Cadmium Substitution - garteur

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GARTEUR LIMITED cadmium although it can be seen that as the nickel content was increased so the time to red rust was extended. J.3.2 Neutral salt fog tests on repaired panels The re-plated panels as described by the matrix in table J1 were subjected to neutral salt fog for a period of 336 hours and then inspected for signs of corrosion. It was found that the Garteur cadmium coatings were satisfactorily re-plated with cadmium, zinc-nickel or zinc-cobalt alloy. Adhesion tests carried out on re-plated coatings showed no signs of failure. On exposure to neutral salt fog, the repair coatings of zinc-nickel and zinc-cobalt failed within 336 hours, as anticipated from the data shown in table J2, where in each case red-rust occurred after 110 and 310 hours respectively. However no red-rust was observed on the Garteur coatings. When re-plating a metallic layer with a coating of a dissimilar metal there is always the risk that galvanic corrosion may occur at the interface between the two coatings. In the present study, no evidence of galvanic corrosion was observed where the coatings overlapped for the cadmium - cadmium or cadmium - zinc-nickel combinations, indicating that they were fully compatible. In contrast, when the zinc-cobalt alloy was employed in re-plating, the cadmium coating was badly pitted around the interface, suggesting that galvanic corrosion had occurred leading to attack on the cadmium plating. For cadmium, which had been repaired with zinc-cobalt, it was found that after 380 hours, red rust started to bleed from the interface showing that the patch had broken down. Table J3 summarises the occurrence of galvanic corrosion between the original coatings and the repair coatings. The repair of PVD aluminium coatings presented particular difficulties due to adhesion problems and the zincate treatment was applied in each case. For cadmium, it was found that the coatings could not be brush plated onto aluminium despite the use of the zincate treatment. However it was found that the zincate treatment could be successfully employed in the brush plating of alkaline zinc-nickel coatings onto PVD aluminium. In the absence of the zincate treatment adhesion failures occurred. In contrast, for the acidic zinc-nickel and zinc-cobalt electrolytes no adhesion problems were observed even in the absence of the zincate treatment. In the neutral salt fog tests the zinc-nickel and zinccobalt coatings were found to afford good protection with no red rust on the re-plated areas. In the absence of the zincate treatment, the more electrochemically active zinccobalt coating caused some sever galvanic corrosion on the aluminium coating, however, when the zincate treatment was applied this effect was considerably reduced. J.4 Conclusions 1. In neutral salt fog trials, the zinc-cobalt alloy was found to provide the highest levels of corrosion protection to a mild steel substrate giving greater protection than either cadmium or zinc-nickel alloy plating. 2. Simulated corrosion damage and re-plating tests showed that brush plated zinccobalt and zinc-nickel coatings could be used to re-protect a range of Garteur coatings including zinc-nickel, zinc-cobalt-iron, PVD aluminium and cadmium. 3. Some evidence of galvanic interactions were observed when using the brush plated zinc-cobalt alloy coating for repair. These effects were minimised when a zincate chemical treatment was used. J.5 Tables Page 106 GARTEUR SM/AG17 TP128
GARTEUR LIMITED Garteur coatings <strong>Cadmium</strong> Acid Zn -Co Brush - plated coatings applied (electrolyte used shown in brackets) Acid Zn-Ni Alkaline Zn- Ni ED cadmium 4 4 4 ED Zn-Co-Fe 4 4 ED Zn-Ni 4 4 IVD Aluminium 4 4 4 4 Table J1; Brush-plated coatings applied to Garteur coatings Electrolyte Alloy composition (wt%) Thickness Time to red Zn Ni Co Cd μm rust (hrs) Acid zinc-nickel 89.5 10.5 7.5 158 Alkaline zinc-nickel 92.2 7.8 8.0 110 Acid zinc-cobalt 99.0 1.0 7.5 310 <strong>Cadmium</strong> 100 8.1 148 Table J2; Corrosion data for 8μm brush -plated coated steel exposed to neutral salt fog GARTEUR SM/AG17 TP128 Page 107
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GARTEUR LIMITED Executive summary B
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GARTEUR LIMITED G.1 Introduction 89
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GARTEUR LIMITED Coating Thickness
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GARTEUR LIMITED As plated Passivate
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GARTEUR LIMITED Properties ED Zn-Co
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GARTEUR LIMITED A.2.4 Delta-tone/De
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GARTEUR LIMITED A.4 Tables Coating
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GARTEUR LIMITED 1 Abrasive blast us
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GARTEUR LIMITED 1 Activation bath (
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GARTEUR LIMITED B.3.3 Coating Adhes
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GARTEUR LIMITED Type A Measurements
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GARTEUR LIMITED B.7 Figures Hi - Lo
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Page 118 and 119: GARTEUR LIMITED Cadmium coated bolt
Page 122 and 123: GARTEUR LIMITED Garteur main coatin
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