Electrodeposited coatings corrosion mechanisms

An important category of the surface finishing industry is plating. This includes the process of electroplating which provides metals with surface coatings (usually by electrodeposition) for corrosion protection, wear resistance, improved electrical and mechanical properties, etc. Ferrous and non-ferrous materials can be electroplated with copper, nickel, chromium, zinc, lead, cadmium, etc. They can also be electroplated with precious metals such as gold and silver. 

A major advantage of the electroless nickel process is that deposition takes place at an almost uniform rate over surfaces of complex shape. Thus, electroless nickel can readily be applied to internal plating of tubes, valves, containers and other parts having deeply undercut surfaces where nickel coating by electrodeposition would be very difficult and costly. The resistance to corrosion of the coatings and their special mechanical properties also offer advantages in many instances where electrodeposited nickel could be applied without difficulty. 

S. Wencheng and O.I. Jude. Electrodeposition mechanism, adhesion and corrosion performance of polypyrrole and poly(N-methylpyrrole) coatings on steel substrates. Synth. Metals, 2000, Vol. 114, pp. 225-234. 

Ni-B and Ni-B-Zn alloy coatings were electrodeposited on low carbon steel substrates (20 mm diameter and 5 mm thick). All substrates were mechanically cleaned from corrosion products prior to the coating process. After mechanical cleaning, all the samples were degreased with acetone, cleaned in an alkaline solution and thoroughly washed with distilled water. Finally, the surfaces of all the above prepared substrates were activated with a 20 % solution of HCl for about 1 minute and then rinsed with distilled water. The chemical composition and operating conditions for electrodeposition of coatings are presented in Table 1. Hot... 

There have been several recent reports of corrosion studies involving polypyrrole or polypyrrole composites on iron or steel [217-226]. One study examined the influence of preparation method on the morphology, mechanical properties, and corrosion inhibition of PPy films on steel, concluding that the best mechanical properties (microhardness. Young s modulus, and elastic recovery) and the best corrosion protection were obtained for coatings electrodeposited at constant current and then thermally treated at 80°C for 1 d [219]. 

Nickel contents of up to about 15% are incorporated in a zinc coating, usually by electrodeposition but also by mechanical coating. (The addition of 0.06-0.11% nickel in hot dip galvanizing, to suppress excessive Zn alloy formation, does not influence the corrosion resistance Van Eijnsbergen unpublished report). 

Li, J. Sim, Y. Sun, X. Qiao, J. (2005). Mechanical and corrosion-resistance performance of electrodeposited titania-nickel nanocomposite coatings. Surface Coatings Technology, Vol. 192, No. 2-3, pp. 331- 335, ISSN 2578972... 

The early applications of electrodeposition were mainly confined to situations where relatively thick polycrystalline metal deposits were needed. These included protective or sacrificial metal layers for corrosion protection, decorative applications, and for coatings with specific mechanical properties. However, in the last several decades, electrodeposition has been proven to be one of the enabling fabrication methods behind the train of hi-tech enterprises. There are many examples where electrodeposition is used as convenient if not the approach to deliver the desired structures, mate-... 

Research work conducted over many years [118-121] has demonstrate that through composite electrodeposition, significant improvements in the properties of the pure metal matrix can be achieved, including hardness, wear resistance, and corrosion behavior. In particular, the incorporation of ceramic or other hard particles is an effective way to improve coating hardness and wear resistance. Hardness increase can be explained according to the Orowan mechanism of dispersion hardening [122], as long as particle size is less than 1 pm [119]. This increase depends on the interparticle distance, i.e., on particle size and volume fraction of the hard phase. 

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