadmium Substitution - garteur

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GARTEUR LIMITED properties and sacrificial properties of the coating. The thickness of the coating will also have a major impact on the performance. The results in table 5 indicate that the corrosion resistance of the electro-deposited zinc alloy coatings, in both the as plated and passivated conditions, are similar to that of electroplated cadmium. The Delta-tone coating, which contains zinc flakes, gives a corrosion current similar to the two zinc alloy coatings. The aluminium based coatings all gave much lower corrosion currents indicating that they are more effective barrier coatings. Corrosion current data obtained by NLR and BAe for different chloride concentrations gave similar results. An indication of the sacrificial properties of the coatings can be obtained from the measurement of the open circuit potential in sodium chloride solution. Tests were made by DBAA, DERA, NLR and BAe in various concentrations of sodium chloride solution. Detailed results are given in table C11 in Annex C. Figure 1 shows the differences in open circuit potential compared to cadmium plating. The data in figure 1 indicate that the zinc rich coatings i.e. ED zinc-nickel, ED zinccobalt-iron and Delta-tone have lower open circuit potentials that ED cadmium and would be expected to provide a greater degree of sacrificial protection than cadmium. Two sets of experiments were conducted at DERA in order to quantify the sacrificial properties of the coatings. The first approach was to use a scratch model test piece. This allows the galvanic current developed between the coating and the steel substrate to be monitored. Details of the specimen design and the technique are given in Appendix C. The second approach used was to measure the distance over which the coating is able to provide protection to the exposed substrate. Again details of the technique are given in Appendix C. Figures 2 and 3 compare the galvanic currents and the protection distances for both the unpassivated and passivated coatings respectively. The results presented in figures 2 and 3 indicate that the two pure aluminium coatings together with the SermeTel coating are less effective as sacrificial coatings than cadmium plating. Some improvement is achieved through the addition of magnesium. The results are generally in line with the behaviour predicted from the open circuit corrosion potential measurements given in figure 1. A low value for the open circuit potential of the ED aluminium was recorded in the present study. However previous electrochemical studies on pure aluminium have shown that the potential may fall rapidly with the initiation of corrosion pits [1]. Overall the electrodeposited zinc alloy coatings and the zinc containing Delta-tone coating give a greater level of sacrificial protection than the pure aluminium coatings. Values for the open circuit potential shown in figure 4.1 for these coatings are much lower than for cadmium plating indicating that they should provide a high degree of sacrificial protection. 4.2.2 Corrosion tests on undamaged panels Accelerated corrosion tests and outdoor exposure trials were made on both undamaged and scratched coated test panels. The time to first rust was used to compare the performance of the different coating systems. In the case of the undamaged panels this will depend both on the barrier and the sacrificial properties of the coatings. Their relative importance will depend on the nature of test, for example whether the coating is continuously exposed to a corrosive environment or whether there is a drying cycle. With the scratched panels, the occurrence of rusting in the scratch is used to assess the sacrificial properties of the coating. A) Accelerated corrosion tests Page 8 GARTEUR SM/AG17 TP128
GARTEUR LIMITED The results of the neutral salt fog tests undertaken by DERA are given in figure 4. The data show that only the two commercial metallic ceramic coatings, SermeTel and Deltatone out performed cadmium when tested in the as plated condition. In every case carrying out post plating treatments such as passivating considerably improved the performance of the coating but only the SermeTel 984 coating with SermeTel 985 gave a higher time to first red rust than passivated cadmium plating. Neutral salt spray tests were conducted by two other laboratories Shorts and Fokker on a limited number of coatings. Although the actual times to red rust for undamaged panels varied considerably between the three laboratories, there was general agreement regarding the relative performances of the coatings. As plated SermeTel 984 > ED cadmium >ED zinc – nickel ˜ ED zinc – cobalt – iron Plated + post plating treatment SermeTel 984 + 985 > Pass. ED cadmium > Pass. ED Zn-Ni ˜ Pass. ED Zn-Co-Fe As indicated in section 4.1, the thicknesses of the different coatings studied varied considerably. The thicknesses were specified by the various coating suppliers to give the desired combinations of corrosion protection, adhesion, frictional properties etc. The SermeTel 984 coating for example has a mean thickness of about 40μm compared with PVD aluminium, which was found to be 18 – 22μm thick. Previously published work has looked at the dependence of time to red rust on coating thickness [2]. This shows that for relatively thin coatings an almost linear relationship exits. However for thicker coatings, small increases in coating thickness have a large effect on the time to red rust. In the present study, the results given in figure 4 have been compared taking into account the differences in coating thicknesses. A “normalised” time to red rust was calculated for each coating system by dividing the time to red rust by the coating thickness. The values obtained are compared in figure 5. From figure 5 it is apparent that whilst the as plated UMS Al-Mg, Delta-tone and SermeTel coatings out perform cadmium plating in the neutral salt fog tests, the passivated cadmium plating gives the highest level of corrosion protection. In the present programme two additional accelerated corrosion tests were employed to evaluate the coatings. These were • Exposure to intermittent acidified salt spray (MASTMAASIS test) • Exposure to 100% humidity MASTMAASIS testing was undertaken by DERA and NLR whilst exposure to 100% humidity was investigated by Fokker. The main observations from the MASTMAASIS tests are summarised in table 6. Whilst good agreement was obtained for tests conducted on as plated panels some differences were found on tests carried out on passivated panels. Humidity tests were conducted on the ED zinc alloy coatings and on the two metal – ceramic coatings. Testing was continued for 2000 hours and at this point only the Deltatone showed evidence of rusting. B) Outdoor exposure trials GARTEUR SM/AG17 TP128 Page 9
Page 2 and 3: GARTEUR LIMITED This page is intent
Page 6 and 7: GARTEUR LIMITED Executive summary B
Page 8 and 9: GARTEUR LIMITED nickel coatings cou
Page 10 and 11: GARTEUR LIMITED Page 9 Recommendati
Page 12 and 13: GARTEUR LIMITED G.1 Introduction 89
Page 16 and 17: GARTEUR LIMITED The various aims of
Page 18 and 19: GARTEUR LIMITED 3 Summary of work p
Page 20 and 21: GARTEUR LIMITED The adhesion of con
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Page 26 and 27: GARTEUR LIMITED coatings showed an
Page 28 and 29: GARTEUR LIMITED torques of 7 and 17
Page 30 and 31: GARTEUR LIMITED 5.3 Conclusions The
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Page 36 and 37: GARTEUR LIMITED 11 Tables Coating t
Page 38 and 39: GARTEUR LIMITED Coating Thickness
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Page 48 and 49: GARTEUR LIMITED Properties ED Zn-Co
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Page 66 and 67: GARTEUR LIMITED Type A Measurements
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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GARTEUR LIMITED the salt spray was
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GARTEUR LIMITED c) Exposure to 100%
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GARTEUR LIMITED [C7] Baldwin K.R.,
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GARTEUR LIMITED Coating rust (hours
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GARTEUR LIMITED Post plating treatm
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GARTEUR LIMITED DBAA DERA NLR NLR B
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GARTEUR LIMITED D.3 Results and dis
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GARTEUR LIMITED D.6 Tables Time hou
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GARTEUR LIMITED E.6 Tables Laborato
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GARTEUR LIMITED I.7 Tables Coating
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GARTEUR LIMITED D-28183 Bremen, GER
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