Anodized coatings mechanics

The standard potential for the anodic reaction is 1.19 V, close to that of 1.228 V for water oxidation. In order to minimize the oxygen production from water oxidation, the cell is operated at a high potential that requires either platinum-coated or lead dioxide anodes. Various mechanisms have been proposed for the formation of perchlorates at the anode, including the discharge of chlorate ion to chlorate radical (87—89), the formation of active oxygen and subsequent formation of perchlorate (90), and the mass-transfer-controUed reaction of chlorate with adsorbed oxygen at the anode (91—93). Sodium dichromate is added to the electrolyte ia platinum anode cells to inhibit the reduction of perchlorates at the cathode. Sodium fluoride is used in the lead dioxide anode cells to improve current efficiency. 

Available forms Structural shapes of all types, plates, rods, wire foil flakes, powder (technical and USP). Aluminum can be electrolytically coated and dyed by the anodizing process (see anodic coating) it can be foamed by incorporating zirconium hydride in molten aluminum, and it is often alloyed with other metals or mechanically combined (fused or bonded) with boron and sapphire fibers or whiskers. Strengths up to 55,000 psi at 500C have been obtained in such composites. A vapor-deposition technique is used to form a tightly adherent coating from 0.2 to 1 mil thick on titanium and steel. 

Pitting of aluminum occurs at defects in the anodized coating due to poor process control or mechanical forces in service such as stone pecking.3,4 characterized by a central pit which is surrounded with a white halo of corrosion product. Pitting is more prevelant in high chloride containing environments. 

Corrosion inhibition of chemically and electrochemically synthesized coatings of PANI, poly (o-methoxyaniline), and their copolymers on stainless steel and on aluminum alloy (6061-T6) was evaluated by immersion in 3% NaCl (steel) and 0.1 M NaCl (A1 alloy). These authors concluded from polarization studies that protection involved an anodic protection mechanism [206]. 

Many protection mechanisms related to ICPs have been proposed in the literature, and in some cases opposing evidence exists. This confusion may be attributed to the wide variations in experimental procedures used (coating type, substrate preparation, corrosive environment, test method). In the following, we just list the main mechanisms that have appeared in the literature. Further details can be found in the review by Spinks et al. [7]. However, the anodic protection mechanism will be discussed at length because it is the most commonly accepted mechanism in previous studies. 

The protection mechanism is similar to that of metallic zinc coatings zinc is less noble than steel and protects the substrate by forming a galvanic corrosion cell, in which zinc is the anode. This mechanism is particularly active in presence of defects in the coating that provide an electrolyte path to the substrate surface. In addition, the atmospheric corrosion of zinc yields voluminous solid corrosion products (oxides, carbonates, etc.) that are capable of blocking pores or small defects in the coating, thereby reinforcing its barrier effect. 

Ma.nga.neseDioxide. Graphite plates used as anodes in this process are coated with MnO during electrolysis. The anodes are removed from the solution periodically and the MnO is removed by mechanical methods. Graphite can also be used as the cathode material. Titanium is used as anode materials where high quaHty MnO is desired. 

A single or multicored plastic-coated cable of the type NYY or NYY-O is used as the connecting cable between a protected object and an anode in soils and fresh water, and particularly in seawater, medium heavy or heavy rubber-sheathed connections of an NSHou or NSSHou type are used. Heavy welded connections of type NSLFSou are used for severe mechanical loading. In addition to these, for ships, marine cable of type MGCG or watertight cables must be considered. 

---