Electrolyte passivation

Gu, W.Y., Lai, W.M, and Mow, V.C. (1998) A mixture theory for charged-hydrated soft tissues containing multi-electrolytes passive transport and swelling behaviors. Journal of Biomechanical Engineering 120, 169-180... 

Gu, W.Y., W. M. Lai, and V. C. Mow. (1998) A Mixture Theory for Charged-hydrated Soft Tissues Containing Multi-electrolytes Passive Transport and Swelling Behaviors. J. Biomech. Eng. 120, 169-180... 

Copper follows the predictions of its diagram. For this seminoble metal, the potential has to be sufficiently positive to get anodic oxidation. In acidic electrolytes this causes anodic dissolution, and the oxide layers dissolve sufficiently fast so that they are non-protective. For pH > 5.0 these oxides are insoluble, i.e. they are in dissolution equilibrium with the electrolyte. Passivity is achieved when oxide formation occurs at sufficiently positive potentials according to the reactions of Eqs. (1) and (2). 

Anodizing is an electrolytic passivation process that increases the thickness of natural oxide layers on the surface of metals. It basically forms an anodic oxide finish on a metal s surface to increase corrosion resistance. For the anodizing process, the metal to be treated serves as the anode (positive electrode) of an electrical circuit. 

Anodizing is an electrolytic passivation process that increases the thickness of natural oxide layers on the surface of metals [13]. It basically forms an anodic oxide finish on a metal s surface to increase corrosion resistance. For the anodizing process, the metal to be treated serves as the anode (positive electrode, where electrons are lost) of an electrical circuit. Anodized films are most often applied to protect aluminum alloys. An aluminum alloy is seen on the front bicycle wheel in Fig. 2 [14]. For these alloys, aluminum is the predominant metal. It typically forms an alloy with the following elements copper, magnesium, manganese, silicon, tin, and zinc [15]. Two main classifications for these alloys are casting alloys and wrought alloys, both of which can be either heat treatable or non-heat treatable. 

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. 

Anodizing, or anodizing, is an electrolytic passivation process used to increase the thickness of the natural oxide l er on the surface of metal parts. The process... 

Electrolytic passivation is illustrated in figure P.l, which shows the behaviour of an iron anode immersed in any suitable electrolyte solution when an increasingly positive potential is imposed on it. At first the current increases in the usual way as metal ions go into solution, but at the passivation potential there is an abrupt change, and the current falls to a very low value which does not increase until some other anodic reaction takes over. The curve can be retraced, and the metal reactivated by holding it at a lower potential. Unless the potential change is very slow, the curve will show some hysteresis while the metal surface reverts to its original condition. Some passivation potentials, on the hydrogen scale, are iron +0.58 V, nickel +0.36 V,... 

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

The protective quality of the passive film is detennined by the ion transfer tlirough the film as well as the stability of the film with respect to dissolution. The dissolution of passive oxide films can occur either chemically or electrochemically. The latter case takes place if an oxidized or reduced component of the passive film is more soluble in the electrolyte than the original component. An example of this is the oxidative dissolution of CrjO ... 

From an electrochemical viewpoint, stable pit growtli is maintained as long as tire local environment witliin tire pit keeps tire pit under active conditions. Thus, tire effective potential at tire pit base must be less anodic tlian tire passivation potential (U ) of tire metal in tire pit electrolyte. This may require tire presence of voltage-drop (IR-drop) elements. In tliis respect the most important factor appears to be tire fonnation of a salt film at tire pit base. (The salt film fonns because tire solubility limit of e.g. FeCl2 is exceeded in tire vicinity of tire dissolving surface in tlie highly Cl -concentrated electrolyte.)... 

Sometimes the formation of oxide films on the metal surface binders efficient ECM, and leads to poor surface finish. Eor example, the ECM of titanium is rendered difficult in chloride and nitrate electrolytes because the oxide film formed is so passive. Even when higher (eg, ca 50 V) voltage is apphed, to break the oxide film, its dismption is so nonuniform that deep grain boundary attack of the metal surface occurs. 

The occurrence of pitting seems to stem from the differential stabiUty of the passive film that forms on the titanium alloy. This film does not break down uniformly even when the electrolytes are fluoride and bromide based. The pitting can be so severe that special measures are needed to counteract it. 

Hot Dip Tin Coating of Steel and Cast Iron. Hot dipping of tin [7440-31 -5] has been largely superseded by electrolytic coating techniques, especially for sheet. However, hot dipping can be the method of choice for complex and shaped parts. Very thin layers of tin are extensively used to passivate steel used for canned goods. Tin is essentially nontoxic, is nearly insoluble in almost all foods, and easily wets and completely covers steel with a pinhole-free coating. 

Stainless steel develops a passive protective layer (<5-nm thick) of chromium oxide [1118-57-3] which must be maintained or permitted to rebuild after it is removed by product flow or cleaning. The passive layer may be removed by electric current flow across the surface as a result of dissinulat metals being in contact. The creation of an electrolytic cell with subsequent current flow and corrosion has to be avoided in constmction. Corrosion may occur in welds, between dissimilar materials, at points under stress, and in places where the passive layer is removed it may be caused by food material, residues, cleaning solutions, and bmshes on material surfaces (see CORROSION AND CORROSION CONTROL). 

Fig. 2. Schematic representation of relevant electrolyte transport through the renal tubule, depicting the osmolar gradient ia medullary iaterstitial fluid ia ywOj yW where represents active transport, —passive transport, hoth active and passive transport, and passive transport of H2O ia the presence of ADH, ia A, the cortex, and B, the medulla. An osmole equals a mole of solute divided by the number of ions formed per molecule of the solute. Thus one mole of sodium chloride is equivalent to two osmoles, ie, lAfNaCl = 2 Osm NaCl. ADH = antidiuretic hormone.

Electrolytic corrosion occurs in regions I and IV with the formation of soluble iron ions. Solid corrosion products which can have a protective effect are formed in region II. This is the region of surface film passivity. Certain corrosive sub-... 

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