Chromium coatings porous

Corrosion resistance. Chromate coatings are mostly applied on aluminum components, particularly on aircraft components and on aluminum objects which are to be coated with paints. They are beneficial in marine environments. Chromium coatings provide an non-porous bond for all paints. 

Most cast irons, except those fully decarburised during malleabilising, give coatings of the chromium carbide type. In view of the great variations in composition of cast irons, reproducibility of results can be achieved only by careful control of specification. High phosphorus and sulphur contents are detrimental to the formation of non-porous coatings. 

The case is different again under micro-discontinuous (i.e. micro-cracked or micro-porous) chromium, on which a definite improvement in corrosion resistance can be achieved when copper is present under the nickel coating . 

Sunderman FW Jr, Hopfer SM, Swift T, et al. 1989c. Cobalt, chromium, and nickel concentrations in body fluids of patients with porous-coated knee or hip prostheses. J Orthop Res 7 307-315. 

Coating of polymer surfaces with thicker metal layers (10-30 pm) is a much more complicated operation. Several pre-treatment steps are required first the polymer surface has to be modified in such a way that, by a chemical process, a layer of copper or nickel can be deposited. With ABS use is made of the circumstance that it is a two-phase system, consisting of a hard matrix in which rubber particles are dispersed. The rubber particles present at the surface, are etched away, leaving a rough, porous surface, which offers a good adhesion to the chemically deposited copper or nickel. Thereafter the application by electrolysis of further layers of other metals (e.g. chromium) is simple. Also for PP, PMMA and polyamides, methods have been developed for chemical deposition of the first metal layer. 

Ducheyne, P. and Healy, K.E. (1988) The effect of plasma-sprayed calcium phosphate ceramic coatings on the metal ion release from porous titanium and cobalt-chromium alloys. J. Biomed. Mater. Res.,... 

Sunderman FW Jr, et al. (1987) Cobalt, Chromium, and Nickd Concentrations in Body Fluids of Patients with Porous-coated Co/Cr Prostheses. In Proceedings Second Nordic Symposium on Trace Elements, Odense. WHO, Copenhagen. 

Thin coatings of chromium [139] are applied over another electrodeposited layer, usually nickel or copper, as a decorative, nonstaining finish these thin (250 pm) layers are porous. Hard chromium plating is used where advantage may derive from the high hardness and wear resistance, or the low coefficient of friction, of such deposits. Chromium electrodeposits are not suitable for use at high... 

Chromium is cathodic to steel however, economic electrodeposited coatings of chromium are too thin and porous to give protection on their own and are used in conjunction with thick (25 pm) deposits of copper or nickel. Chromized coatings, on the other hand, are relatively thick and can confer protection by their barrier action, although corrosion of the steel substrate will be increased at any discontinuities in the coating. 

An interesting variation of the effect of galvanic coupling occurs with metals that exhibit active-passive transitions. When noble metals such as platinum, which are good catalysts for hydrogen reduction, are coupled to a metal with an active-passive transition below the reversible proton-hydrogen potential, spontaneous passivation may ensue (Fig. 7). Thus, a porous coating of noble metal on titanium, chromium, or stainless steels will result in anodic protection of the substrate. 

In the anodic atmosphere, metallic nickel is stable, and so nickel coating improves the durability of the stainless steel. On the cathode side, chromium easily dissolves from the stainless steel. Therefore, the outer layer and inner layer are porous LiFe02 and dens chromium-rich oxide layer as the corrosion protection layer, respectively. The corrosion of the current collectors increases the electronic resistance and decreases the amount of electrolyte. Therefore, the material is selected with a lower electronic resistance of the corrosion layer and lower crmsumption of electrolyte. Based on these requirements, SUS316L is better than SUS310 even it has higher Ni and Cr crmtents [2, 4, 5]. 

---