Cadmium coatings corrosion resistance

Zinc—Nickel. Steel has the best salt spray resistance when the nickel is 12—13% of the alloy. At increasing nickel contents, the deposit becomes more difficult to chromate and more noble, eventually becoming cathodic to steel. At those levels and above, corrosion resistance usually decreases and is dependent on a complete lack of porosity for protection of the steel. In efforts to replace cadmium and nickel—ca dmium diffused coatings in the aircraft industry, 2inc—nickel has insufficient wear properties for some appHcation, but is under study as an undercoat to various electroless nickel top coats (153). 

Compared with other methods, vacuum evaporation produces coatings that have a most satisfactory corrosion resistance, e.g. 0.005 mm of evaporated Cd gives a degree of protection to steel similar to that afforded by 0.01 mm of electrodeposited Cd. Cadmium coatings on ferrous and other substrates can meet authoritative specifications concerning corrosion... 

The aluminium coatings are highly corrosion resistant, and are less liable to contact corrosion than cadmium when in contact with light alloys. They can also be used at temperatures of up to 496°C as against 232°C for cadmium. The aluminium coating is also unaffected by aviation fuels, unlike cadmium. 

The degree of protection given in practice by zinc and cadmium, whether by physical exclusion or by sacrificial action at gaps, depends on the durability of the coatings themselves against corrosive attack. It is now well established that, thickness for thickness, cadmium is more resistant to... 

Full chromate passivation (Section 15.3) improves the corrosion resistance of both zinc and cadmium towards all environments and is applied for a wide range of applications. Clear and olive-coloured chromate coatings can also be applied for certain purposes. The highest degrees of corrosion protection... 

Post-Treatments. Although many post-treatments have been used over plated metals, chromate conversion coatings remain as the most popular. Chromates are used to improve corrosion resistance, provide good paint and adhesive base properties, or to produce brighter or colored finishes. Formulations are usually proprietary, and variations are marketed for use on zinc, zinc alloys, cadmium, copper and copper alloys, and silver (157). Chromates are also used on aluminum and magnesium alloys (158,159). More recently, chromate passivation has been used to extend salt spray resistance of autocatalytic nickel plated parts. 

Galvanoaluminum layers thicker than 12 pm are extraordinarily corrosion resistant to exposure to SO2, in which they surpass the performance of cadmium and zinc layers [31]. After 20 rounds of the SFW 2.0 L, DIN 50018 Kesternich test, in contrast to the ca. 12 pm thick, yellow chromated galvanoaluminum layers, the zinc and cadmium coatings were already heavily corroded. The SO2 resistance of anodized galvanoaluminum can also be considered excellent in comparison to cadmium and zinc coatings. 

The elements may be obtained by reduction of sulfides or oxides (e.g. ZnO with C). Zinc and cadmium are used for corrosion-resistant coatings. The metals have melting and boiling points that are lower than for group 2 elements, especially with Hg, which is one of two elements (Br being the other) existing as a liquid at... 

Cadmium Cadmium plating is used to increase the corrosion resistance of steel fasteners and electrical connectors. However, the Cd plate is itself subject to oxidation and tarnishing in humid and tropical environments to such an extent that electrical and mechanical functionality may be adversely affected. CCCs may are applied to Cd to resist oxidation, or to increase paintability [81]. Cd plating can be successfully conversion coated using chemistries applicable to other metals. 

Where the corrosion resistance of a coating depends upon its passivity, it is common to follow plating with a conversion coating process to strengthen the passive film. Zinc, cadmium and tin in particular are treated with chromate solutions which thicken their protective oxides and also incorporate in it complex chromates (see Section 1S.3). There are many proprietary processes, especially for zinc and cadmium. Simple immersion processes are used for all three coatings, while electrolytic passivation is us on tinplate lines. Chromate immersion processes are known to benefit copper, brass and silver electrodeposits, and electrolytic chromate treatments improve the performance of nickel and chromium coatings, but they are not used to the extent common for the three first named. 

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